Proc. of Second World Avocado Congress 1992 pp. 25-35
The Avocado and Human Nutrition. I. Some
Bob Bergh
Department
of Botany and Plant Sciences, University of California,
Riverside,
CA 92521, USA
Abstract. Avocado usage is reduced
because of negative misconceptions. The avocado has various uses as a natural
cosmetic, with advantages in rapid skin penetration, and as a superior natural
sunscreen. Avocado oil has several culinary and health benefits. The greater
use of whole fruit has important advantages: usefulness in human weight
control, high nutritional density, source of major antioxidants, stroke
prevention, fruit protein source, fiber source; as baby food, and other dietary
benefits. Erroneous public perceptions of avocado calorie content and of cancer
influences are noted.
Many people
appreciate the delicious flavor of avocado, but are afraid of them for
(mistaken) dietary reasons. They therefore avoid avocados or eat less than they
could, because of incomplete or erroneous nutritional information. Most of us
connected with the avocado industry have encountered such errors in
understanding among friends or chance acquaintances. Indeed, on occasion, one
even encounters them among "avocado" people.
These errors are diverse and
recurrent. One sometimes feels a bit like Hercules of classical mythology
battling the ugly Hydra: as soon as one evil head is cut off, two more grow in
its place! Its seems worthwhile, therefore, to make a summary of the scientific
findings concerning the avocado. We will begin with a brief look at cosmetics,
then examine the more important food attributes, and close with an analysis of
misconceptions in the press and among the public.
Avocado Cosmetics
Increasing recognition of
unhealthful consequences from additives, preservatives, processing, and
artificial products generally gives the avocado a major advantage as a food and
also as a cosmetic. With increasing concern for the environment, an
added plus for avocado cosmetics is that they are biodegradable.
Consumers are beginning to favor basic, natural ingredients. Swisher (1988)
discusses use of the avocado as a skin moisturizer, cleansing cream, makeup
base, sunscreen, lipstick, bath oil, and hair conditioner. Toxicological tests
of avocado oil products have provided an official health/safety assessment.
In skin care, the two major
advantages of the avocado are its marked softening and soothing nature and its
notable absorption. Compared with almond, corn, olive, and soybean oils,
avocado oil had the highest skin penetration rate (Swisher, 1988).
In sunscreens, chemicals
like PABA have superior effectiveness but cause skin irritation in some people.
Because they are synthetic, there are lingering questions about long-term
safety. Among eight plant
oils, avocado oil proved the most effective sunscreen (Swisher, 1988).
Avocado
Oil
Like olive oil, avocado oil
is predominantly monounsaturated, a property which is thought to confer
distinct health benefits. The avocado as a refined cooking oil has additional
advantages (Swisher, 1988).
1.
It
is unusually light, so it mixes well with other foods.
2.
2.
It has a mild, delicate flavor, which enhances and brings out the flavor of
other foods, instead of dominating them such as olive oil and other oils.
3.
3.
Avocado oil withstands a high cooking temperature before breaking down, i.e.,
its "smoke point" is about 255C (490F), which is much higher than
that of olive oil.
Fried foods presently have an undesirable health
reputation, but use of olive and avocado oil could change that.
Whole fruit pulp
This is by far the most important human use of the
avocado and will
constitute the remainder of this article. We will look at several nutritional
qualities which sometimes overlap.
Weight control. Contrary to popular assumptions, the avocado
can be a helpful part of a successful weight-management program. It brings
several advantages.
- Its monounsaturated fat speeds up the basal metabolic rate, as
compared with saturated fat.
- Its high fat content gives a quicker feeling of satiation
("fullness"), thus helping to reduce overeating.
- Its high fat content makes an overall sound diet more palatable,
reducing the temptation to binge on foods high in sugars or saturated
fats.
- Its rich supply of
vitamins and minerals also makes the diet more wholesome and satisfying
and thus more conducive to overall health and to moderation in
consumption.
The high fat content of most
avocados (up to 90% of its calories) will make some people dubious about its
positive effects on weight control. Not only does fat have over twice as many
calories per unit weight as does carbohydrate or protein, but also if excess
calories are consumed, the body apparently burns about three times as many
calories when storing calories from carbohydrate or protein as it burns when
calories from fat are stored. Recent evidence, however, suggests that calories
are not the whole story for body weight, with timing of both consumption and
exercise as factors and with calories from fat also a possible added handicap.
