Why Healthy Foods Can Make You Fat

New research concludes that there's no such thing as a food that's healthy for everyone.
New research concludes that there's no such thing as a food that's healthy for everyone.James Baigrie / Getty Images

We all know that certain foods tend to raise blood sugar more than others — constantly eating junk clearly can lead to insulin resistance, Type-2 diabetes, and obesity. Conventional wisdom also tells us that we all react to the same foods in the same way, something that can be measured on a scale called the glycemic index. A new study says that's all wrong, and one person's healthy diet could be another's road to weight gain.

When Israeli researchers fed the same foods to different people, they noted wildly divergent blood-sugar responses. One woman in the study with high blood sugar, for example, was eating lots of tomatoes (low on the glycemic index) prior to the trial and tests showed her blood sugar skyrocketed after eating them. Another patient ate ice cream daily and didn't get near the glucose response that this treat's high glycemic index predicted.

A body's reaction to foods, the researcher found, is dependent upon a host of factors such as body-mass index, gut bacteria, and what else you're eating. Pooling 137 factors, the researchers successfully crafted a personalized meal plan for each participant that did not jack up his or her blood sugar. Shockingly, these diets sometimes included chocolate, ice cream, and other foods that usually send levels through the roof. In other words, when it comes to dieting, one size clearly does not fit all. 

So how can you personalize your diet? Here are some of the rules gleaned from the study.

Fat Improves Your Response to Food
The glycemic index scores foods individually. That's a problem. Aside from having a banana for a snack or frozen yogurt at 9 p.m., most foods you eat are paired with something else. Think veggies and hummus, Greek yogurt and blueberries, or a complete dinner. A meal's fat-to-carbohydrate ratio makes a big difference in glycemic response. "As this ratio increases, such as when bread is spread with butter, we usually see a beneficial effect on [the response]," says lead researcher Eran Segal. It still varies from person to person, but generally, tortilla chips with guacamole should elicit a lower glycemic response than the same chips dry. 

BMI and Age Matter
If you're a svelte 155 pounds, you'll likely have a lower glycemic response to a pizza than will the chunky buddy you're sharing it with. If you and your dad crack into a six-pack, you may have two very different blood-sugar responses to the same beer. This study linked age, high BMI, and other risk factors for obesity, diabetes, and early death to higher overall glycemic responses.

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Gut Bacteria Plays a Roll
The health and diversity of your gut microbiome is dictated by what you eat. In turn, these bacteria influence how you process food. And since everyone's microbiome is unique, two people might have a different response to the same food. But it gets even trickier. If you make some big dietary changes, your own microbiome will also change, says Segal, as will your response to a variety of foods.

Don't Toss out the Glycemic Index Entirely Yet
The GI is limited, but it can still be a useful tool to guide food choices if you're worried about blood sugar. Despite variations, most people will respond to a given food in a similar way, says Segal. Chances are, your actual glycemic response to that food will be closer to its GI than not.

Obesity, Diabetes, and Diet
"For years we've thought people develop obesity and diabetes because they're not following our dietary advice," Segal says. "But based on our study, it's possible people are in fact compliant but our advice is inappropriate." A prime example is the tomato lady. She thought tomatoes were safe because they don't raise most people's blood sugar, but for her they were bad news. "If a diet didn't work for you, it may be the diet's fault, not yours," says Segal.

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