Low carb diet increases heart disease risk | Low-Carb Diets Hit the Headlines
United Kingdom :— A small study in only 24 patients and lasting just eight weeks has reignited debate over the relative benefits and possible harms of low-carb vs low-fat diets .
In the December 2009 issue of Diabetes, Dr Una Bradley (Royal Victoria Hospital, Belfast, UK) and colleagues found that the low-carb diet they studied significantly increased a little-known measure of arterial stiffness, the aortic-augmentation index. “This observation is of concern and, if confirmed, would suggest a potentially negative effect of a low-carbohydrate diet on long-term vascular health,” they conclude. The findings have prompted media reports with headlines such as “Low-carb diets hurt hearts.”
One expert contacted by heartwire takes issue with the findings, claiming that the study is being reported as negative for low-carb diets, when in fact there was no difference in weight loss or insulin resistance between the two groups (low-carb diets and low-fat diets). Bradley et al are accused of naivety at best or–to take it to the other extreme–being unduly influenced by the UK Sugar Bureau, which sponsored the study.
The UK Sugar Bureau press release on the study, issued this week , concludes: “Worryingly, the research . . . revealed a significant difference in overall systemic arterial stiffness and pointed to increased cardiovascular risk factors from high-fat, low-carbohydrate diets.”
Both Diets Prompted Weight Loss, Improved Insulin Resistance
The study randomized 24 overweight/obese people to either a low-fat (20% fat, 60% carbohydrate) or a low-carbohydrate (60% fat, 20% carbohydrate) diet for eight weeks, with all food being weighed and intake calculated to produce a 500-kcal/day energy deficit.
Significant weight loss occurred in both groups (p<0.01), with no significant difference between the two. And for the primary outcome of the study, insulin resistance–as determined by glucose infusion rate (GIR) during euglycemic clamp–there was also no difference between the two groups (p=0.36).
But the aortic-augmentation index was significantly increased with the low-carb diet, whereas it was reduced with the low-fat diet (between groups, p=0.04).
Senior author of the study, Dr Steven J Hunter (Royal Victoria Hospital, Belfast, UK), told heartwire : “What we are doing is just introducing some degree of caution, in that there do seem to be negative effects [of the low-carb diet] on what’s regarded as a risk factor for cardiovascular disease, the aortic-augmentation index. We can’t be as bold as to say that will translate in the long term into heart disease, but it’s certainly recognized as a risk factor.”
But Dr Eric C Westman (Duke University Medical Center, Durham, NC), who was not involved in the study, begs to differ. Westman acknowledges he is a consultant for Atkins Nutritionals, “the one company that would be upset” by this research, but adds, “I work at Duke, I’m independent, and my reputation is more important to me than my consultation to Atkins.”
Westman says: “What is puzzling to me is that this was a positive study for a 20%-carbohydrate low-carb diet, and yet the authors conclude it was a negative study in that the diet was possibly harmful. The researchers clearly have not read the current literature in this area and do not cite important studies in this field.
“The conclusions did not logically follow from the data they presented, which makes this article quite puzzling, because it is in a peer-reviewed journal,” he adds, noting that there are studies out to a year and two years showing improved insulin resistance with low-carb diets. But Hunter remarked, “To my knowledge, none of these have used the ‘gold-standard test’ of insulin resistance, which is the glucose clamp.”
“It appears the discussion was influenced by the outdated paradigm ‘dietary fat is bad,’ ” Westman added. “Is it possible that the researchers were influenced in the interpretation of their findings by the UK Sugar Bureau funding?” he wonders.
Hunter denies being influenced by the UK Sugar Bureau, an organization that partly funded the study; he notes that he has no relationship with this body or any other conflicts of interest.
What Value the Aortic-Augmentation Index?
Westman also takes issue with the researchers pulling out the aortic-augmentation index “from at least 30 comparisons” and using it as a concern. “I’m not even sure what this measurement [aortic-augmentation index] means; it’s not something we use clinically,” he observes.
Other experts who have studied low-carb diets but were not involved in the current study also commented on the new findings for heartwire .
Dr Christopher Gardner (Stanford University, CA) told heartwire the p value between the groups for the aortic-augmentation index “would be expected by chance alone” and “the standard deviations around the means are huge, reinforcing my first reaction that this could be due to chance.”
Dr Gary Foster (Temple University, Philadelphia, PA), an obesity expert, said: “It always concerns me when there are many variables measured and only one is significantly different; that creates pause for me. The issue is whether this aortic-augmentation index has clinical significance, and as a noncardiologist, I don’t know.”
Hunter replied: “There is always the possibility that any significant difference can be due to chance, but this was below the predefined level of statistical significance.” He also agrees that the aortic-augmentation index “is not something that’s used clinically, but it is quite extensively used in research.” The same could be said of insulin resistance, he notes, “which is not used clinically, but the term is widely talked about, and it’s a valid research measure.”
He also pointed heartwire in the direction of a recent study, in mice, “which again raised concerns about low-carb diets. Taken together, there are just some concerns,” Hunter says. “Low-fat diets have been tried and tested over time. Any rival needs to be tested against it so we can be sure.”