So, what is known from
experiments with the avocado? The next part of this article will discuss in
detail results from the three studies of known changes in cholesterol resulting
from the addition of avocado to the diet. In each of the three studies, there
was an average small weight loss associated with avocado consumption. In
the most pertinent experiment (Grant, 1960), a mean of just over one California
avocado a day for a mean of 33 days increased average daily calories by a
calculated 24% and fat by 54% but resulted in a weight loss averaging
approximately 1 kg (2.2 lbs). This remarkable result (under exceptional and
tightly controlled hospital conditions) should not be taken as a universal
guarantee; individual results will vary depending upon complex individual
metabolic histories and interactions. What can be said is that eating avocado
has been shown to be fully compatible with good weight control.
Very few people will have rapid weight loss as a
result of increasing their consumption of avocado; however, the more slowly one loses weight,
the less likely one is to regain it. It has become clear that an effective
solution to the widespread overweight problem will not come from simply eating
less. Indeed, such 'dieting' can actually make things worse by causing muscle
loss so that the usual 'yo-yo' weight rebound leaves one fatter than ever and
subsequent weight loss more difficult to achieve than ever. Goodrick and Foreyt
reported in the October, 1991, issue of the American Dietetic Journal that
even a combination of behavioral self-management training plus sharply reduced
calories gave a discouraging 90% eventual relapse level. Part of the reason is
genetic: we tend to inherit our body fat amount and distribution. Part of the
solution is more exercise which, of course, is good for us for other reasons.
The emerging picture is quite complex. In the
February, 1990, issue of the American Journal of Clinical Nutrition, Bouchard
reported from Canada that women who were small eaters (mean of 1488 calories/day)
as compared with similarly exercising 2393 calories/day eaters, actually
weighed 10 lbs (22 kg) more on the average, plus having 22% more body fat. The
explanation is probably a combination of average differences in both genes and
food consumption: i.e., reducing calorie intake can lower an individual's
metabolic rate and thus defeat its purpose. We need to eat enough healthful
food to keep our metabolism high, and to provide the sated feeling that
eliminates binge eating.
Calorie-counting,
dieting, advertised gimmicks and quick weight loss are recipes for failure;
whereas exercise plus healthy food habits as a permanent way of life can bring
success. The situation is analogous to that of personal philosophy: Focus on
happiness and it will probably elude you; focus on wholesome living and
happiness is a likely by-product. Similarly, focus on weight loss and it will
probably elude you; focus on wholesome eating and weight control is a likely by-product.
This insight was expressed
years ago by Wood (1983) in a book hailed by one health magazine as the finest
method for weight reduction known to them. Wood emphasized 'playful' exercise,
but his introduction begins: "The solution to our national overweight
problem is to encourage people to eat more." He counseled eating fresh
plant food "heartily." And his recommended list of "ingredients
for a healthy diet" included the avocado.
Americans spent
an estimated $32 billion in 1989 on diet foods and products with the annual
amount increasing at a rate that could reach $50 billion by 1995. It is
difficult for me to avoid the conclusion that we would be healthier as a nation
if we spent a tiny fraction of that amount to buy more avocados and pocketed
most of the remaining money.
High
nutritional density. The fourth reason given above for the avocado's role in weight control
is its "rich supply of minerals and vitamins." More important than a
food's calorie content is its total nutritional contribution to human needs. A
good measure is nutrients per calorie. Different avocado analyses have given
somewhat variable results. The most detailed publication is that of Slater et
al. (1973). Their data indicate that one half of a 'Hass' avocado, about 80
g edible fruit, provides a substantial percentage of the daily nutritional
needs of a child aged 7 to 10 (adult percentages are generally a little lower,
especially iron for females) (Table 1).
The avocado contains little vitamin B 12 and calcium, limited zinc and modest phosphorus. Its half-a-fruit quota of riboflavin and thiamine for children is about 9.5 and 8%, respectively, hence about equal to its relative calorie contribution. But the striking thing is that no less than eight essential nutrients are apparently present in about a 2: 1 calorie ratio. Nor is that all; three additional nutrients, potassium, copper and pantothenic acid, (for which I could not find precise Recommended Dietary Allowances) are also estimated to be present in avocado at about twice the calorie content. Per calorie, the avocado is indeed remarkably nutritious.
