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.)
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.