Maybe people shouldn't be eating high-carbohydrate, low-fat
diets after all. According to a new study, replacing saturated fats
with carbohydrates did not have the beneficial effects that experts
thought it would.
Researchers led by Dr. Gerald Reaven, professor emeritus in the
department of endocrinology, gerontology and metabolism at the
Stanford University School of Medicine in Stanford, Calif., found
that a high-carbohydrate, low-fat diet resulted in higher levels in
the blood of certain triglycerides -- a type of fat -- and lower
levels of high-density lipoproteins (HDL), the so-called good
cholesterol.
The diet, however, did not change the concentrations of
low-density lipoproteins (LDL), the so-called bad cholesterol. High
levels of LDL are associated with an increased risk of heart
disease.
The researchers studied the effects of a high-carbohydrate,
low-fat diet on eight healthy volunteers. The subjects were
randomly assigned to either a high-carbohydrate diet, where 60
percent of the total calories came from carbohydrates, or a
low-carbohydrate diet, where 40 percent of the calories came from
carbohydrates. Each diet contained the same amount of total
calories.
The volunteers stayed on their respective diets for two weeks,
ate normally for two weeks and then were again randomly assigned to
a diet.
Those who were on the 60 percent carbohydrate diet had higher
levels of triglycerides and another type of cholesterol called
remnant lipoprotein cholesterol (RLP), compared with those on the
40 percent carbohydrate diet. These levels remained elevated over
an eight-hour period, during which breakfast and lunch were given.
The study is published in the January issue of the American
Journal of Cardiology (www.elsevier.com/locate/amjcard).
``It confirms what has been previously reported,'' said Alice
Lichtenstein, professor of nutrition at the Jean Mayer USDA Human
Nutrition Research Center on Aging at Tufts University in Boston,
and spokesperson for the American Heart Association
(www.americanheart.org).
Past studies have shown that high-carbohydrate, low-fat diets
decrease levels of HDL in the blood. Increases in triglyceride and
RLP levels as a result of the diet are new findings. RLP consists
of concentrated remnants of fat that remain in the blood after
cells take the components of fat that they need. These increases in
triglyceride and RLP levels, along with lower HDL levels, have been
found to be associated with blockages in the blood vessels.
Lichtenstein noted that the two diets used in the study were
opposite extremes in terms of carbohydrate intake. ``The critical
points are that they kept the calories the same and they kept the
saturated fat the same,'' she said. Saturated fat is a major
determinant of blood cholesterol levels.
Scientists have, for the most part, agreed that the intake of
saturated fat should be reduced. However, a suitable substitute for
saturated fat has been the source of debate.
``The bottom line is that when you drop the saturated fat in the
diet, what do you replace it with?'' Lichtenstein remarked. ``A
very high carbohydrate diet was detrimental in respect to
lipoprotein profiles. The evidence suggests that it should be
replaced with unsaturated fat.''
``It is reasonable to suggest that the clinical use of current
dietary guidelines aimed at decreasing risk of coronary heart
disease be re-examined,'' the researchers concluded.
