Toast and a lunchtime sandwich is a standard part of most people’s daily diet, but could bread be changing your metabolism and making you more prone to gaining weight – even without overeating?
The claim that ‘even just looking at bread makes me fat’ has been a popular excuse for years for those struggling with their weight.
Once dismissed by experts, the idea that bread is ‘fattening’ has now, however, been backed by a study. This has found that not only bread, but other carbohydrates – including rice and noodles – appear to alter how the body burns energy, setting you up for weight gain without a forkful more.
The researchers, from Osaka Metropolitan University in Japan, say their work is the first to identify this metabolism-altering impact of carbohydrates.
While the study was only on mice, given how central carbs are to our national diet (latest data from UK Flour Millers shows nearly 11million loaves are sold every day in the UK, mostly processed white bread) – and the population’s expanding waistlines (two thirds of adults are overweight or obese) – this research could have huge implications.
For the study, the team divided mice into groups given different meals – bread, wheat flour, rice flour or high-fat food, plus their usual pellets of soya bean and fishmeal. All meals contained similar calories.
Once dismissed by experts, the idea that bread is ‘fattening’ has now, however, been backed by a study
Over ten weeks the mice given refined carbs rejected their usual food and piled on the pounds, even though they ate roughly the same amount of calories
Over ten weeks the mice given refined carbs – bread, wheat flour and rice flour – rejected their usual food and piled on the pounds, even though they ate roughly the same amount of calories as the other mice. Those that ate the high-fat foods were not significantly affected.
‘These findings suggest that weight gain may be due to a strong preference for carbohydrates and associated metabolic changes,’ said Shigenobu Matsumura, an associate professor specialising in nutrition and metabolic health, who led the study.
Furthermore, blood tests found raised levels of fatty acids – which can lead to fatty liver disease and diabetes – among the mice with the carb-heavy diets.
The good news is the study, published in the journal Molecular Nutrition & Food Research, found that when the mice went back to their previous diet, their weight reduced and their blood fats returned to normal.
But why might refined carbohydrates have this effect?
Previous studies have found that a low-carb diet (where carbs accounted for less than 10 per cent of calories) led to improved metabolism and weight loss in humans and mice.
But not all carbs are equal.
Diets high in refined carbs – i.e. carbs with the benefits stripped from them, such as white rice and white flour found in white bread, cakes and biscuits – raise blood sugar rapidly, causing the pancreas to produce high levels of insulin.
This in turn can lead to a build up of abdominal fat, which is strongly linked to type 2 diabetes, heart disease, stroke, high blood pressure and certain cancers.
Dr Maria Chondronikola, research scientist in human nutritional physiology at Cambridge University, says she ‘isn’t surprised’ carbs affect metabolic responses
Meanwhile, studies have found diets high in refined carbs may reduce levels of ‘good’ HDL cholesterol, increasing the risk of cardiovascular disease.
And for years, proponents of low-carb diets have eschewed carbs for this reason.
Professor Matsumura says that his team will now shift their research to the impact on humans, as other factors may affect how carbohydrates impact the body – including what you eat them with and when you eat them.
Previously research has suggested that in terms of weight loss and blood sugar control, it is better to eat your carbs earlier in the day.
That’s because the body is more sensitive to the effects of the hormone insulin (which keeps blood sugar levels stable) in the morning than it is in the evening.
‘We also intend to investigate how factors such as wholegrains, unrefined grains and foods rich in dietary fibre – as well as their combinations with proteins and fats, food processing methods and timing of consumption – affect metabolic responses to carbohydrate intake,’ said Professor Matsumura.
Maria Chondronikola, a research scientist in human nutritional physiology at the University of Cambridge, says she ‘isn’t surprised’, adding that it underlines the link high-carb diets have to obesity and fatty liver disease.
However, she says, the research has some important limitations.
‘Research in rodents is very useful for understanding how biological processes work, but the results don’t always carry over to human health,’ she explains.
Meanwhile, Gary Frost, a professor of nutrition and dietetics at Imperial College London says: ‘All the scientific evidence suggests that weight gain only happens when you consistently consume more calories than your body burns, regardless of the source of those calories.
‘If the researchers are suggesting that high-carb diets affected metabolism independently of energy content, this has never been shown before.’
He believes a drawback of the study is that ‘more than 80 per cent of the mice’s energy intake came from either wheat or noodles – which would not happen in human diets.
‘For example, this would mean a person with an energy expenditure of 2,000 calories a day would have to eat 1,600 from bread.’
Until further studies unpick these results further, the expert view is that the best way to lose weight is to focus on eating fibre (the official advice is to have roughly 30g a day).
‘Fibre slows down digestion, which helps us feel fuller for longer, prevents sharp rises in blood sugar and reduces the number of calories absorbed from different foods,’ says Dr Chondronikola.
‘High-fibre foods are also typically richer in vitamins and minerals that are often stripped away during the processing of refined products.
‘On top of this, fibre plays an important role in regulating cholesterol levels and supporting gut health.
‘And the double benefit is that eating high-fibre, wholegrain foods that have intact cell structures also helps to suppress appetite in the future.’
