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Ernährungsmedizin |
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Image:
RICHARD BORGE |
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Rebuilding
the Food Pyramid
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The dietary guide introduced a decade ago has led people
astray. Some fats are healthy for the heart, and many carbohydrates
clearly are not.
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By
Walter C. Willett and Meir J. Stampfer
©
1996-2002 Scientific American, Inc., December 15, 2002 |
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In
1992 the U.S. Department of Agriculture officially released
the Food Guide Pyramid, which was intended to help the American
public make dietary choices that would maintain good health
and reduce the risk of chronic disease. The recommendations
embodied in the pyramid soon became well known: people should
minimize their consumption of fats and oils but should eat
six to 11 servings a day of foods rich in complex carbohydrates--bread,
cereal, rice, pasta and so on. The food pyramid also recommended
generous amounts of vegetables (including potatoes, another
plentiful source of complex carbohydrates), fruit and dairy
products, and at least two servings a day from the meat and
beans group, which lumped together red meat with poultry,
fish, nuts, legumes and eggs.
Even when the pyramid was being developed, though, nutritionists
had long known that some types of fat are essential to health
and can reduce the risk of cardiovascular disease. Furthermore,
scientists had found little evidence that a high intake of
carbohydrates is beneficial. Since 1992 more and more research
has shown that the USDA pyramid is grossly flawed. By promoting
the consumption of all complex carbohydrates and eschewing
all fats and oils, the pyramid provides misleading guidance.
In short, not all fats are bad for you, and by no means are
all complex carbohydrates good for you. The USDA's Center
for Nutrition Policy and Promotion is now reassessing the
pyramid, but this effort is not expected to be completed until
2004. In the meantime, we have drawn up a new pyramid that
better reflects the current understanding of the relation
between diet and health. Studies indicate that adherence to
the recommendations in the revised pyramid can signif- icantly
reduce the risk of cardiovascular disease for both men and
women.
How did the original USDA pyramid go so wrong? In part, nutritionists
fell victim to a desire to simplify their dietary recommendations.
Researchers had known for decades that saturated fat--found
in abundance in red meat and dairy products--raises cholesterol
levels in the blood. High cholesterol levels, in turn, are
associated with a high risk of coronary heart disease (heart
attacks and other ailments caused by the blockage of the arteries
to the heart). In the 1960s controlled feeding studies, in
which the participants eat carefully prescribed diets for
several weeks, substantiated that saturated fat increases
cholesterol levels. But the studies also showed that polyunsaturated
fat--found in vegetable oils and fish--reduces cholesterol.
Thus, dietary advice during the 1960s and 1970s emphasized
the replacement of saturated fat with polyunsaturated fat,
not total fat reduction. (The subsequent doubling of polyunsaturated
fat consumption among Americans probably contributed greatly
to the halving of coronary heart disease rates in the U.S.
during the 1970s and 1980s.) |
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 Image:
USDA/DHHS
OLD
FOOD PYRAMID conceived by the U.S. Department
of Agriculture was intended to convey the message
"Fat is bad" and its corollary "Carbs
are good." These sweeping statements are now
being questioned.
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The notion that fat in general is to be avoided stems
mainly from observations that affluent Western countries
have both high intakes of fat and high rates of coronary
heart disease. This correlation, however, is limited
to saturated fat. Societies in which people eat relatively
large portions of monounsaturated and polyunsaturated
fat tend to have lower rates of heart disease. On the
Greek island of Crete, for example, the traditional
diet contained much olive oil (a rich source of monounsaturated
fat) and fish (a source of polyunsaturated fat).
Although fat constituted 40 percent of the calories
in this diet, the rate of heart disease for those who
followed it was lower than the rate for those who followed
the traditional diets of Japan, in which fat made up
only 8 to 10 percent of the calories. |
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Furthermore, international comparisons can be misleading:
many negative influences on health, such as smoking, physical
inactivity and high amounts of body fat, are also correlated
with Western affluence. Unfortunately, many nutritionists
decided it would be too difficult to educate the public about
these subtleties. Instead they put out a clear, simple message:
"Fat is bad." Because saturated fat represents about
40 percent of all fat consumed in the U.S., the rationale
of the USDA was that advocating a low-fat diet would naturally
reduce the intake of saturated fat. This recommendation was
soon reinforced by the food industry, which began selling
cookies, chips and other products that were low in fat but
often high in sweeteners such as high-fructose corn syrup.
