The pants you wore 20 years ago - do they fit today? Probably not, if you're like most Americans.
That's because as we age, we slowly pack on extra pounds even though we swear we're not eating any differently.
Now a new Harvard study has looked at why we get fatter as we get older.
Researchers tracked the weight and eating habits of 120,877 Americans every four years for two decades and say they have found the main culprit: Potatoes.
The study, published in the New England Journal of Medicine, found that people who ate an extra daily serving of potatoes put on more weight over a four-year period than people who snacked on an extra serving of yogurt or even nuts.
Snacking on potato chips, to absolutely no one's surprise, led to an extra 1.7 pounds every four years, the study found.
But the single worst food for weight gain may be french fries. When the researchers only looked at potatoes, they found that an extra daily serving of fries added more than three pounds every four years.
By contrast, eating an extra serving of yogurt a day helped people lose the most - just under a pound every four years. Snacking on nuts lowered weight by a little more than half a pound.
While we may think we haven't really changed how we eat over the years, "these small choices add up," study author Dariush Mozaffarian, MD, of Harvard's School of Public Health, said in a statement.
The researchers analyzed the nearly 121,000 men and women from three large studies between 1986 and 2006, tracking their weight and other factors, like exercise, every four years. All the participants were healthy and normal weight when they started, but by the end of the 20 years, they had gained an average 16.8 pounds.
Chips, potatoes and soda were the top three contributors to weight gain, the study found, followed by extra helpings of red meat and processed meats, like sausage or bacon. On the other hand, those who added more vegetables, fruits, grains, nuts and yogurt to their daily diet lost weight.
The findings also shoots some holes in those simplistic adages like "eat less, exercise more," and the thinking that all calories are the same.
"All foods are not equal," Mozaffarian told the Washington Post. Different foods have different effects on the body. The starch in potatoes can cause blood sugar and insulin to rise, which leaves people feeling less satisfied and still hungry. Higher fiber foods, like fruit and vegetables, can make you feel fuller.
Of course, the big question is, why is yogurt so helpful?
Researchers aren't really sure. They think perhaps it's because yogurt adds helpful microbes to the digestive tract.
Or maybe it's simply because people who eat more yogurt also tend to do other healthy things. Like not snack on chips.
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