High-carb, low-protein diet 'may keep brain young'

"Low-protein, high-carb diet may help ward off dementia," reports The Guardian.

Researchers studying mice kept on different diets found that mice on either restricted-calorie diets or low-protein, high-carbohydrate diets showed differences in the hippocampus region of the brain compared with mice fed on other diets.

The hippocampus plays an important part in memory, especially long-term memory, and the differences seen in this brain region suggest that either of these diets might have a protective effect.

Also, mice on the low-protein, high-carb diet performed slightly better than all the other mice on tests of memory and behaviour.

Previous studies have shown that restricted-calorie diets (20% lower in calories than standard diets) improve memory in older mice.

But restricting food intake is more difficult in humans than in mice. Researchers set out to see if other diets that might be more adaptable to humans were also linked to better brain ageing.

The problem with animal studies is that we don't know how well they translate to humans.

We need to see more convincing research before we can recommend that older people all switch to a low-protein, high-carbohydrate diet to prevent dementia.

But there are other things you can do to reduce your dementia risk.

Where did the story come from?

The study was carried out by researchers from the University of Sydney, Australia, Harvard Medical School and the National Institute of Ageing in the US.

It was funded by the Australian National Health and Medical Council, the Ageing and Alzheimer's Association, McKnight Bequest, Sydney Medical School Foundation and the American Australian Association.

It was published in the peer-reviewed science journal Cell Reports and is free to read online.

The Guardian, the Mail Online and the Sun carried reasonably accurate news stories covering the study.

But the headlines overstated the strength of the research and don't mention mice, although the stories make it clear that the research was carried out in rodents using specifically designed mouse food (chow).

What kind of research was this?

The researchers carried out a series of experiments on laboratory mice.

Animal experiments are useful ways to test treatments or diets that can't be easily tested in humans.

But we don't know if the results apply to humans.

What did the research involve?

Researchers fed mice 1 of 5 diets from the point at which they were weaned:

a calorie-restricted diet with 19% protein, 63% carbohydrate and 18% fat
an unrestricted diet with 5% protein, 77% carbohydrate and 18% fat
an unrestricted diet with 10% protein, 72% carbohydrate and 18% fat
an unrestricted diet with 15% protein, 67% carbohydrate and 18% fat
an unrestricted diet with 19% protein, 63% carbohydrate and 18% fat

The food consisted of starch-based complex carbohydrates and dairy-based casein protein (the main component of cows' milk).

Twelve female mice and 12 male mice from each group had their brain tissue examined at 15 months of age to look for differences in gene activity (gene expression) associated with brain ageing and memory, protein activity and growth of nerve cells (dendritic spines).

Twelve mice per group took part in behavioural tests, using a maze and recognising objects to test their memory at 13 months of age and 23 months of age.

Researchers looked for differences between the mice on different diets, both male and female.

What were the basic results?

Researchers said the behavioural tests "tended to show a benefit for calorie-restricted and lower-protein diets, primarily in female mice".

They said mice on calorie-restricted diets showed the best performance, while lower-protein diets "were associated with some trends suggestive of improvement".

They said they found more differences when they looked at gene expression, proteins and dendritic spines (part of the neurone that helps transmit electrical signals).

They found that the calorie-restricted and low-protein diets were linked to different patterns of gene expression, but "there were similarities when specific genes involved with brain ageing were analysed".

A protein associated with nerve repair after injury was more likely to be expressed in mice fed calorie-restricted or low-protein diets.

Dendritic spines, also thought to be important for formation of memory, were denser in the hippocampus region of the brain in mice fed calorie-restricted or 5% protein diets.

How did the researchers interpret the results?

The researchers said: "A very low-protein, high-carbohydrate diet may be a feasible nutritional intervention to delay brain ageing."

Conclusion

The research adds to knowledge about how diet affects the ageing brain in mice, but doesn't tell us much about how it might affect humans.

The study didn't look specifically at dementia in mice, but at the effects of different diets on genes, proteins and nerve cells in the brain, as well as some tests of memory.

So this study can't tell us what diet humans should be eating to prevent dementia.

Apart from the obvious limitation that the study looked at mice, not people, and also didn't look at dementia, there are a few other things to be aware of.

Brain changes in the mice were measured during the equivalent of late middle age in mice, and we don't know how they changed over time.

The results from the behavioural tests were inconsistent and small, and most results in male mice weren't conclusive.

But we know that lifestyle more generally is likely to affect the chances of getting dementia.

Eating a healthy, balanced diet with plenty of vegetables and fruit (which are largely made up of carbohydrates) is one of the things you can do to keep your brain healthy.

Other ways to lower your chances of dementia are not smoking, not drinking too much alcohol, getting plenty of exercise, and keeping blood pressure within recommended limits.

But the main risk factor for dementia is getting older, so there are no guaranteed ways to avoid it.

Read more about ways to lower the risk of dementia.