The Duke study compared the effects of a low-carbohydrate diet, which included nutritional supplements, with a low-fat, low-cholesterol, low-calorie diet. The two diets improved cardiac risk in different ways, said lead researcher Eric Westman, M.D., associate professor of medicine at Duke University Medical Center.
The low-carb diet improved HDL, or good cholesterol levels, and lowered triglycerides, the researchers found. The reduced fat diet lowered total cholesterol levels and triglyceride levels. Both diets brought down blood levels of small LDL particles, the form of bad cholesterol most likely to lead to hardened arteries, they found.
The results appeared early online November 16, 2005 in the International Journal of Cardiology and will appear in print in 2006. The research was funded by an unrestricted grant from the Robert C. Atkins Foundation. The study authors have no financial interest in Atkins Nutritionals, Inc.
"I think the emerging science shows different diets improve cardiac risk in different ways. We are moving from a one-size-fits-all approach to considering many different diets to fit the many different types of cardiac risk," Westman said.
Overall, both diets had positive effects on cholesterol, Westman said. The triglyceride levels improved significantly in both groups, falling 74.2 points for the low-carb group and 27.9 points for the low-fat group. People on the low-carb diet showed an increase in HDL cholesterol by 5.5 points, a positive change, while those following the low-fat diet did not have a significant change. LDL cholesterol levels did not change significantly in either group but small LDL particles decreased 17.4 points for the low-carb dieters and 19.2 points for the low-fat dieters, a similar improvement. The total cholesterol of the low-fat dieters saw a 13.7 point decline over 6 months but did not change significantly in the low-carb dieters.
The 120 study participants were randomly assigned to either the low-carbohydrate diet or the low-fat, low-cholesterol, low-calorie diet. All were between 18 and 65 years old and in generally good health, with a body mass index (BMI) between 30 and 60, indicating obesity, and a total cholesterol level of more than 200 mg/dL. None had tried dieting or weight loss pills in the previous six months.
The low-carbohydrate group was permitted daily unlimited amounts of animal foods (meat, fowl, fish and shellfish); unlimited eggs; 4 oz. of hard cheese; two cups of salad vegetables such as lettuce, spinach or celery; and one cup of non-starchy vegetables such as broccoli, cauliflower or asparagus. They also received daily nutritional supplements -- a multivitamin; essential oils including flax seed oil, borage oil and fish oil; and chromium picolinate. There were no restrictions on total calories, but carbohydrates were kept below 20 grams per day at the start of the diet.
The low-carbohydrate diet appears to have a favorable effect on cardiac risk, Westman said. "While the low-carbohydrate group received extra nutritional supplements, and experienced greater weight loss, these differences did not fully account for the changes in cardiac risk factors that we saw," he said.
The low-fat, low-cholesterol, low-calorie group followed a diet consisting of less than 30 percent of daily caloric intake from fat; less than 10 percent of calories from saturated fat; and less than 300 milligrams of cholesterol daily. They were also advised to cut back on calories. The recommended daily calorie level was 500 to 1,000 calories less than the participant's maintenance diet -- the calories needed to maintain current weight.
Westman noted that the diets have one often-ignored similarity. "It's possible that the common denominator of these diets is what they're not eating – both diets did not allow refined sugar or junk food," Westman said.
Study participants were encouraged to exercise 30 minutes at least three times per week, but no formal exercise program was provided. Both sets of dieters had group meetings at an outpatient research clinic regularly for six months.