Wednesday August 26 2009
An artery with red blood cells
New research suggests that “low-carbohydrate Atkins-style diets could increase risk of heart disease and stroke”, reported the Daily Mail. Research in animals has found that “‘low-carb’, high-protein diets can lead to a 'significant' build-up in plaque in arteries” and “make it harder for the body to form new blood vessels”, the Daily Mail's article on low-carb diets said. It also said one of the researchers has warned that the effects do not show up in the usual tests.
This study looked at the effects of the low-carbohydrate high-protein diet in mice and found it to be associated with greater build-up of fatty deposits in blood vessels than a diet that had more carbohydrate and less protein, but similar amounts of fat.
As this research is in mice, it is unclear to what extent these findings apply to humans. It will no doubt prompt more research into this diet and into identifying an easy way to measure the effects it might have on the blood vessels. People trying to lose weight should try to do so by exercising and eating a healthy balanced diet that can be sustained long term.
Where did the story come from?
The research was carried out by Dr Foo and colleagues from Harvard Medical School and other research centres in the US. The study was funded by the Leducq Foundation Network of Research Excellence, the American Heart Association, the National Institutes of Health, Judith and David Ganz and the Maxwell Hurston Charitable Foundation. It was published in the peer-reviewed scientific journal Proceedings of the National Academy of Sciences of the USA.
What kind of scientific study was this?
This animal study looked at the effects of a low-carbohydrate high-protein diet on the cardiovascular systems of mice. Low-carbohydrate diets, such as the traditional Atkins diet, are used by many as a way of losing weight, but some of these diets are high in protein and fat.
These researchers were particularly interested in the effects of the low-carbohydrate and high-protein aspects of the diet, and so used test diets that had similar amounts of fat. The long-term effects of low-carbohydrate high-protein diets on the cardiovascular system are unknown.
The researchers used male mice that were genetically engineered to develop atherosclerosis when they were given a diet designed to mimic a typical Western diet (43% carbohydrate, 42% fat, 15% protein and 0.15% cholesterol).
Atherosclerosis is the thickening of the walls of the arteries due to the accumulation of fatty substances such as cholesterol. It can lead to heart attacks.
The mice were put on one of three diets one week after weaning. These were:
- the Western diet,
- standard mouse food with more carbohydrate and less fat (65% carbohydrate, 15% fat, 20% protein), or
- a low-carbohydrate high-protein (LCHP) diet consisting of 12% carbohydrate, 43% fat, 45% protein and 0.15% cholesterol.
The Western diet and LCHP diet had similar amounts of calories, fat and cholesterol.
The mice were weighed after 12 weeks on the diet, and their aortas (the main artery leading out of the heart) examined for signs of atherosclerosis after six and 12 weeks. Other signs of heart disease risk were also measured, including levels of cholesterol and other kinds of fats in the blood, levels of insulin and glucose, and levels of chemicals that indicate inflammation (which is involved in the formation of the fatty build-ups in the arteries).
The researchers assessed whether the diets affected cells called endothelial progenitor cells (EPCs). These move into areas where blood vessels are susceptible to atherosclerosis, and may help to repair the blood vessels. A reduction in EPCs indicates greater risk of cardiovascular problems.
The researchers also looked at the effects of the diets on the mice’s ability to form new blood vessels when blood supply (and therefore oxygen) was cut off from the tissues (ischaemia). To investigate this, normal laboratory mice were fed the Western-style or LCHP diet for four weeks. After this period the researchers surgically cut off blood flow to one of the mice’s back legs and looked at how long it took for blood flow to re-establish itself, which indicates how well new blood vessels are being formed.
What were the results of the study?
The genetically engineered mice on the LCHP diet gained less weight over 12 weeks than those on the Western-style diet or standard food. Mice that were fed the LCHP diet had significantly greater fatty deposits in their aortas after six and 12 weeks than mice that were the Western-style diet or standard food. Mice fed on the Western-style diet had larger fatty deposits in their aortas than mice on standard food.
The mice on the LCHP diet did not show differences in the levels of cholesterol or chemicals relating to inflammation in their blood compared to those on the Western-style diet. However, they did show a reduction in the number of EPCs compared to mice that were fed on the Western or standard diets. This suggests that the mice would have limited ability to repair damaged blood vessels.
The researchers also found that mice fed on the LCHP diet showed less ability to regrow new blood vessels in response to a loss of blood supply to their back legs than mice fed on a Western diet.
What interpretations did the researchers draw from these results?
The researchers conclude that in an animal model of atherosclerosis, low-carbohydrate high-protein diets have adverse effects on blood-vessel health, and that typical markers of cardiovascular risk, including cholesterol levels and signs of inflammation, do not show this. They are not sure whether it is the low-carbohydrate or high-protein content that has these effects. They say that caution is needed if these findings are to be extended to humans, but raise their concerns that typical blood markers may not reflect the effects of the low-carbohydrate high-protein diet on cardiovascular risk.
What does the NHS Knowledge Service make of this study?
This study looked at the effects of a low-carbohydrate high-protein diet in mice, and found that it was associated with a greater build-up of fatty deposits than a diet that had more carbohydrate and less protein, but similar amounts of fat. As this research is in mice, it is unclear to what extent these findings apply to humans.
It is important to note that both the LCHP diet and the Western-style diet had a similar fat and cholesterol content. This means that some aspect of protein or carbohydrate metabolism contributed to the increased build-up of the cholesterol-rich fatty deposits in the mice on the LCHP diet. This is clearly a complex effect that will need further evaluation and the finding will prompt more research into how this diet has its effect.
The message for humans remains unchanged: people aiming to lose weight should try to do so by eating a healthy balanced diet that can be sustained in the long term and ensuring that they are physically active.