There is not
space here to discuss either the human body's uses or the status in typical
diets of these various nutrients. Judgments vary; several surveys have
concluded that one or another of them is deficient in a considerable portion of
American diets. Next we will look briefly at one of the least discussed
vitamins in the group of nutrients provided by the avocado.
Vitamin B6 (pyridoxine) is important for
the nervous system, red blood cells, teeth and gums. The Harvard Health Letter
for March, 1991, reported 1990 data indicating that most Americans consume too
little of vitamin B6. Among good sources they list watermelon and
banana (overlooking avocado). Polansky and Murphy (1966) compared the 86
content per unit weight of 26 vegetables and fruits. For total chromatographed
86' banana and avocado had the highest amounts, the remaining 24 vegetables had
from 1/5 to 1/25 of the amount of the avocado, with watermelon 1/6. A more
recent report by William Sears, M.D., privately printed Nature's Guide to First
Foods"(1988) reports that avocado has three times as much B6
per g as banana.
Antioxidants and free radicals: cancer. cataracts.
and aging. The
University of California at Berkeley Wellness Letter for October, 1991,
discusses the basic role of oxygen for human cellular energy and for life, but
notes that an inevitable by-product of this activity is the formation of highly
reactive "free radicals." These unstable troublemakers can affect
various cell constituents: perhaps artery walls or LDL cholesterol advancing
coronary heart disease; eye lens tissue causing cataracts; a critical gene
leading to cancer; perhaps arthritis; or DNA (the genetic material) causing
gradual deterioration and aging.
The human body has two main
defenses against these free radicals: enzymes and other blood compounds that depend on trace
minerals and good general nutrition and three potent antioxidants vitamins C, E,
and beta carotene (vitamin A precursor). An article in the June, 1991,
issue of the American
Journal of Clinical Nutrition indicates that only about 57% of nonsmokers may be
getting enough vitamin C daily (for smokers the deficit is much greater). Men's
Health (June, 1991) concluded that most of us are not getting enough
vitamin E. A number of nutritionists have called for greater consumption of
beta carotene.
As we have discussed, the avocado provides about
twice as high a proportion of our daily needs for the above three antioxidant
vitamins as its calorie proportion. Recall also that the avocado is rich in
copper and iron, two mineral constituents of antioxidant enzymes. Nutritionists
usually recommend that we get our needs met from basic food rather than from
supplements. And while the risk of colon cancer, for example, has been
associated with fat consumption, the Harvard Health Letter for March,
1991, reported that while meat indeed increased that risk sharply, there was no
association with plant fat. Thus, eating avocados could be an enjoyable way to
help protect ourselves against cancer, heart disease, arthritis, and eye
cataracts; it may even delay the processes of aging.
Stroke prevention. Heart disease, cancer and
stroke are the three leading causes of death in the United States.
Additionally, stroke can cause many years of partial or total incapacitation
and tragic crippling. The Associated Press, 15 October, 1987, reported an
interview with Dr. Louis Tobian of the University of Minnesota on protection
from strokes by fruit and vegetable consumption, in particular, by high
potassium intake. Dr. Tobian is quoted as suggesting that bachelors may die
earlier than married men because they have less balanced diets, specifically
insufficient in potassium. As the best sources of potassium, he included
strawberry, banana, citrus juice, potatoes, and milk (avocado is not
mentioned).
Smith et a/. (1983)
measured potassium in 100 g portions of 10 tropical fruits (including avocado),
seven common fruits, nine common vegetables, and eight tropical vegetables. The
seven common fruits, including strawberry, banana and orange, ranged from 1/5
to less than half the potassium content of avocado. Similarly, the nine common
vegetables ranged from less than 1/4 (summer squash) to less than 2/3 (carrots)
the avocado level. The other, lesser used, tropical fruits ranged from 1/2 to
just over half the potassium supplied by avocado. Tropical vegetables, which
are rare in the United States, ranged upward from 1/5 to about the same
potassium richness of avocado.