When the food pyramid was being developed, the typical American
got about 40 percent of his or her calories from fat, about
15 percent from protein and about 45 percent from carbohydrates.
Nutritionists did not want to suggest eating more protein,
because many sources of protein (red meat, for example) are
also heavy in saturated fat. So the "Fat is bad"
mantra led to the corollary "Carbs are good." Dietary
guidelines from the American Heart Association and other groups
recommended that people get at least half their calories from
carbohydrates and no more than 30 percent from fat. This 30
percent limit has become so entrenched among nutritionists
that even the sophisticated observer could be forgiven for
thinking that many studies must show that individuals with
that level of fat intake enjoyed better health than those
with higher levels. But no study has demonstrated long-term
health benefits that can be directly attributed to a low-fat
diet. The 30 percent limit on fat was essentially drawn from
thin air.
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The wisdom of this direction became even more questionable
after researchers found that the two main cholesterol-carrying
chemicals--low-density lipoprotein (LDL), popularly
known as "bad cholesterol," and high-density
lipoprotein (HDL), known as "good cholesterol"--have
very different effects on the risk of coronary heart
disease. Increasing the ratio of LDL to HDL in the blood
raises the risk, whereas decreasing the ratio lowers
it. By the early 1990s controlled feeding studies had
shown that when a person replaces calories from saturated
fat with an equal amount of calories from carbohydrates
the levels of LDL and total cholesterol fall, but the
level of HDL also falls. Because the ratio of LDL to
HDL does not change, there is only a small reduction
in the person's risk of heart disease. Moreover, the
switch to carbohydrates boosts the blood levels of triglycerides,
the component molecules of fat, probably because of
effects on the body's endocrine system. High triglyceride
levels are also associated with a high risk of heart
disease.
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Image: RICHARD BORGE
NEW
FOOD PYRAMID outlined by the authors distinguishes
between healthy and unhealthy types of fat and carbohydrates.
Fruits and vegetables are still recommended, but the
consumption of dairy products should be limited.
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The
effects are more grievous when a person switches from either
monounsaturated or polyunsaturated fat to carbohydrates. LDL
levels rise and HDL levels drop, making the cholesterol ratio
worse. In contrast, replacing saturated fat with either monounsaturated
or polyunsaturated fat improves this ratio and would be expected
to reduce heart disease. The only fats that are significantly
more deleterious than carbohydrates are the trans-unsaturated
fatty acids; these are produced by the partial hydrogenation
of liquid vegetable oil, which causes it to solidify. Found
in many margarines, baked goods and fried foods, trans fats
are uniquely bad for you because they raise LDL and triglycerides
while reducing HDL. |
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Image: CORNELIA BLIK
INTERNATIONAL
COMPARISONS reveal that total fat intake
is a poor indicator of heart disease risk. What is
important is the type of fat consumed. In regions
where saturated fats traditionally made up much of
the diet (for example, eastern Finland), rates of
heart disease were much higher than in areas where
monounsaturated fats were prevalent (such as the Greek
island of Crete). Crete's Mediterranean diet, based
on olive oil, was even better for the heart than the
low-fat traditional diet of Japan.
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Because of practical constraints and cost, few such
studies have been conducted, and most of these have
focused on patients who already suffer from heart
disease. Though limited, these studies have supported
the benefits of replacing saturated fat with polyunsaturated
fat, but not with carbohydrates.
The best alternative is to conduct large epidemiological
studies in which the diets of many people are periodically
assessed and the participants are monitored for the
development of heart disease and other conditions.
One of the best-known examples of this research is
the Nurses' Health Study, which was begun in 1976
to evaluate the effects of oral contraceptives but
was soon extended to nutrition as well. Our group
at Harvard University has followed nearly 90,000 women
in this study who first completed detailed questionnaires
on diet in 1980, as well as more than 50,000 men who
were enrolled in the Health Professionals Follow-Up
Study in 1986. After adjusting the analysis to account
for smoking, physical activity and other recognized
risk factors, we found that a participant's risk of
heart disease was strongly influenced by the type
of dietary fat consumed. Eating trans fat increased
the risk substantially, and eating saturated fat increased
it slightly.
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In contrast, eating monounsaturated and polyunsaturated
fats decreased the risk--just as the controlled feeding
studies predicted. Because these two effects counterbalanced
each other, higher overall consumption of fat did not lead
to higher rates of coronary heart disease. This finding
reinforced a 1989 report by the National Academy of Sciences
that concluded that total fat intake alone was not associated
with heart disease risk.
