Eating three portions of whole grains a day slows the onset of middle-age spread by half, a study has found.

Many people in their 50s suffer from an expanding midriff and struggle to shift the excess fat, but it also increases a person’s risk of cardiovascular disease, stroke or heart attack.

Researchers from Tufts University in Boston, in the United States, studied more than 3,000 people for 18 years, tracking their health and body size as well as their diet.

They analysed how whole grains affected five risk factors for heart disease – waist size, high blood pressure, blood sugar levels and the amount of harmful fats called triglycerides, as well as the amount of "good" (HDL) cholesterol.

Regular assessments found people who ate the fewest whole grains saw their waist size swell by around 3cm (1.2 inches) every four years. However, for people who regularly ate three portions of whole grains a day this figure dropped to around 1.4cm (0.5 inches).

The health benefits also extended beyond waist size. Fasting blood sugar levels increased 3.5 times more in the low intake group compared to high intake individuals, and blood pressure was also more stable for whole grain rich diets.

Whole grain foods feature the entire kernel, including the bran and germ. The most common whole grain foods in the study were brown breads and porridge oats. Whole grain is healthier than more processed grains because the outer layer is packed with fibre while the inside is rich in nutrients such as B vitamins, antioxidants and healthy fats.

Milling removes these components, leaving only the starch-packed refined grain behind.

Dr Nicola McKeown of Tufts University, the report’s senior author, said: "Our findings suggest eating wholegrain foods as part of a healthy diet delivers benefits beyond just helping us lose or maintain weight as we age.

"In fact, these data suggest people who eat more whole grains are better able to maintain their blood sugar and blood pressure over time. Managing these risk factors as we age may help to protect against heart disease."

First author Dr Caleigh Sawicki, also of Tufts, said: "There are several reasons whole grains may work to help people maintain waist size and reduce increases in the other risk factors. The presence of dietary fibre in whole grains can have a satiating effect, and the magnesium, potassium, and antioxidants may contribute to lowering blood pressure.

"Soluble fibre in particular may have a beneficial effect on post-meal blood sugar spikes."

Dr McKeown said: "The average American consumes about five servings of refined grains daily, much more than is recommended, so it’s important to think about ways to replace refined grains with whole grains throughout your day. Small incremental changes in your diet to increase wholegrain intake will make a difference over time."

The study, in the Journal of Nutrition, used data from the Framingham Heart Study Offspring Cohort. It has been following people who were born in Boston during the 1970s.

Previous research by Scottish scientists has found a diet high in whole grains is as effective at reducing blood pressure as medications.