Low-Fat Diet Good, But Even Lower One Isn’t Always Better
The often repeated advice that everyone should eat less fat and more carbohydrates is coming under challenge.
New research finds that cutting fat levels much below the typical American diet probably won’t lower the risk of heart disease for the majority of adults, and it might even increase the hazard for some.
On average, fat makes up 34 percent of the calories in the American diet, and this is already close to the level recommended by major health organizations. The American Heart Association suggests that 30 percent or less of calories should come from fat.
Nevertheless, “there is a movement toward even further fat restriction,” said Dr. Ronald Krauss of the Lawrence Berkeley National Laboratory at the University of California.
He noted that many individuals, sometimes with the encouragement of doctors and other health professionals, believe that if a low-fat diet is good, an even lower fat one should be better still.
Krauss is head of the heart association’s nutrition committee, which writes the organization’s dietary guidelines. At a meeting Saturday of the American Association for the Advancement of Science, he presented new evidence of why very low fat diets may be good for some but bad for others.
He said his data suggest that about one-third of the U.S. population may benefit from reducing fat intake below 30 percent. Another one-third will neither be helped nor harmed by this. And the final third may actually increase their risk of heart disease.
Krauss noted that evidence in favor of lowering fat and replacing it with carbohydrates comes from studies that look at average cholesterol levels across large population groups.
“These fail to reflect the tremendous amount of individual variation,” he said.
Just how someone’s cholesterol level responds to diet depends, at least in part, on the genes that he or she inherits. Probably many genes are involved, and no one knows what they are.
However, once these genes are identified, Krauss said it should be possible to tailor people’s diets to match the needs of their genetic profiles.
One factor in all this appears to be the kind of low-density lipoprotein cholesterol, or LDL, that people produce. While LDL is generally known as the bad cholesterol, since it increases the risk of clogged arteries, it comes in two varieties.
By far the worse is what experts call small, dense LDL. Between 20 percent and 30 percent of adults make this kind of LDL. It is part of a syndrome that also often includes low levels of HDL, the protective good cholesterol, and high triglycerides, among other things.
Those with small, dense LDL have what’s known as a pattern B cholesterol profile. Their risk of heart disease is about three times higher than the majority of Americans, who have larger LDL and are considered to be pattern A.
Krauss put 105 men on a low-fat diet in which 24 percent of calories came from fat. This is close to what’s typically achieved with AHA’s more rigorous Step 2 diet, intended for those who have trouble lowering their cholesterol.
He found that men with pattern B cholesterol responded strongly to the low-fat diet, significantly improving their cholesterol levels. However, those with more normal pattern A showed little or no benefit.
Indeed, about 40 percent of them actually shifted from pattern A to pattern B. Their protective HDL dropped significantly, and they developed the small, dense LDL.
“This may give some caution to very low-fat diets in these individuals,” said Krauss.
For now, doctors cannot easily determine whether their patients are pattern A or pattern B, although Krauss said such tests should soon be available.
Dr. Jan Breslow of Rockefeller University questioned whether the apparently damaging effects seen among the pattern A men on low-fat diets truly will put them at higher risk of heart trouble.
He noted that large population studies generally conclude that the less fat people eat, the lower their risk.
“This is a big raging debate,” he said, that will require more research to settle.