Just a few bad habits - watching TV, eating potato chips, having a sugary soda at lunch or staying up too late at night - can add up to a steady creep of kilos over the years, US researchers reported in the New England Journal of Medicine.
While most studies on diet focus on changes needed to help obese people lose weight, the study by the Harvard team showed tiny changes in diet and lifestyle can make a big impact.
The study focuses on specific lifestyle choices - foods, activity, sleep habits - that slowly pack on the kilograms. The researchers stressed that the quality of food choices, and not just kilojoules, are key to maintaining a healthy weight.
Small choices add up
"These small choices add up," said Dr Dariush Mozaffarian of the Harvard School of Public Health and Brigham and Women's Hospital, who led the study.
"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," Mozaffarian said.
To get at this, the team analysed data on 120,877 US women and men from three large studies of health professionals that tracked changes in lifestyle factors and weight every four years over a 20-year period.
All study participants were normal-weight and healthy when they started. Over time, they gained an average of 1.52kg during each 4-year period for a total average weight gain of 7.6kg at the end of the 20-year study.
Foods that added most to weight gain over a four-year period included daily consumption of potato chips (0.77kg), potatoes (0.58kg), sugar-sweetened beverages (0.45kg), unprocessed red meats (0.43kg) and processed meats (0.42 kg).
Mozaffarian said understanding ways to keep people from becoming obese may be more effective than getting people to lose weight as US policymakers attempt to turn the tide.
Those in the study who lost or maintained their weight over time tended to eat minimally processed foods.
"Fruits, vegetables, whole grains and nuts - if you increase their intake, you had relative weight loss, presumably because you are replacing other foods in the diet," Mozaffarian said.
The study contradicts the notion that all foods are good for you in moderation.
Food myths busted
"The idea that there are no 'good' or 'bad' foods is a myth that needs to be debunked," said Frank Hu of Harvard, who worked on the research.
Mozaffarian said different foods have a different effect on the body. "You can't just say a calorie is a calorie. It doesn't address your feelings of fullness, your blood glucose levels, your blood insulin levels and the other biological responses in your body," he said.
In the study, dietary changes appeared to have the biggest impact on weight gain over time, but other lifestyle changes also were a factor.
For example, watching one hour of TV per day added 0.14kg over a four-year period.
Sleep also played a role. People in the study who got between six and eight hours of sleep were less likely to gain weight over the study period. But people who got less than six hours or more than eight hours tended to gain weight.
And when people increased their physical activity, they tended to gain less weight during the study period. - (Julie Steenhuysen/Reuters Health, June 2011)
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