Food and diet

Switching to wholegrains may boost metabolism

"Eating more wholegrain foods can help to speed up weight loss, scientists claim," the Daily Mail reports.

Researchers found that people who ate a diet high in wholegrains absorbed less energy from food than people who ate a similar diet, but with refined grains (such as white flour).

The study included 81 men and women in the US, who were each allocated to eat a wholegrain or refined grain diet for six weeks, after a two week run-in period of eating no wholegrain. All food and drink was provided during the study.

People provided stool samples, gave blood samples, and had their metabolic rate tested. On the latter, researchers were interested in what is known as the resting metabolic rate (RMR), which is the amount of energy the body burns during rest. RMR is sometime used as a benchmark for how efficiently the metabolism is working.

People who ate the wholegrain diet were found to pass more stools (poo). They also had a higher RMR: although this difference was so small it could have been down to chance. These two factors combined amounted to an average difference in energy balance between the two groups of about 92 calories a day.

Assuming people didn't eat more to make up the difference, the study authors said, this would amount to a weight loss of about 2.5kg over a year.

If this modest amount of weight loss doesn't seem inspiring, wholegrain foods include more micronutrients than refined grains, and may improve digestion and reduce the risk of bowel cancer.

Where did the story come from?

The study was carried out by researchers from Tufts University, the University of Minnesota, and the Bell Institute of Health and Nutrition, in the US. It was funded by the Bell Institute of Health and Nutrition. The study was published in the peer-reviewed The American Journal of Clinical Nutrition.

Mail Online concentrated on the findings that a wholegrain diet may speed up metabolism, rather than the effects on stools, saying rather coyly that the wholegrain group "absorbed fewer calories in their digestive systems".

A more accurate, if less palatable, headline would have been "Wholegrains make you poo more".

The report also focused on white versus brown rice, although the study said most of the grain consumed in the study was from wheat, rather than rice.

What kind of research was this?

This was a randomised controlled trial. Although people in the study weren't told whether they were on a wholegrain or refined grain diet, they would have been able to see whether they were eating, say, wholegrain or white bread – so the study was not blinded.

This type of study is a good way of looking at the effects of an intervention such as a diet, especially when (as in this case) all food was provided.

What did the research involve?

Researchers recruited 49 men and 32 women, all aged 40 to 65. All the women were postmenopausal. They were supplied with all food and drink for an eight-week period. For the first two weeks they ate an identical low-fibre diet, then were randomly allocated to a diet similar in nutrients, except that one group ate wholegrain and the other refined grain.

The wholegrain diet included an average 207g of wholegrain daily, including 40g dietary fibre. The refined grain diet included no wholegrain and an average 21g of dietary fibre.

People gave stool samples after two weeks and eight weeks that were collected in separate containers over a 72-hour period and stored chilled until transported to the laboratory. They also had blood tests, tests of metabolic rate, and filled in questionnaires about their appetite, feelings of hunger and satisfaction with their diet. Researchers compared test results taken at weeks two and eight of the study, between the two groups.

The diet was designed to keep their weight steady, not to lead to weight loss or gain. People were excluded from the study for a range of reasons, including if:

  • their weight had fluctuated over the past year
  • they had certain diseases, including cancer or gastrointestinal disease
  • they took medicines affecting appetite or digestion
  • they took supplements (except calcium and vitamin D)
  • they had recently taken antibiotics
  • they drank two or more alcoholic drinks per day

The researchers looked at a wide range of confounding factors that they thought could be affected by wholegrain diet, including people's digestion, metabolic rate, appetite, gut bacteria and the body's regulation of glucose.

Study results were adjusted for people's age, sex and body mass index (BMI). Researchers also looked at the effect of daily adherence to the diet (ie whether people stuck to the diet, or ate other things as well) on the results.

What were the basic results?

After six weeks of the diet, people who ate the wholegrain diet:

  • passed more stool, with a higher total energy content, than those who ate the refined grain diet (an additional 96 calories/day, plus or minus 18 calories)
  • had a resting metabolic rate that burned 48 calories a day more (plus or minus 23 calories) than those who ate the refined grain diet. However, this finding did not hold true when the researchers excluded people who did not stick to the diet
  • after adjusting for other factors, the combined average daily energy loss for people eating a wholegrain diet was 92 calories a day, or 108 calories a day if you only look at people who stuck to the diet
  • the study found no effect on control of glucose in the blood, and no differences in appetite, hunger, satisfaction with diet or eating behaviour

How did the researchers interpret the results?

The researchers said they’d found "new evidence of energetic benefits" if you replace refined grain with wholegrain in the diet.

"We showed that dietary substitution of wholegrains for refined grains conferred favourable energetic benefits that were primarily attributable to a greater energy excretion in the stool," they said.


The suggestion that you can lose weight by simply swapping refined grains like white bread and rice for wholegrains like wholemeal bread and brown rice is attractive if you're planning to shift a few pounds. But there are some things to remember before relying on the study results:

  • People didn't lose weight during the study. Indeed, it was designed to make sure they didn't lose or gain weight, with a dietitian adjusting their daily calories if they started to gain or lose weight.
  • The daily extra amount of calories that the researchers estimate people in the wholegrain group lost is modest – the equivalent of two small ginger nut biscuits, or a matchbox-sized piece of cheddar cheese. Relying on this alone is unlikely to help you lose significant amounts of weight, especially as it would be easy to eat that much extra food each day without realising.
  • Some of the results, such as the effects on metabolic rate, were on the border of being too small to be reliable.

However, we already know that wholegrain foods provide more micronutrients and that the fibre they contain may help digestion. Choosing wholegrain over refined grain foods is a healthy choice.

While they may not automatically melt away the weight, wholegrain foods are a good choice as part of a balanced weight loss diet, such as the NHS Choices weight loss plan.

NHS Attribution