An unfortunate drawback to the popular high−protein, low−carbohydrate diet is that it may increase the LDL form of cholesterol – also known as “bad” cholesterol. Researchers at the University of Colorado found that while the diet is as effective for weight loss as a low−protein, high−carbohydrate diet, it does have the unwanted effect of increasing certain blood fats.

Participants on the low−carb diet showed increases in LDL cholesterol of 12 mg/dl, while the LDL cholesterol of those on the high−carb diet decreased by 7 mg/dl.

Teri L. Hernandez and her colleagues studied the effects of the two diets in 32 obese participants. They randomly assigned half of the subjects to the low−carb diet and half to a high−carb diet, for a period of six weeks. Those on the low−carb diet were instructed to consume 20 grams or fewer of carbohydrates per day, while participants on the high−carb diet took in 55% of their daily calories from carbs.

People in both groups lost about the same amount of weight – on average, about 13 pounds over the six−week period. But changes in their cholesterol levels were very different. Participants on the low−carb diet showed increases in LDL cholesterol of 12 mg/dl, while the LDL cholesterol of those on the high−carb diet decreased by 7 mg/dl. Another effect of the low−carb diet was that the blood levels of free fatty acids increased in this group of participants, whereas this was not true for the high−carb dieters.

The findings are intuitive in the sense that when one limits the amount of carbohydrates consumed, the protein and fat content of the diet tends to increase, particularly for those following the well−known Atkins™ diet. When protein sources like red meat and dairy make up a larger percentage of one’s daily calories, it makes sense that rising cholesterol levels would follow. The research team writes that their “data suggest that a high−fat diet may have adverse metabolic effects during active weight loss.”

The study was published in the January 27, 2010 online issue of the American Journal of Clinical Nutrition.