On average, American adults gain nearly one pound per year as they age, and much of that weight gain is caused by subtle behaviors that add up to extra baggage. A new study provides the strongest evidence yet that weight gained with aging is primarily due to dietary and lifestyle choices, according to researcher Frank Hu, a professor of nutrition and epidemiology at the Harvard School of Public Health.

Every extra serving of potato chips led to a 1.7 pound weight increase every four years while sugary beverages were associated with a one pound weight gain.

Researchers at Brigham and Women's Hospital and the Harvard School of Public Health used data from three separate studies, the Nurses' Health Study, the Nurses' Health Study II, and the Health Professionals Follow-Up Study, to assess the relationships between changes in diet, lifestyle behaviors, and weight changes in over 120,000 men and women. Participants were tracked every four years for 20 years.

The data found that adults gained an average of 3.35 pounds every four years. The increased consumption of potato chips, French fries, sugary sodas, refined grains, and red meat were identified as major causes of steady weight gain. Over four years, people who ate an extra serving of these foods gained anywhere from just under a pound to 1.7 pounds for every extra serving consumed. For example, every extra serving of potato chips led to a 1.7 pound weight increase every four years while sugary beverages were associated with a one pound weight gain.

Dariush Mozaffarian, MD, PhD, lead researcher and an associate professor of Medicine and Epidemiology at Brigham and Women's Hospital, said that this is not a case of a small segment of the population gaining a lot of weight quickly. It's a matter of everyone gaining weight slowly as they get older.

Lifestyle factors such as increased television watching, decreased physical activity, and getting less than 6 hours or more than 8 hours of sleep were also linked to gradual and steady weight gain. The consumption of alcohol was associated with a .41 pound weight gain per drink per day.

Nuts, despite the fact that they are high in fat, helped prevent weight gain in the study as did the consumption of fruits, vegetables, yogurt, and minimally processed foods. Those who participated in more physical activity gained 1.76 fewer pounds during each four-year period of the stud than those who did less physical activity.

According to Mozaffarian, every lifestyle factor has a pretty small effect on weight when looked at individually, but the combined effect can explain the gradual weight gain that aging adults experience. So it seems that it's the little things we do or don't do every day that can make a big difference in how much weight we gain as we age. Eating better, i.e. more fruits, vegetables, whole grains, nuts, and minimally processed foods, being active, turning off the television, and getting enough, but not too much, sleep may help stave off those unwanted pounds that tend to accumulate with age.

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