Prevention
magazine
for August, 1987, reported the results of a 1 2-year study of stroke entitled "Potassium
was the key.", conducted jointly by the Schools of Medicine of the
University of California San Diego and Cambridge University in England. A 40%
reduction in stroke risk was associated with an average daily increase in
potassium consumption of about 400 mg, the amount supplied by less than half an
avocado! Moreover, blood pressure, a stroke factor, was linked in the same
article to potassium-sodium imbalance, namely to a sodium excess. The avocado has about 52 times as much
potassium as sodium. For the touted potassium-rich carrot, the potassium to
sodium ratio is less than 7 to 1 according to the brochure by Dr. William Sears
cited earlier. Moreover, vegetables high in potassium which include the common potato and winter squash,
are commonly
eaten cooked. Boiling can remove up to 30% of potassium originally present,
according to a researcher cited in the above Prevention article, giving
the avocado an additional advantage.
Finally, avocado and olive oils are the two chief
foods that are very high in monounsaturated fats, while being comparatively low
in both polyunsaturated
and saturated fats. An Italian epidemiological survey of 4,903 people (reported in a press
release in February, 1990, based on an article in the Journal of the
American Medical Association), found that while both monounsaturated and
polyunsaturated fat were associated with lower blood cholesterol as compared
with saturated,
only monunsaturated fat was also associated with lower blood pressure. The next part of this paper will look at factors
contributing to a healthy heart and related to avocado consumption. There is
evidence to indicate that eating avocado may reduce the incidence of all three
of the major causes of death in the United States.
Protein. Cultivars vary somewhat but the dominant 'Hass' is
about 2.4% protein on a fresh weight basis (Slater et al., 1975). This
is unusually high for a fruit. Hall et al. (1980) compared the essential
amino acid content of 15 fruits: the avocado was second only to the rare Tucuma
from Brazil which had about three times as much as mango, orange, peach and persimmon, and about six
times as much as the two other common fruits listed, apricot and apple.
Polansky and Murphy (1966) compared the protein content of 26 more common
fruits and vegetables (41 entries counting separate cultivars and years). The
avocado was in first place with about two to ten times the protein content of
the others. The avocado is a "complete food" in terms of protein,
containing all 9 essential amino acids, although not in the ideal proportions.
The fact that the avocado is
an excellent source of essential protein is of little interest to most
residents of the United States and other industrialized countries who are
already consuming more protein than they need. It could be of increasing
importance as health concerns move people away from meats and toward more
vegetarian dishes, especially for those who do not care to consume many
legumes. It could already be of significant benefit in tropical areas where
protein consumption is insufficient (Hall et al., 1980).
Fiber. A review paper by Anderson (1990) noted that
"fiber has emerged as a leading dietary component in chronic disease
prevention. High fiber intake lowers the risk for cardiovascular disease, some
cancers, hypertension (high blood pressure), diabetes, and obesity" He
gives references for each of these; as well as for therapeutic benefits from
fiber treatment of each of these conditions, except cancer, and with the addition
of gastrointestinal diseases. For some disorders, a mixture of both soluble and
insoluble fiber appears to be most beneficial. He also noted that "most
individuals in the West ingest suboptimal amounts of dietary fiber."
Smith et a/. (1983) compared the fiber
content of 16 fruits and 18 vegetables. Of the 34 food sources, only the
avocado had large amounts of both soluble and insoluble fiber (it had,
respectively, 2.1 % and 2.7% by fresh weight). Guava had the highest fraction
of soluble fiber by a wide margin, pears had 2/3 as much as avocado. The others
all had less than half as much, with the common fruits ranging from 16% to 30%
of the avocado level. Among vegetables, available data indicate that only
broccoli surpassed avocado in soluble fiber, the others ranged downward to 21
%. For the complementary benefits of insoluble fiber, no fruit and only peas
and among vegetables pigeon peas are listed with levels equal to or greater than avocado, the others
ranged downward
to 15%.
A frequent recommendation is that the typical
American should double his or her fiber intake. The avocado would be a very
pleasant means toward that end.
Various dietary benefits. The avocado is a mild-bland,
oil-rich, nutrient-rich deliciously-flavored food. This combination gives it an
exceptionally diverse range of dietary advantages. Just in terms of usage
alone, the avocado
can be served as an hors d'oeuvre, soup, salad, dip, sandwich spread, garnish, half-shell
spoon-out, entree, dessert, or beverage with various kinds of use in each
category. Such eating versatility makes it easy to increase consumption in
order to gain greater benefit from its numerous advantages to the human diet.
