As some of us have been maintaining for years now, the so-called "Food
Pyramid" is in need of major revision. Now, scientists are doing exactly
that - at long last! The latest issue of the Scientific American, a journal
to which I have subscribed for many years (annual cost is a fraction of the
cost of belonging to most fitness and strength associations), presents the
case for a revised Food Pyramid. To subscribe to this journal or read other
articles from its archives, go to: http://www.sciam.com .
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<http://www.sciam.com/print_version.cfm?articleID=0007C5B6-7152-1DF6-9733809EC
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[Excerpts given ....]
Scientific American, Dec 17, 2002
Rebuilding the Food Pyramid
The dietary guide introduced a decade ago has led people astray. Some fats
are healthy for the heart, and many carbohydrates clearly are not
By Walter C Willett & Meir J Stampfer
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 6 - 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% from fat. This 30% 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% 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.......
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 [in most margarines, snacks and fast foods] 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........
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. ..... 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..........
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 [fruit sugar] 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.......
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......
Yet another concern regarding the USDA pyramid is that it promotes over
consumption 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.....
A Healthier Food 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......
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........
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.......
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Dr Mel C Siff
Denver, USA
http://groups.yahoo.com/group/Supertraining/ (sports science, fitness and
rehab forum)
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