A new study from the Harvard School of Public Health has found that small changes in diet, lifestyle and sleep patterns are strongly linked with long term weight gain, with humble potato the single biggest offender.

Where previous studies have examined patterns of weight loss, the Harvard study examined factors in weight gain, finding changes in diet to have the biggest association with gaining weight.

“An average adult gains about one pound per year. Because the weight gain is so gradual and occurs over many years, it has been difficult for scientists and for individuals themselves to understand the specific factors that may be responsible,” said lead author Dariush Mozaffarian, associate professor in the Department of Epidemiology at HSPH and Division of Cardiovascular Medicine, Brigham and Women’s Hospital (BWH), and Harvard Medical School.

The foods associated with the biggest weight gain over the 20-year study period included potato chips, other potato foods, sugar-sweetened beverages, unprocessed meats and processed meats. On the other hand, certain foods were linked with reduced weight gain – yoghurt, nuts, fruits, wholegrains and vegetables.

One extra daily serve of potato chips alone was associated with as much as an extra 0.76kg (1.69lb) every four years, and one extra serve of potatoes in other forms, just over half a kilogram.

The research was conducted as part of the Nurses’ Health Study (NHS), the Nurses’ Health Study II (NHS II), and the Health Professionals Follow-up Study (HPFS), evaluating changes in lifestyle factors and weight gain every four years in just under 100,000 women and 22,500 men in the US.

Study participants gained an average of 1.51kg (3.35 lb) during each four-year period, which corresponded to a weight gain of 7.62kg (16.8 lb) over the 20-year period.

According to the researchers, just counting calories may not be the best way to keep a healthy diet, and other yardsticks such as fat, sugars or energy density may be misleading. Instead, they said that focusing on overall dietary quality and healthier foods and beverages is the best option.

“These findings underscore the importance of making wise food choices in preventing weight gain and obesity,” said Frank Hu, professor of nutrition and epidemiology at HSPH and senior author of the paper. “The idea that there are no ‘good’ or ‘bad’ foods is a myth that needs to be debunked.”

In particular, the researchers said that the most useful changes appeared to be reducing liquid sugars (such as soft drink) and other sweets, reducing starches and refined grains like potatoes, white bread, white rice and low-fibre cereals, reducing processed foods, and increasing minimally processed foods like fruit, vegetables, whole grains, nuts and yoghurt.

The research also showed that changes in physical activity and TV-viewing influenced changes in weight. Also, those who slept 6-8 hours a night gained less weight than those who slept less than 6 or more than 8 hours.

Overall, the weight-changes associated with any one lifestyle change were fairly small. However, together they added up, especially for diet.

“Small dietary and other lifestyle changes can together make a big difference – for bad or good,” said Mozaffarian. “This makes it easy to gain weight unintentionally, but also demonstrates the tremendous opportunity for prevention. A handful of the right lifestyle changes will go a long way.”

The full results of the study are published in the June 23, 2011 issue of the New England Journal of Medicine.

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