The blandness, especially of some varieties, has
sometimes been denigrated. But this very quality makes it soothing to the
alimentary tract. Perhaps that is partly why American Indians for hundred of
years have
regarded the avocado as especially desirable at times of illness. Linoleic fatty acid is an
essential polyunsaturate, i.e., the human body cannot manufacture it. Our
analyses (unpublished) indicate that the linolenic oil content of the 'Hass'
avocado averages just over 21 %, only the content of the monounsaturate fat
oleic acid was higher.
Diabetes. The 27 September, 1988, issue of the New England
Journal of Medicine reported a 4-week comparison of individuals on the
traditional type II diabetic's diet of low fat-high carbohydrate with those on
a diet
lower
in carbohydrates and higher in monounsaturated fat (the dominant kind in avocados). The
monounsaturated diet offered better control of blood sugar levels, accompanied
by lower triglycerides and higher HDL ("good" cholesterol). However,
this needs confirmation, including more careful monitoring. Moreover, there is
a report that avocado has an odd sugar type that depresses insulin production.
Diabetics probably should consume avocados cautiously.
Baby food. Dr. William Sears (1988) notes that avocado is
"one of the first fresh fruits a baby can enjoy." It is a time-saver,
served raw. "Low in sodium and cholesterol-free, avocados contain [many]
valuable nutrients ". For example, "Ounce for ounce, avocados contain
more potassium than 45 other fruits, juices or vegetables... and they are one
of the only fruits or vegetables which contain monounsaturated fats, essential
for baby's development.”
In an interview with the California Grower (October,
1989), Dr. Sears stated, "When you think about it, ...avocados are an
ideal first food for infants. Avocados have a delicate flavor and a smooth,
creamy consistency which makes them a perfect food for babies. ...Avocados
provide infants with more vitamin B1 B2 niacin, folacin,
potassium and magnesium per 1 5 gram serving than any of the other frequently
recommended fruits and vegetables [and are second to the highest in several
other vitamins and minerals.]"
Seventy
years earlier, long before these dietary details were understood, Pasadena nurseryman
D. W. Coolidge
addressed the annual meeting of what was then the California Avocado
Association: "The fruit of the avocado is about the most tasteful and
nourishing that grows out of the ground The strongest people physically and
mentally, the happiest and most beautiful children, will be those who make the
avocado, instead of meat, their daily diet. I have often marveled how babies
and very young children take to the avocado at once. If I have a greater love
for anything than the avocado, it is for ruddy, happy children."
Misconceptions
"Avocados are high in fat and therefore bad for
the heart"
Right
premise, wrong deduction. A number of articles discusses this misconception in
detail. A brief answer is given here: avocados have been shown to maintain good
cholesterol while reducing bad cholesterol. They are good for your
cardiovascular system. For a more complete explanation, please see the Avocado
and Human Nutrition part II, Avocados and Your Heart.
"But their high fat content makes them very high
in calories"
We have already noted that
increased avocado consumption has been shown experimentally to be compatible
with good weight control, and we have suggested why the very fact of its high
fat content may help in human weight control. Nevertheless, one keeps
encountering warnings, even from dieticians, about the avocado being "very
high" in calories, so let's look at the facts.
The number of kilocalories per avocado fruit varies
with race, variety, size, and season; an average for a California-grown avocado
might be 300 (Slater et al., 1975). Number of kilocalories per person
per day for normal maintenance varies with weight, age, sex, activity level,
and personal metabolic rate. For moderately active individuals aged 19 to 50, a
125 lb (57 kg) woman needs about 2100 kilocalories, and a 157 lb (71 kg) man
about 2800 kilocalories (University of California, Berkeley, Wellness Letter
for May, 1991). Thus, our "average" people, if they were to get
all their daily calories from avocado, would have to eat 7 or 9, respectively.
That is a lot of avocado! With its high fat content, one would expect
pronounced satiation well before the total "allowed" was consumed.
