Eating too much white bread could expand your waistline, US research shows.
White bread leads to expanding waistlines
People who ate whole grain foods, such as brown bread, did not have the same gain in waist size, the scientists at Tufts University in Boston found.
This might be because whole grain foods are higher in fibre which give a feeling of fullness so you eat less, say Professor Katherine Tucker's team.
The study appeared in the American Journal of Clinical Nutrition.
The researchers looked at five different diets where one food-type was prominent - healthy food, white bread, alcohol, sweets or meat and potatoes - in 459 healthy men and women.
The people who ate a diet of predominantly white bread saw the biggest increases in their waist measurement.
In a year their girth increased by an average of about a centimetre, which was three times more than people who ate a healthy diet that was high in fibre and whole grain foods such as brown bread.
Professor Tucker said: "Waist circumference was very much associated with this high-refined grains pattern."
White bread is the most commonly consumed food in the UK. We eat an average of three slices a day.
The researchers do not know why high-refined grain foods like white bread increases waist size, but it could be linked to fibre content and how the food is broken down by the body for energy.
Many of the foods in the healthy diet were high in fibre.
Not only do these foods fill you up more quickly, they also have a low glycaemic index (GI), said the researchers.
The GI is a relative measure of how fast a given food raises blood sugar.
It compares foods gram for gram for carbohydrate. Carbohydrates that breakdown quickly during digestion have the highest GI value and blood glucose response is fast and high.
Carbohydrates that breakdown slowly release glucose gradually into the blood stream and have low GI values.
In turn, the level of blood sugar affects the amount of insulin produced by the body which is linked with appetite.
"Many of the foods in the healthy pattern are low in glycaemic load, which evokes a decreased insulin response and therefore decreases hunger and energy intake.
"Those in the white-bread pattern received almost 16% of their daily energy intake from white bread - the food with the highest GI value," said the researchers.
But they said because foods are not eaten in isolation but as part of an overall diet, it might be difficult to extend their findings to people's natural diets.
A spokeswoman from the British Nutrition Foundation said: "Looking at the data, those that ate white bread consumed more calories, more fat and less fibre, than those that ate wholemeal bread, so it is hard to say that white bread is the reason for this weight gain.
"This type of diet may be associated with a more sedentary lifestyle - people who eat wholemeal bread may be more health conscious overall and exercise more.
She said the way people gain weight is by taking in more calories than they burn up, rather than eating specific foods or nutrients.
"We should be basing our diets on starchy carbohydrates (breads, cereals, potatoes and other cereal products) and we should be including wholegrain versions of these foods in the diet as well as refined.
"Consumption of wholegrain foods, such as wholemeal bread, is associated with reduced rates of heart disease, some cancers, type II diabetes and such foods may play a role in weight maintenance."
She said the GI was a very complicated concept.
"White bread has a higher GI than wholemeal bread, and consumption of high GI foods is associated with a quick rise in blood glucose.
"However, high GI foods are not necessarily unhealthy. The GI of a food can be altered by combining it with another food. Consuming white bread with chicken and salad as a sandwich for example would lower the GI of the meal.
"Consumption of all bread and white bread in the UK has been declining since the 1940s. At this same time the prevalence of overweight and obesity has been increasing.
"Obesity is a multifactorial disorder and it is hard to find a reason for the increase in prevalence," she said.