But what about illnesses besides coronary heart disease?
High rates of breast, colon and prostate cancers in affluent
Western countries have led to the belief that the consumption
of fat, particularly animal fat, may be a risk factor. But
large epidemiological studies have shown little evidence
that total fat consumption or intakes of specific types
of fat during midlife affect the risks of breast or colon
cancer. Some studies have indicated that prostate cancer
and the consumption of animal fat may be associated, but
reassuringly there is no suggestion that vegetable oils
increase any cancer risk. Indeed, some studies have suggested
that vegetable oils may slightly reduce such risks. Thus,
it is reasonable to make decisions about dietary fat on
the basis of its effects on cardiovascular disease, not
cancer.
Finally, one must consider the impact of fat consumption
on obesity, the most serious nutritional problem in the
U.S. Obesity is a major risk factor for several diseases,
including type 2 diabetes (also called adult-onset diabetes),
coronary heart disease, and cancers of the breast, colon,
kidney and esophagus. Many nutritionists believe that eating
fat can contribute to weight gain because fat contains more
calories per gram than protein or carbohydrates. Also, the
process of storing dietary fat in the body may be more efficient
than the conversion of carbohydrates to body fat. But recent
controlled feeding studies have shown that these considerations
are not practically important. The best way to avoid obesity
is to limit your total calories, not just the fat calories.
So the critical issue is whether the fat composition of
a diet can influence one's ability to control caloric intake.
In other words, does eating fat leave you more or less hungry
than eating protein or carbohydrates? There are various
theories about why one diet should be better than another,
but few long-term studies have been done. In randomized
trials, individuals assigned to low-fat diets tend to lose
a few pounds during the first months but then regain the
weight. In studies lasting a year or longer, low-fat diets
have consistently not led to greater weight loss.
Carbo-Loading
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let's look at the health effects of carbohydrates. Complex
carbohydrates consist of long chains of sugar units
such as glucose and fructose; sugars contain only one
or two units. Because of concerns that sugars offer
nothing but "empty calories"--that is, no
vitamins, minerals or other nutrients--complex carbohydrates
form the base of the USDA food pyramid. But refined
carbohydrates, such as white bread and white rice, can
be very quickly broken down to glucose, the primary
fuel for the body. The refining process produces an
easily absorbed form of starch--which is defined as
glucose molecules bound together--and also removes many
vitamins and minerals and fiber. Thus, these carbohydrates
increase glucose levels in the blood more than whole
grains do. (Whole grains have not been milled into fine
flour.)
Or consider potatoes. Eating a boiled potato raises
blood sugar levels higher than eating the same amount
of calories from table sugar.
Because potatoes are mostly starch, they can be rapidly
metabolized to glucose. In contrast, table sugar (sucrose)
is a disaccharide consisting of one molecule of glucose
and one molecule of fructose. Fructose takes longer
to convert to glucose, hence the slower rise in blood
glucose levels. A rapid increase in blood sugar stimulates
a large release of insulin, the hormone that directs
glucose to the muscles and liver. As a result, blood
sugar plummets, sometimes even going below the baseline.
High levels of glucose and insulin can have negative
effects on cardiovascular health, raising triglycerides
and lowering HDL (the good cholesterol). The precipitous
decline in glucose can also lead to more hunger after
a carbohydrate-rich meal and thus contribute to overeating
and obesity.
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HEALTH
EFFECTS of the recommendations in the revised
food pyramid were gauged by studying disease rates
among 67,271 women in the Nurses' Health Study and
38,615 men in the Health Professionals Follow-up Study.
Women and men in the fifth quintile (the 20 percent
whose diets were closest to the pyramid's recommendations)
had significantly lower rates of cardiovascular disease
than those in the first quintile (the 20 percent who
strayed the most from the pyramid). The dietary recommendations
had no significant effect on cancer risk, however.
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In
our epidemiological studies, we have found that a high intake
of starch from refined grains and potatoes is associated with
a high risk of type 2 diabetes and coronary heart disease.
Conversely, a greater intake of fiber is related to a lower
risk of these illnesses. Interestingly, though, the consumption
of fiber did not lower the risk of colon cancer, as had been
hypothesized earlier. |
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Overweight, inactive people can become resistant to insulin's
effects and therefore require more of the hormone to regulate
their blood sugar. Recent evidence indicates that the adverse
metabolic response to carbohydrates is substantially worse
among people who already have insulin resistance. This finding
may account for the ability of peasant farmers in Asia and
elsewhere, who are extremely lean and active, to consume large
amounts of refined carbohydrates without experiencing diabetes
or heart disease, whereas the same diet in a more sedentary
population can have devastating effects.
