Sept. 28, 2023 – The message about weight gain in midlife is probably familiar: Avoid excess sugar, starchy vegetables and refined carbohydrates. At the same time, eat more fruit, whole grains, and green, leafy vegetables rich in fiber. 

A new large study confirms these recommendations, but researchers also calculated, on average, how the quantity and quality of what you eat can affect weight gain over time. 

Eating an extra 100 grams a day of starch from vegetables like corn, green peas, or potatoes was linked to 3.3 pounds more weight gain over 4 years, for example. In contrast, adding 10 grams per day more of fiber was linked to 1.75 pounds less weight gain at the same time. (For comparison, one medium russet potato is about 170 grams, one cup of green peas is 150 grams, and one ear of corn is about 100 grams)

The researchers found that women with excess weight gained more weight, compared to men. 

Not All Vegetables Are Created Equal

The findings are generally consistent with previous research, said study author Yi Wan, PhD, a postdoctoral research fellow at the Harvard T.H. Chan School of Public Health in Boston. But unlike many previous studies, Wan and colleagues make the distinction between unfavorable, starchy vegetables and favorable, non-starchy ones. Finding a larger effect among people with excess weight has not been widely reported either, he said. 

The study was published online Wednesday in the journal BMJ

The researchers also looked at extra sugar, including sugary drinks. Adding 100 grams per day of sugar (about 24 teaspoons, or about three 12-ounce cans of soda per day) was linked to almost 2 pounds more weight gained over 4 years.

“The quality and source of carbohydrates is crucial for long-term weight management, especially for people already with excessive body weight,” Wan said. “Transitioning from low-quality carbohydrate food sources to high-quality sources may support efforts to control body weight.” 

Specifically, shifting away from added sugar, sugar-sweetened beverages, refined grains, and starchy vegetables and toward adding whole grains, fruits, and non-starchy vegetables can help,  he said.

The advantages could go beyond less midlife weight gain, Wan said, “Other studies have shown that this shift would also reduce risks of diabetes, cardiovascular disease, and some cancers.”

Wan and colleagues followed 136,432 men and women who were healthy and younger than 65 when they enrolled in 1986 or 1991 in one of three long-term health studies. Researchers checked in on their health, diet, and wellness every 2 to 4 years over 24 years. 

Overall weight gain was common. On average, people gained 3.3 pounds every 4 years or 19.4 pounds over the 24 years. 

The Advice is Not Anti-Potato 

“People can take away from this research that a diet rich in non-starchy vegetables and whole grains may result in a healthier weight,” said registered dietitian Kristen Smith, a spokesperson for the Academy of Nutrition and Dietetics, who was not involved in the study. “This research supports my current practice with clients who are seeking to maintain a healthier weight.”

The researchers are not recommending people cut out all peas, corn, and potatoes, said registered dietitian Jessica Alvarez, PhD, “but more about making sure to incorporate other kinds of vegetables.” 

This research reinforces the messages health professionals give us, said Alvarez, a spokesperson for the Obesity Society, “and they show it in a very large, well-designed study.”

Some messages about diet and weight management can be oversimplified or misleading, she said. But this study reinforces the “tried and true” advice about eating whole grains and leafy vegetables. “I feel like that needs to be reinforced more often than it is.”

Smith offered a couple of caveats. The research focused on preventing weight gain and not weight loss. Also, the study had an observational design, meaning the links between quantity and quality of the foods and weight gain were not cause-and-effect relationships. Wan and colleagues also noted that people self-reported their diets, which is also a potential limitation. 

Alvarez said research findings like this can help people interested in gaining less weight in midlife adjust their diets. For example, someone may have already reduced excess sugar in their diet, but they continue to eat a lot of starchy vegetables. This gives them a chance to “see what you are eating more or less of and try to tweak it.”