Rinzler (1987) gives a modern comparative food
evaluation. The avocado is rated only moderate in calories per serving,
with such foods as beans, bread, oatmeal, pasta, peas, and rice; less caloric than peanuts, most nuts and seeds, most
cheeses, sugar,
and butter, etc. Yet, the avocado is high enough in calories to be helpful to
the estimated 18% of Americans who are underweight. It is apparently not so
high in calories to be of concern to normal individuals as shown by the actual
weight results cited earlier. In Fit health magazine for August, 1982,
an article on the avocado concluded that in the public eye, it has somehow
become packed "with fictitious calories, ...avocados have received the
undeserved reputation of being fattening."
Why has this happened? One possibility is that even
diet specialists tend toward the oversimplification that "fat is fat"
, overlooking not only the fact that avocado fat is predominantly
monounsaturated, but also that it has a very high nutrient density per fat
calorie. Another possibility is the lament that "everything I love is
illegal, immoral, or fattening." Preferred, high-fat foods from steaks to
deserts now have deservedly bad reputations, and many people automatically conclude
that anything as "sinfully" rich and delicious as avocado must be fattening and cannot be good
for us ("Good"
for us are rather the foods with little appeal to most people, like carrots,
celery, leaf lettuce and spinach). It is an unfortunate irony that two major
advantages of the avocado, high monounsaturated fat content and exquisite nutty
flavor, are in many people's minds twisted into disadvantages.
"I've heard that avocados can cause cancer. "
This rumor resulted from some
dubious epidemiological findings in Shanghai, China; findings which a few
superficial commentators then extrapolated to even more dubious conjecture. The
study, by Shun-Zhang et al. (1990) was on breast cancer. The authors
summarized: "[Our cancer] cases have a significantly increased intake of
total calories compared to.. .controls, this excess primarily coming from
monounsaturated fat." This is the basis for the speculation that avocado,
which is rich in monounsaturated fats, is somehow linked to breast cancer. There
are major logical problems with this speculation.
1. Correlation does not prove causation. (The
classic statistical example was the strong correlation between the salaries of
Methodist ministers and the price of rum in Jamaica!) Increased breast cancer
and higher calorie consumption may both be the result of an unknown third
factor. That this may indeed be the case here is indicated by another of their
findings: the added breast cancer risk associated with a university education
(compared with less than full high school) was more than five times as large as
that associated with additional calories or monounsaturated fat!
2. Even if extra calories were to blame for increased risk of cancer, the specific
link to monounsaturates may well be entirely spurious. The calculated added
risk from added protein is nearly as large as that from added monounsaturated fat, but the lesser
variability in protein consumption reduces its statistical significance. Also,
both polyunsaturated and saturated fats are relatively minor dietary
constituents in this Chinese population. The authors note: "Given these
limitations, the primary dietary finding is the positive association with total
calorie intake." (Moreover, instead of a gradual dosage response, the
statistically significant increase in cancer risk was observed only for the 20%
of women with highest consumption a very high 3142 kilocalories per day
average.) In addition, the individuals having the highest calories and cancer
rate (20% of the group), when compared with the 20% having the lowest of both,
averaged 3.6 times more monounsaturated fat, but also 3.9 times more polyunsaturated and 13.2 times
more saturated fat!
3. Even if somehow
monounsaturated fat were to blame, extrapolating this to the avocado is
logically hazardous. Almost none of the monounsaturated fat eaten by this
population would be from avocado. Unlike most oils, avocado monounsaturates are
usually consumed as part of the total fruit, with its rich supply of minerals
and vitamins (including the antioxidants thought to reduce cancer risk). Any
conceivable added cancer risk from monounsaturates could well be real only in
the absence of other avocado nutrients.
4. As the authors
acknowledge, their hypothesis implicating monounsaturated fat in breast cancer
is contrary to results elsewhere: "...this finding contrasts with the
relatively low risks seen in Mediterranean populations with high
monounsaturated fat intake." and "The present results are
inconsistent with the only other reported controlled case study of diet and
breast cancer in a low-fat intake, low-risk population, which was carried out
in Japan." There have been suggestions elsewhere of possible linkage with
total fat, but "...two recent cohort studies... in the United States have
failed to find any positive association between fat intake and breast cancer
risk."
