The Surprising Reason Healthy Food Can Make You Gain Weight

There’s a reason why you can follow the same diet as your friend and still gain weight while she doesn’t: Healthy foods impact everyone differently.

That’s the major finding from new research published in the journal Cell.

For the study, scientists tracked the diets and blood sugar levels of 800 people for a week. The participants answered health questionnaires and underwent body measurements, blood tests, and glucose monitoring, among other tests, and used a mobile app to report their lifestyle and food intake during the study. Scientists then provided feedback to let participants know what they had found about their body’s response to what they ate. 

Researchers found that a person’s age and body mass index (BMI) were associated with their blood sugar levels after they ate. But they also discovered that people had different blood sugar responses from other study participants after eating the exact same food — and some worked against them.

One study participant, for example, learned that one of her healthy eating habits could have been contributing to her obesity: Her blood sugar levels spiked after eating tomatoes, which she did regularly.

Blood sugar spikes are problematic because they can leave you feeling famished when they inevitably plummet, registered dietitian-nutritionist Karen Ansel, co-author of The Calendar Diet: A Month by Month Guide to Losing Weight While Living Your Life, tells Yahoo Health.

“If this happens once in a while it’s no big deal, but if you’re eating foods day in and day out that send your blood sugar on a roller coaster ride, it can wreak havoc on your appetite and eventually your weight,” she says.

But blood sugar spikes can do more than impact your waistline — they can also cause serious health repercussions, registered dietitian nutritionist Beth Warren, author of Living a Real Life with Real Food tells Yahoo Health. “They lead to an increased risk to develop insulin resistance, which can lead to diabetes,” she says.

Researchers say the new study results make the case for a personalized approach to diet — but not everyone can afford to undergo the same testing and monitoring to see how certain foods may impact their waistline.

Luckily, experts say there are some healthy foods that are less likely to cause blood sugar spikes (and eventual weight gain). “Usually these foods are rich in slowly digested complex carbohydrates plus fiber, which we can’t digest, so it slows down digestion,” Ansel says. 

However, there are ones to avoid — if you’re eating them on their own. Ansel lists rice, rice cakes, watermelon, white potatoes, and pretzels among them.

If you’re eating healthy but can’t seem to maintain a healthy weight, she recommends trying beans, lentils, peas, whole (but not quick cooking) oats, barley, most vegetables, nuts, low-fat milk, unsweetened yogurt, and even pasta. “The less you cook pasta (or the more al dente it is), the less likely it is to raise blood sugar,” Ansel says.

Warren also recommends pairing foods with a high-glycemic index (like watermelon) with a protein and healthy fat (like half an ounce of walnuts) to stabilize blood sugar and avoid sharp spikes and dips.

While Ansel calls the study “thought provoking,” she says its findings may be just one small piece of a person’s weight loss puzzle. “If someone is having trouble losing weight, I’d recommend using a smart phone app designed to track everything they eat,” she says. “By looking at what — and how much — you’re eating, you can fine tune your diet to help you loose those stubborn pounds.”