Low Fat, High Sugar Diets Prompt Production of Saturated Fats

Eating a low-fat diet may not always be as healthy as people wish. Results from a study, reported in the May 1 The Journal of Clinical Investigation by scientists at The Rockefeller University and the University of California, Berkeley, show that people on weight-maintenance diets low in fat but high in sugar increase their production of saturated fat.

“Our study suggests that low-fat diets designed to maintain–not lose–weight could be a hazard for people who also eat lots of simple carbohydrates, mostly sugars,” explains first author Lisa Cooper Hudgins, M.D., assistant professor in the Laboratory of Human Behavior and Metabolism at Rockefeller. “Current public health recommendations suggest that people should reduce fat in their diets and increase their carbohydrates. However, too great a reduction in fats and too much of an increase in simple carbohydrates may prompt the body to make the sugar into saturated fats, which could harm the heart and blood vessels.”

Previous studies showed that people make fats from carbohydrates when they eat more calories than they use. However, prior to the new study, scientists thought people did not make fat when they ate as many calories as they used.

The body stores fat mostly as triglycerides, large molecules containing saturated, monounsaturated and polyunsaturated fats. Saturated fat, lacking a type of bonding among its carbon atoms, is associated with blockages of the heart’s coronary arteries, a disease that affects millions of Americans each year.

The liver readily processes the fats in food for immediate use. Stored body fat is used as energy in between meals. These fats, as well as those the liver can make from carbohydrates, are carried in the blood mainly as triglycerides. Fats in the blood that the body does not use right away are stored.

During the study, 10 healthy participants stayed at The Rockefeller University Hospital for nearly one month while eating low- and high-fat diets. All of the people had minimal changes in the composition of their body fat. However, the seven on low-fat diets increased blood levels of saturated fats, which the body makes, and decreased levels of polyunsaturated fats, which the body cannot make.

Specifically, the composition of blood triglycerides among participants on the low-fat diet dramatically differed from the compositions of their diet and body triglycerides. For example, their circulating levels of palmitate, the major saturated fat, increased an average of 54 percent and circulating linoleate, the major polyunsaturated fat, dropped an average of 44 percent, in comparison to the dietary and stored fats. In contrast, concentrations of the different triglyceride fats were similar in the blood, stored body fats or diets of participants on the high-fat diet.

“We know that most of the saturated fats in the blood triglycerides of the people on the low-fat diets were newly made fats,” explains Jules Hirsch, M.D., senior author of the paper and physician-in-chief at The Rockefeller University Hospital. “Only three sources can be responsible for circulating fat. First, the body stores, which are very slow to change to circulating fat. Second, the diet, which we controlled. This leaves the third source, the body’s production of new fats.”

In the study, the investigators used two new methods to measure the body’s creation of fats. Hudgins established a new way to monitor saturated fats and polyunsaturated fats in blood, using the blood levels of the polyunsaturated fat linoleate as an index of newly created fat. The researchers calculated that newly made fat accounted for 34 to 54 percent of the blood’s triglyceride in those on low-fat diets.

The second new method, developed by coauthor Marc Hellerstein, M.D., Ph.D., of the Department of Nutritional Sciences at the University of California, Berkeley, measures the body’s use of acetate, a building block of fat. By tagging acetate provided in the diets, the research team traced it within the participants for 24 hours, finding that much more blood triglyceride contained the acetate’s tag among participants on the low-fat diets, compared to those on the high-fat diets.

The patients received for 25 days either a low- or high-fat liquid formula diet, with enough calories to maintain constant weights. The low-fat diet had 10 percent of the calories from fat, 75 percent from carbohydrates in the form of the glucose polymers, short chains of glucose sugar molecules, and 15 percent from milk proteins. The high-fat diet had 40 percent calories from fat, 45 percent glucose polymers and 15 percent protein. All participants received daily supplements of multivitamins, iron and 5 grams of salt.

The researchers sampled each participant’s body fat before they began their diets to determine its composition of saturated, monounsaturated and polyunsaturated fats. The scientists then closely matched the dietary fat composition to each participant’s body fat by mixing lard, a saturated fat, with corn oil, a polyunsaturated fat, and olive oil, a monounsaturated fat.

During the study, every two to three days, after an overnight fast and over 24 hours at the end of the study, the scientists took blood samples from the participants to measure cholesterol, triglycerides, glucose and insulin levels.

“We found that after 10 days, each participant’s fat pattern in the blood was constant for the remainder of the study and did not fluctuate with meals,” says Hudgins, who also is an associate member and staff physician at the Comprehensive Lipid Control Center of the Rogosin Institute affiliated with New York Hospital. She is an assistant professor of pediatrics and medicine at New York Hospital.

The participants ranged in age from 20 to 57. The six men and four women were healthy, nonsmokers weighing between 80 to 120 percent of an ideal body weight based on their height.

Hudgins notes that other simple sugars like fructose found in fruits, sucrose found in table sugar or lactose found in milk may behave similarly to the glucose polymers used in the study. Hellerstein has shown that fructose can stimulate fat creation in people after one day of eating, perhaps because of its direct uptake in the body and rapid processing by the liver.

Complex carbohydrates, such as the starches found in breads, vegetables and pasta, may have less of an effect, says Hudgins, who is conducting studies of diets combining simple and complex carbohydrates eaten by people with normal or elevated blood and body fats.

Hudgins, Hirsch and Hellerstein’s coauthors include Cynthia Seidman, M.S., R.D., and Jolanta Diakun, M.S., R.D., of Rockefeller and Richard Neese, Ph.D., of the Department of Nutritional Sciences at the University of California, Berkeley.

The National Institute of Diabetes and Digestive and Kidney Diseases and the National Cancer Institute, both of the National Institutes of Health (NIH), supported the research. NIH also supports the Rockefeller University Hospital. The Nora Eccles Treadwell Foundation, the Frederick P. Rose Clinical Scholars and the American Diabetes Association also supported the work.