5. Breast cancer is several
times as prevalent in developed countries of Western Europe and North America
as in Asia. The authors' Table 1 gives dietary data for Canada and China as
representative of the two contrasted risk groups. In seeming contradiction of
their primary finding, average calorie consumption is actually a bit higher in
Chinese women. True, total fat intake averages higher in Canada, but this is a
result of Canadian women eating 4.2 times as much saturated fat;
polyunsaturated fat is about the same in the two countries, and it is the Chinese, who are known for
their lower risk of cancer, who actually averaged more monounsaturated fat! (Canadians get about
44% of their fat calories from monounsaturated fat, Chinese women about 69%).
The total evidence available
certainly does not support any linkage of monounsaturated fat, let alone
avocado, with increased breast (or any other) cancer risk. In fairness, the
original authors were very cautious in their conclusion; it is some of the
popularizations that went astray.
A much broader Chinese epidemiological survey
(Junshi et a/., 1990), of environmental and behavioral factors related
to mortality, is by far the largest study of its kind, yielding over 100,000
correlations. The authors wisely noted the danger of coincidental correlations
being mistaken for cause and effect, and therefore warned against accepting
even statistically significant correlations unless they are supported by research
and logical explanation, two features that are lacking in the Shun-Zhang et
a/. (1990) report data. Junshi et a/. (1990) drew only one dietary
inference concerning cancer: eat more fresh plant food. (Such as avocado!)
Henderson et a/. (1991)
recently surveyed the literature on cancer and diet. They note the
"inconsistent" results from attempts to relate total fat consumption
to hormone-mediated cancer risk, but conclude "It is difficult to refute a
postulated association between polyunsaturated fat and breast and prostate cancer ," because of
measurement difficulties. (Unlike monounsaturated fat, polyunsaturated fat is
suspected of increasing harmful free radicals). In contrast, the association
between animal
fat
(especially red meat) and colorectal cancer seems clear; "These results
suggest a 50% decrease in consumption of animal fat would result in about the
same reduction in colon cancer risk." "Consumption of fresh fruits
and vegetables has consistently been found to decrease the risk of stomach cancer."
In that vein, the December, 1991, issue of Men's Health reported a
survey of cancer incidence in a sample of over 14,000 people, classified into
three categories based on relative consumption of fruits and vegetables such as
avocado; for no less than 1 2 assorted cancers combined, the people with the
highest consumption averaged only 43% as much cancer as the third of
individuals who consumed the lowest amount. It has been estimated that only 9%
of Americans eat the recommended level of fruits and vegetables.
The avocado differs from most plant foods in its high
oil (fat) content. Nutritionists commonly recommend reducing total fat
consumption including animal and saturated fats, to 30% (or preferably less) of
total calories. The December, 1991, issue of Nutrition Action Healthletter quotes
Frank Sachs, Harvard physician and diet researcher: "The high-fat diets
that are associated with cancer in humans are very low in vegetables that are
cancerprotective. There is no need for total fat to be very low. It could be as
high as 35 to 40 percent. Given a calorie intake that will [avoid obesity], I
don't think it matters whether calories 'Come from vegetable oils or
carbohydrates." The same article quotes physician and epidemiologist Meir
Stampfer: "...in the Nurses Health Study, we found virtually no difference
in the risk of breast cancer with calorie intake between 29 and 49 percent of
calories from fat This country probably would be healthier if we came closer to
a vegetarian diet. "
When suitable experiments are eventually run, I
expect that dietary avocado will prove to be an effective reducer of cancer
risk.
One may hope that this lengthy discussion will lay to
rest a particularly distressing public misconception.
Also,
it will help us to be "inoculated" against any such future
misconception. We must urge investigation and correction of any errors of fact
or interpretation. Most food editors are appreciatively open to factual information.
