A new clinical trial from the US has reported that a low-carb diet improves cholesterol levels, as a second study in a week reports health benefits for this once bright star of the diet world.
Low carb diets, like the once fashionable Atkins diet, have lost popularity amongst the public with critics saying that the approach puts followers at a higher risk of clogged arteries and heart attack in the long-term.
Indeed, Atkins Nutritionals, filed for bankruptcy last year, only to re-emerge in January with a new mission: to promote 'tasty, portable nutrition'.
However, results of a study published in the May issue of The American Journal of Clinical Nutrition (Vol. 83, pp. 989-990), seem to suggest that the public should not be so hasty to ditch the low-carb diet and that a more favourable cholesterol profile is observed when eating a diet low in carbohydrates, even if there is no weight loss.
The researchers, led by Ronald Krauss from the Children's Hospital Oakland Research Institute, randomly divided 178 overweight and mildly obese men with an average age of 50 into four groups. All groups ate a control diet of 54 per cent carbohydrate for one week with low saturated fat content (7 per cent) - one group stayed with this diet, while the other three had carbohydrate intake reduced to 39 per cent, 26 per cent, and 26 per cent plus an increase of saturated fat to 15 per cent.
After three weeks, the calorie intake of all the diets was reduced by about 1000 calories to induce weight loss for a further five weeks, and then the energy intake adjusted to stabilise weight for a final four weeks.
Krauss and his colleagues found that, during the first four weeks, that the 26 per cent carbohydrate diet with low saturated fat resulted in a 2 mg/ML increase in HDL cholesterol, a 8 mg/mL decrease in LDL cholesterol, and a reduction of 0.57 in the ratio of total cholesterol to HDL, compared to the control group. This last measure is reported to be the most specific lipid risk factor for CHD, and a decrease suggests a favourable effect.
Comparing the high and low saturated fat diets with 26 per cent carbohydrate, the researchers found that LDL and total cholesterol levels increased, as did HDL levels. Results from other studies have reported that HDL levels should remain unchanged.
When the subjects were switched to the weight loss and stabilisation regime, the researcher found that lipoprotein effects were more favourable in the high carb group than the low carb groups.
"It is possible that this finding reflects a convergence of the effects of adiposity and carbohydrate intake on common pathways that affect these lipoprotein measures, such that reductions of either can achieve similar results," said the researchers.
In an accompanying editorial, Martijn Katan from the Vrije Universiteit in Amsterdam said the research was valuable as it extended current knowledge of dietary protein and blood lipids.
"The finding adds to the evidence that carbohydrates have adverse effects on triacylglycerols and the ratio of total to HDL cholesterol that are not shared by dietary protein or unsaturated fat," he said.
On the results of the weight loss, stablisation regime, Katan said: "I find it difficult to visualise how a diet can have one effect in men weighing 93 kg, but a different effect in these same men when they weigh 87 kg. Future studies will hopefully clarify this."
Katan said however that we do not currently know if excessive consumption of proteins is safe for the kidneys and bones, and that clinical trials are urgently needed.
Yet only this week, researchers from the University of South Florida College of Medicine reported results from an intervention trial of a diet low in carbohydrates on bone loss and found that there was no significant difference in bone turnover between people on a 'normal' diet and those on the low-carb (on-line in Osteoporosis International, ISSN: 1433-2965).
"I was surprised by the results," admitted lead researcher John Carter. "Patients on low carbohydrate diets absorb less calcium through the gut and excrete more calcium in the urine, so you'd expect they would be leaching their bones."