"It's pretty clear low-carb is effective for weight loss," said study author Kerry J. Stewart, director of clinical and research exercise physiology at the Johns Hopkins University School of Medicine and its Heart and Vascular Institute. "The concern has been that because you are eating more fat this is going to put stress on your blood vessels."
So, Stewart and his team evaluated the short-term effects of a low-carb, higher-fat diet after a single meal. The researchers also compared a low-carb diet with a low-fat diet in dieters. In each case, they found no ill effects on blood vessel health.
Stewart is due to present his findings Friday at the American College of Sports Medicine meeting in Denver.
However, one nutrition expert said longer-term research is needed before concluding that high fat intake doesn't hurt blood vessel health.
Blood vessel health and high fat diets
For the first study, Stewart's team looked at the effects of eating an extremely high-fat McDonald's breakfast. The breakfast had more than 3,780kJ and 50 grams of fat. "That's half of what you should eat in a whole day," Stewart said.
The researchers then evaluated a marker of arterial stiffness and another measure of blood vessel health, known as endothelial function. "Even after eating this one meal, we didn't find any vascular changes from before to after," he said.
The arterial stiffness, in fact, improved, he noted, although he is not sure why.
Neither study had industry funding; both were financed by the US National Heart, Lung, and Blood Institute.
For the diet study, Stewart assigned 55 men and women who were overweight or obese to the low-carb diet or a low-fat diet. They also had abdominal obesity and a large waist circumference (35 inches or more for women, 40 or more for men). Both are risk factors for heart disease.
The low-carb plan included up to 55% fat at the beginning, and phased down to about 40%. It had about 15% carbs initially, and then went to 40%. The other dieters followed the American Heart Association's low-fat diet, with no more than 30% fat a day.
Both groups had supervised exercise three times a week.
Low-carb dieters lost the most, fastest
At the meeting, Stewart will report on 46 dieters, 23 from each group, who lost about 4,5kg. "In the low-carb group, they reached the 4,5kg loss at 45 days," he said. The low-fat group needed 70 days to shed 4,5kg.
Their kilojoule intake was similar, whichever diet they were on.
Stewart performed the same blood vessel measures as in the breakfast study. "There were no differences in any of the vascular measures," he said.
The researchers will continue the study for six months. While Stewart cautioned that these are initial findings, he added, "We are pretty confident this is a real result. At the 4,5kg weight-loss mark, we don't see any harm to the vasculature."
Stewart said he put on weight a few years ago, went on a low-carb plan while also exercising and dropped 18 kg. He has kept if off for four years.
Long-term effect not known
While the study is intriguing, long-term research is crucial, said Connie Diekman, director of university nutrition at Washington University in St. Louis and past president of the American Dietetic Association.
"When it comes to the impact on blood vessel functioning, as a registered dietitian I would like to see more studies in healthy and unhealthy subjects and longer-duration studies before concluding that this high-fat intake does not impact blood vessel health," she said, although the study does show that exercise is important. The breakfast study, with its one-time test, does not provide much information about what impact these diets will have long-term, she added.
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