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Switching the types of carbs you eat may help prevent weight gain, large study finds

Avoiding weight gain in middle age doesn’t require a complicated change in diet, a new study suggests.

Researchers in the United States analyzed more than two decades of data from almost 137,000 people and found less weight gain among people who ate more whole grains, fruit and nonstarchy vegetables — such as broccoli, carrots and spinach — and fewer refined grains, starchy vegetables and sugary drinks.

The study, published Wednesday in the BMJ, shows “the quality of the carbohydrates in a person’s diet is much more important than the amount,” said its senior author, Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health. “You want to increase whole grains and limit starchy vegetables.”

Until now, Willetts said, there hasn’t been a study showing the impact of lowering the amount of refined grains, starchy vegetables and sugary drinks on weight gain.

“Is sugar the villain? It should be behind bars,” he said. “But interestingly, there are bigger villains. Overall, starch is a bigger villain.” Starchy foods include white bread and vegetables, such as peas, corn and potatoes.

While this study looked at long-term weight gain, earlier research showed that better quality carbohydrates were associated with a lower risk of heart disease and certain cancers, Willett said.

The study found less weight gain among people who avoided:

  • Foods made from refined grains, especially white flour products.
  • Starchy vegetables.
  • White rice.
  • Sugar-sweetened beverages.

These foods have a high glycemic index, which is a measure of the increase in blood sugar after consumption, Willett said. When a person eats a food like potatoes with a high glycemic index, the starch is easily and quickly absorbed, causing a spike in blood sugar leading to more fat being produced and stored. Many starchy foods are also low in fiber.

Willett advised people to eat more:

  • Whole fruits. Fruit juices should be avoided because the fiber has been removed.
  • Nonstarchy vegetables, including spinach, kale, broccoli and carrots.
  • Whole grains. The way to tell if a food is a whole grain is to check the label. Whole grain foods include whole wheat bread, whole grain pasta and oatmeal, as well as, popcorn, quinoa, barley and farro.

The sugars and starches in these foods take longer to be absorbed by the body, which means they have a much smaller immediate impact on blood sugar and thus lead to less fat being produced and stored, Willett said.

It’s not the carbs, it’s the type of carbs

To take a closer look at carbohydrate quality on weight gain, Willett and his colleagues examined participants enrolled in one of three long-term studies: the Nurses’ Health Study, which has provided major research on women’s health; the Nurses’ Health Study II; and the Health Professionals Follow-Up Study, which was developed to track men’s health.

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At the outset, the participants in each were, on average, in their 40s, 50s or 60s. When they joined the research, they were free of chronic health conditions such as diabetes, cancer or heart disease. Researchers followed them for at least two decades.

The researchers excluded from their analysis participants who were 65 or older at baseline because at that age, weight loss could be due to other factors associated with aging.  

At the beginning and every two to four years, participants filled out questionnaires that asked about medical history, lifestyle and other health-related factors.

They were also asked, every four years, to fill out a form that assessed diet.

Overall, the average weight gain was 3.3 pounds every four years — which over an average of 24 years amounted to 19.4 pounds.

The results showed that:

  • A 3.5 ounce a day increase in starch — the equivalent of about 6 slices of white bread or 2 cups of white rice — was associated with a 3.3 pound weight gain every four years.
  • The same increase in sugar — or two to three cans of sugary drinks — was associated with a 1.9 pound weight gain over the same time period. 

In contrast:

  • One-third of an ounce per day of added fiber — around three-to-four slices of multi-grain bread — was linked to 1.7 pounds less weight gain over four years.
  • Replacing two servings of starchy vegetables with equal servings of whole grains daily meant 4.1 pounds less weight gain over four years.
  • Consuming whole fruits was associated with 4.4 pounds less weight gain over four years.
  • Switching to nonstarchy vegetables was associated with 4 pounds less weight gain over four years. 

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The findings were stronger in women than men and people who were already overweight or obese compared to those who were normal weight.

Why whole foods are the healthiest

Dr. Holly Lofton, director of the medical weight management program at NYU Langone Health, has seen similar results in her patients.

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“I was glad to see this study,” Lofton said. “And I was surprised that no one had done it before.”

Lofton applauded the researchers for the large number of participants and the length of time those participants were followed.

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The findings underscore the value of eating not only whole grains, but also whole fruits and vegetables, she said. When fruits and vegetables are pureed, most of the fiber is lost, meaning smoothies aren’t as healthy as people would like to think. she added.

“Think about it,” said Lofton, who was not associated with the new study. “A fruit is fiber, water, vitamins, minerals and natural sugars. When you turn it into juice, it’s like taking vitamins and minerals and drinking Kool-Aid afterwards.”

She suggests pairing healthy carbohydrates with protein because it helps avoid a rapid spike in insulin, which can cause more weight gain and fat storage.

Dr. Michelle Thompson, medical director of the UPMC Lifestyle Medicine in Pittsburgh, said the study’s overall message is that people should be eating real food instead of processed foods.  

“One of our biggest problems is that our families didn’t teach us this,” said Thompson, who is not associated with the new research. “I grew up in an Italian American family where we ate a lot of pasta and meatballs. This led to obesity in my family and diabetes, hypertension and stroke.”

“The beauty of this study is the number of years it was going on,” she said. 

The study shows how important fiber is, said obesity specialist Dr. Sahar Takkouche, an assistant professor of medicine in the department of diabetes, endocrinology and metabolism at the Vanderbilt University Medical Center.

“People don’t realize that when you remove fiber, it’s easier to break down sugars in the mouth with the digestive enzymes in our saliva,” said Takkouche, who was not involved in the new research. “That allows us to get a punch of glucose much faster and get a dopamine hit.” And that effect is similar to what happens with substances of abuse. 

It’s not clear why the association between high starch diets and weight gain is stronger in women, she said. “The cause is likely related to hormones that may be making us more sensitive,” she said.

Linda Carroll

Linda Carroll is a regular health contributor to NBC News. She is coauthor of “The Concussion Crisis: Anatomy of a Silent Epidemic” and “Out of the Clouds: The Unlikely Horseman and the Unwanted Colt Who Conquered the Sport of Kings.”