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 significantly 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.)
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. 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.
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.
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.
The Big Picture
To evaluate fully the health effects of diet, though, one
must look beyond cholesterol ratios and triglyceride levels.
The foods we eat can cause heart disease through many other
pathways, including raising blood pressure or boosting the
tendency of blood to clot. And other foods can prevent heart
disease in surprising ways; for instance, omega-3 fatty acids
(found in fish and some plant oils) can reduce the likelihood
of ventricular fibrillation, a heart rhythm disturbance that
causes sudden death.
The ideal method for assessing all these adverse and
beneficial effects would be to conduct large-scale trials in
which individuals are randomly assigned to one diet or
another and followed for many years. 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. 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
Now 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.
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.
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.
The real value of eating fruits and vegetables 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.