Eat Your Veggies
High intake of fruits and vegetables is perhaps the least
controversial aspect of the food pyramid. A reduction in cancer
risk has been a widely promoted benefit. But most of the evidence
for this benefit has come from case-control studies, in which
patients with cancer and selected control subjects are asked
about their earlier diets. These retrospective studies are
susceptible to numerous biases, and recent findings from large
prospective studies (including our own) have tended to show
little relation between overall fruit and vegetable consumption
and cancer incidence. (Specific nutrients in fruits and vegetables
may offer benefits, though; for instance, the folic acid in
green leafy vegetables may reduce the risk of colon cancer,
and the lycopene found in tomatoes may lower the risk of prostate
cancer.)
The
best way to avoid obesity is to LIMIT YOUR TOTAL CALORIES,
not just the fat calories.
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The
real value of eating fruits and vegetable may be in reducing
the risk of cardiovascular disease. Folic acid and potassium
appear to contribute to this effect, which has been seen in
several epidemiological studies. Inadequate consumption of
folic acid is responsible for higher risks of serious birth
defects as well, and low intake of lutein, a pigment in green
leafy vegetables, has been associated with greater risks of
cataracts and degeneration of the retina. Fruits and vegetables
are also the primary source of many vitamins needed for good
health. Thus, there are good reasons to consume the recommended
five servings a day, even if doing so has little impact on
cancer risk. The inclusion of potatoes as a vegetable in the
USDA pyramid has little justification, however; being mainly
starch, potatoes do not confer the benefits seen for other
vegetables.
Another flaw in the USDA pyramid is its failure to recognize
the important health differences between red meat (beef, pork
and lamb) and the other foods in the meat and beans group
(poultry, fish, legumes, nuts and eggs). High consumption
of red meat has been associated with an increased risk of
coronary heart disease, probably because of its high content
of saturated fat and cholesterol. Red meat also raises the
risk of type 2 diabetes and colon cancer. The elevated risk
of colon cancer may be related in part to the carcinogens
produced during cooking and the chemicals found in processed
meats such as salami and bologna. Poultry and fish, in contrast,
contain less saturated fat and more unsaturated fat than red
meat does. Fish is a rich source of the essential omega-3
fatty acids as well. Not surprisingly, studies have shown
that people who replace red meat with chicken and fish have
a lower risk of coronary heart disease and colon cancer. Eggs
are high in cholesterol, but consumption of up to one a day
does not appear to have adverse effects on heart disease risk
(except among diabetics), probably because the effects of
a slightly higher cholesterol level are counterbalanced by
other nutritional benefits. Many people have avoided nuts
because of their high fat content, but the fat in nuts, including
peanuts, is mainly unsaturated, and walnuts in particular
are a good source of omega-3 fatty acids. Controlled feeding
studies show that nuts improve blood cholesterol ratios, and
epidemiological studies indicate that they lower the risk
of heart disease and diabetes. Also, people who eat nuts are
actually less likely to be obese; perhaps because nuts are
more satisfying to the appetite, eating them seems to have
the effect of significantly reducing the intake of other foods.
Yet another concern regarding the USDA pyramid is that it
promotes overconsumption of dairy products, recommending the
equivalent of two or three glasses of milk a day. This advice
is usually justified by dairy's calcium content, which is
believed to prevent osteoporosis and bone fractures. But the
highest rates of fractures are found in countries with high
dairy consumption, and large prospective studies have not
shown a lower risk of fractures among those who eat plenty
of dairy products. Calcium is an essential nutrient, but the
requirements for bone health have probably been overstated.
What is more, we cannot assume that high dairy consumption
is safe: in several studies, men who consumed large amounts
of dairy products experienced an increased risk of prostate
cancer, and in some studies, women with high intakes had elevated
rates of ovarian cancer. Although fat was initially assumed
to be the responsible factor, this has not been supported
in more detailed analyses. High calcium intake itself seemed
most clearly related to the risk of prostate cancer.
Men and
women eating in accordance with THE NEW PYRAMID had a lower
risk of major chronic disease.
More research is needed to determine the health effects of
dairy products, but at the moment it seems imprudent to recommend
high consumption. Most adults who are following a good overall
diet can get the necessary amount of calcium by consuming
the equivalent of one glass of milk a day. Under certain circumstances,
such as after menopause, people may need more calcium than
usual, but it can be obtained at lower cost and without saturated
fat or calories by taking a supplement.
