"Eat more 'good' carbohydrates and less protein for a longer life," reports the Mirror.
It seems like only last week that the media was advising us to eat less carbohydrates. The reality is that neither today's "pro-carbs" or recent "anti-carbs" news stories have changed government food advice.
Today’s news refers to a short study of different diets on a relatively small number of mice – not people. It's always a fairly safe bet that studies in mice have few implications for the British human public – and today’s news is no exception.
And the study did not look at "good" or "bad" carbohydrates, nor the effect of the diet on lifespan. Instead, mice randomly got to eat one of three diets, with either unlimited access or in a calorie restricted fashion. All diets had 20% fat content, and then either a high, low or medium protein to carbohydrate ratio.
As expected, mice fed calorie restricted diets lost the most weight and had good metabolic status. Mice fed unlimited low protein, high carbohydrate diets ate the most amount of food and had the highest energy intake, but did not put on as much weight as the other two groups of mice fed unlimited food. The researchers say this was because they burned off more energy. These mice also had improved insulin and cholesterol levels similar to that observed for mice with calorie restricted diets.
Find out the truth about carbs.
Where did the story come from?
The study was carried out by researchers from the University of Sydney, Australia and the National Institutes of Health in Baltimore, US. It was funded by the National Health and Medical Research Council of Australia, the Ageing and Alzheimer’s Institute of Concord Hospital and the US National Institutes of Health.
Both the Mirror and the Daily Mail reported that "healthy" carbs such as fruit and veg may be the key to living longer. While this may be the case, neither the type of carbohydrates nor lifespan were studied in this piece of mouse-based research.
What kind of research was this?
This was an animal study using mice. The researchers wanted to directly compare the effects of different diets on indicators of health.
The authors point to previous research in which restricting calories by 30 to 50% increased "healthspan", delayed the onset of ageing and age-associated diseases and improved metabolic health in most animal species that have been tested. They wanted to see if other diets could have the same effects without the calorie restriction, because most people find it difficult to limit the amount of calories they consume.
What did the research involve?
The researchers randomly assigned 90 mice to have either calorie restricted diets or diets with unlimited access to food for eight weeks. At the end of the eight weeks, the researchers compared the weight and metabolic status of mice between the groups.
All of the diets had 20% fat content and the same number of calories per gram. The diets were split into:
- low protein, high carbohydrate
- medium protein, medium carbohydrate
- high protein, low carbohydrate
Mice fed the calorie restricted diets were given 40% of the average amount of food that the other group of mice ate.
What were the basic results?
After eight weeks:
- Mice fed calorie restricted diets lost the most weight.
- Mice fed unlimited low protein, high carbohydrate diets ate the most amount of food and had the highest energy intake, but did not put on as much weight as the other two groups of mice fed unlimited food. They had the highest level of energy expenditure out of all groups studied.
- Mice fed unlimited low protein, high carbohydrate diets also had improved insulin and cholesterol levels similar to that observed for mice with calorie restricted diets, regardless of the type of diet.
- Mice fed unlimited high protein and low carbohydrates had higher insulin levels and impaired glucose tolerance compared to mice in the other groups.
How did the researchers interpret the results?
The researchers concluded that after eight weeks, mice fed unlimited low protein, high carbohydrate diets had similar metabolic improvements as seen under calorie restriction. This was despite increased energy intake. Importantly, this was these mice also didn’t develop increased body fat and fatty liver that is observed in longer-term low protein, high carbohydrate feeding. They say that these results "suggest that it may be possible to titrate the balance of macronutrients to gain some of the metabolic benefits of [calorie restriction], without the challenge of a 40% reduction in caloric intake".
This study has found that over a short time period, mice fed a diet that’s low in protein and high in carbohydrates gained less weight than those fed diets with higher levels of protein. It also found that mice lost weight regardless of the amount of protein and carbohydrate if the number of calories was restricted.
The researchers say that the mice fed unlimited low protein, high carbohydrate diets did not gain as much weight because they burned off more calories. In this study, their "metabolic status" improved compared to mice with unlimited higher protein diets. However, previous research has shown that low protein, high carbohydrate diets consumed over longer time periods have been associated with weight gain, increased body fat and fatty liver.
While this study has produced interesting findings, its use is severely limited because it had no control group to compare the diets against.
It is also not clear how the results of this study would be applicable to humans. Calorie restriction does cause weight loss in humans, but in the long term this can reduce the metabolic rate. Humans are also more complex and less likely to naturally burn off any excess calories consumed from a low protein, high calorie diet.
In conclusion, this study in mice does not change current human dietary advice to eat plenty of starchy carbohydrates, fresh fruit and vegetables, some milk and dairy foods, lean meat and fish (or other protein), and foods high in sugar, fat or salt to be eaten little or in small amounts.
The eatwell plate highlights the different types of food that make up our diet, and shows the proportions we should eat them in to have a well balanced and healthy diet.