Conclusion
Purseglove (1968), In his
authoritative textbook, labeled the avocado "...the most nutritious of all
fruits." Noted food writer Gaylord Hauser once said of it: "In this
single delectable fruit are combined the protein of meat, the fat of butter
[but much more wholesome!], the vitamins and minerals of green vegetables, the
flavor of nuts, a six course dinner ". America's greatest plant explorer,
David Fairchild is attributed "...the avocado is a food without rival
among the fruits, the veritable fruit of paradise." And in 1959, cereal
pioneer John H. Kellogg maintained that "of all edible fruits, it stands
pre-eminent as a source of concentrated nutriment adapted to human use. For
purity, wholesomeness, ease of digestibility, and adaptation to human needs, it
has few rivals and none that can fill its place." (Quotation courtesy of
J. S. Shepherd).
Anecdotes lack scientific weight, yet sometimes
capture reality more effectively than long scientific discourse. In the Los
Angeles HeraldExaminer for November 9, 1969, writer Emily Wilkens had a
4-column article, "How One Woman Keeps a Youthful Look." It tells the
story of her friend Lisa, who was "a super traveling saleswoman in the
cosmetics field and could buy every known external beauty aid at a fraction of
its cost. [Yet] day by day she looked worse even with the most careful skin
care and makeup. At the same time, her energy for "just living" was
flagging, and she was having a weight problem She consulted eminent diet
specialists, ...all to no avail. One day while out west, she invited a very
famous movie star to have dinner with her to discuss a special promotion for her company. Lisa watched
with wonder as the actress reached into her handbag and produced a luscious avocado.
On the way home the actress...revealed her [secret] of beauty through
healthful eating [emphasizing] fresh fruits and vegetables. The very
next day, Lisa embarked on the diet suggested by the
actress...Lisa has lost weight and inches. Her eyes sparkle and her skin glows,
and she grows...lovelier and more vibrant each year."
Avocado health benefits are
gradually becoming more widely recognized. For example, Prevention health
magazine for June, 1988, had an article on 'Nutritional nuggets from California
cuisine' ("with Stanford University"). It speaks of the
"light" and the "lean," "sparkling with beauty, flavor
and good health," an eating style that "may help prevent heart
disease, obesity and cancer, while delighting the senses." A recommended
component: avocado. Similarly, Men's Health Newsletter for February,
1992, discusses health-conscious sandwiches, to "keep your energy level
high, your wallet fat and your stomach lean." They recommend picking one
item of bread, condiment and filling from select lists; then "add as many
extras as you like" from a list that includes various superior vegetables,
low-fat cheese and avocado slices.
Yes, the avocado indeed is "the veritable fruit
of paradise."
Literature Cited
Anderson, J. W. 1990. Dietary
fiber and human health. HortScience 25: 14881494.
Grant, W.C. 1960. Influence
of avocados on serum cholesterol. Proc. Soc. Exper. Bioi. Med. 104:45-47.
Hall, N. T., J.M. Smoot, R.J.
Knight, Jr. and S. Nagy. 1980. Protein and amino acid composition of ten
tropical fruits by gas-liquid chromatography. J. Agric. Food Chern.
28:1217-1221.
Henderson, B.E., R.K. Ross,
and M.C. Pike. 1991. Toward the primary prevention of cancer. Science
Junshi, C., T.C. Campbell, L.
Junyao, and R. Peto. 1990. Diet, life-style, and mortality in China: a study of
the characteristics of 65 Chinese counties. Oxford, New York.
Polansky, M.M., and E.W.
Murphy. 1966. Vitamin B 6 components in fruits and nuts. J. Amer. Dietetic
Assoc. 48: 1 09-111 .
Purseglove, J. W. 1968.
Tropical crops: Dicotyledons. Wiley, New York.
Rinzler, C.A. 1987. Avocados,
p. 16-17. In: The complete book of food: a nutritional, medical, and culinary
guide. World Almanac, New York.
Shun-Zhang, Y., L. Rui-Fang,
X. Da-Dao, and G.R. Howe. 1990. A case-control study of dietary and nondietary
risk factors for breast cancer in Shanghai. Cancer Research 50:5017-5021.
Slater, G.C., S. Shankman,
J.S. Shepherd, and R.B. Alfin-Slater. 1975. Seasonal variation in the
composition of California avocados. J. Agr. Food Chern. 23:468-474.
Smith, J., S. Goldweber, M.
Lamberts, R. Tyson, and J.S. Reynolds. 1983. Utilization potential for
semi-tropical and tropical fruits and vegetables in therapeutic and family
diets. Proc. Fla. State Hort. Soc. 96:241-243.
Swisher, H.E. 1988. Avocado
oil: from food use to skin care. J. Amer. Oil Chemists' Soc. 65: 1704-1706.
Wood, P. 1 983.
California diet: and exercise program. Anderson World Books, Mountain View,
Calif.