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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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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.
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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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