August 25, 2009 -- Low-carbohydrate diets, popular because they can work well for weight loss, have been linked by researchers to diseased arteries, a risk factor for stroke and heart disease. But the study looked at the effect of these diets on mice. We don't know whether people would be affected in the same way.

What do we know already?

Low-carbohydrate weight-loss diets, such as the Atkins diet, have become popular in recent years. In a low-carbohydrate diet, people cut down on carbohydrates such as pasta and bread, rather than cutting down on fat. The diet normally includes more protein from meat and dairy products than a low-fat diet. Although there is no agreed definition of a low-carb diet, it is typically less than 20 percent carbohydrates. Studies have shown that low-carb diets can work as well as, if not better than, traditional low-fat diets in terms of losing weight. That's an attractive prospect when so many people are overweight. But questions remain about how healthy these diets are in the long term. Some people worry that replacing carbohydrates with protein and fats could be dangerous for the heart. Studies so far have found no increases in known risk factors for heart disease. People eating low-carbohydrate diets don't have higher blood pressure or higher cholesterol levels than people eating low-fat diets. One study suggested they might have a better balance of 'good' (HDL) cholesterol and 'bad' (LDL) cholesterol than people on a low-fat diet. But worries persist. Now scientists have looked at what happens to mice eating a low-carbohydrate diet, compared with two other types of diet. They used mice because they could be killed and dissected at the end of the study, to see what had happened to their blood vessels.

What does the new study say?

Mice who'd been fed on a low-carbohydrate diet had narrower blood vessels and more fatty lumps (known as plaques) sticking to the inside of their blood vessels, compared with mice on a typical 'Western' diet high in carbohydrates. Having narrow blood vessels with lots of plaques increases the risk of heart attacks and strokes. Interestingly, the mice on the low-carb diet didn't have higher levels of cholesterol in their blood or higher levels of other markers for artery disease. A third group of mice were given a standard mouse diet, and showed no evidence of artery damage.

How reliable are the findings?

The trouble with studies in mice is that we don't know whether or not the results will also apply to people. These particular mice were bred with a high genetic chance of developing arterial disease for the study. They were given food specially prepared to mimic either a low-carbohydrate diet or a typical 'Western' diet – but neither of these is the type of diet a mouse would usually eat. So it's hard to know whether we can rely on this study to tell us how low-carbohydrate diets affect humans.

Where does the study come from?
The study was done by researchers from Harvard Medical School in Boston, in the U.S. It was published in an online medical journal called the Proceedings of the National Academy of Sciences. The research was funded by a number of research institutes in the U.S.

What does this mean for me?

The study raises questions about the safety of low-carbohydrate diets, but it doesn't give us any firm answers. Research so far in humans hasn't found any major problems with low-carbohydrate weight-loss diets, but previous studies have not looked specifically at what happens to people's arteries. They have only looked at 'markers' for artery damage, such as blood cholesterol levels. If you need to lose weight, you need to find a diet that works well for you, and that you can stick to over a long period. The important thing is to stick to the diet you choose, and to take exercise. Other studies have shown that getting help and support during your diet can help you stay on track.

What should I do now?

For anyone who needs to lose weight and is considering a low-carbohydrate diet, this study raises more questions than it answers. It's important to bear in mind it was carried out in mice. More research needs to be done in people to find the long-term health effects of this type of diet. If you are worried about your weight, a good place to start is your GP. Your GP will be able to help you work out your body mass index (BMI) to see whether you are overweight. He or she may also be able to recommend local classes or support groups that can help you plan and stick to a diet.

From:

Foo S, Heller ER, Wykrzykowska J, et al. Vascular effects of a low-carbohydrate high-protein diet. Proceedings of the National Academy of Science. Published online 25 August 2009. Available at Proceedings of the National Academy of Sciences.