A Healthier Pyramid
Although the usda's food pyramid has become an icon of nutrition
over the past decade, until recently no studies had evaluated
the health of individuals who followed its guidelines. It
very likely has some benefits, especially from a high intake
of fruits and vegetables. And a decrease in total fat intake
would tend to reduce the consumption of harmful saturated
and trans fats. But the pyramid could also lead people to
eat fewer of the healthy unsaturated fats and more refined
starches, so the benefits might be negated by the harm.
To evaluate the overall impact, we used the Healthy Eating
Index (HEI), a score developed by the USDA to measure adherence
to the pyramid and its accompanying dietary guidelines in
federal nutrition programs. From the data collected in our
large epidemiological studies, we calculated each participant's
HEI score and then examined the relation of these scores to
subsequent risk of major chronic disease (defined as heart
attack, stroke, cancer or nontraumatic death from any cause).
When we compared people in the same age groups, women and
men with the highest HEI scores did have a lower risk of major
chronic disease. But these individuals also smoked less, exercised
more and had generally healthier lifestyles than the other
participants. After adjusting for these variables, we found
that participants with the highest HEI scores did not experience
significantly better overall health outcomes. As predicted,
the pyramid's harms counterbalanced its benefits. Because
the goal of the pyramid was a worthy one--to encourage healthy
dietary choices--we have tried to develop an alternative derived
from the best available knowledge. Our revised pyramid emphasizes
weight control through exercising daily and avoiding an excessive
total intake of calories. This pyramid recommends that the
bulk of one's diet should consist of healthy fats (liquid
vegetable oils such as olive, canola, soy, corn, sunflower
and peanut) and healthy carbohydrates (whole grain foods such
as whole wheat bread, oatmeal and brown rice). If both the
fats and carbohydrates in your diet are healthy, you probably
do not have to worry too much about the percentages of total
calories coming from each. Vegetables and fruits should also
be eaten in abundance. Moderate amounts of healthy sources
of protein (nuts, legumes, fish, poultry and eggs) are encouraged,
but dairy consumption should be limited to one to two servings
a day. The revised pyramid recommends minimizing the consumption
of red meat, butter, refined grains (including white bread,
white rice and white pasta), potatoes and sugar.
Trans fat does not appear at all in the pyramid, because it
has no place in a healthy diet. A multiple vitamin is suggested
for most people, and moderate alcohol consumption can be a
worthwhile option (if not contraindicated by specific health
conditions or medications). This last recommendation comes
with a caveat: drinking no alcohol is clearly better than
drinking too much. But more and more studies are showing the
benefits of moderate alcohol consumption (in any form: wine,
beer or spirits) to the cardiovascular system.
Can we show that our pyramid is healthier than the USDA's?
We created a new Healthy Eating Index that measured how closely
a person's diet followed our recommendations. Applying this
revised index to our epidemiological studies, we found that
men and women who were eating in accordance with the new pyramid
had a lower risk of major chronic disease. This benefit resulted
almost entirely from significant reductions in the risk of
cardiovascular disease--up to 30 percent for women and 40
percent for men. Following the new pyramid's guidelines did
not, however, lower the risk of cancer. Weight control and
physical activity, rather than specific food choices, are
associated with a reduced risk of many cancers.
Of course, uncertainties still cloud our understanding of
the relation between diet and health. More research is needed
to examine the role of dairy products, the health effects
of specific fruits and vegetables, the risks and benefits
of vitamin supplements, and the long-term effects of diet
during childhood and early adult life. The interaction of
dietary factors with genetic predisposition should also be
investigated, although its importance remains to be determined.
Another challenge will be to ensure that the information about
nutrition given to the public is based strictly on scientific
evidence. The USDA may not be the best government agency to
develop objective nutritional guidelines, because it may be
too closely linked to the agricultural industry. The food
pyramid should be rebuilt in a setting that is well insulated
from political and economic interests.
Walter
C. Willett and Meir J. Stampfer are professors of epidemiology
and nutrition at the Harvard School of Public Health. Willett
chairs the school's department of nutrition, and Stampfer
heads the department of epidemiology. Willett and Stampfer
are also professors of medicine at Harvard Medical School.
Both of them practice what they preach by eating well and
exercising regularly.
©
1996-2002 Scientific American, Inc. |
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