Why Some People Can Eat a Lot of Fast Food — And Not Gain Weight

The average fast-food meal packs 1,750 milligrams of sodium — more than the American Heart Association recommends adults eat in an entire day. (Photo: Joho/cultura/Corbis)

Know how annoying it is that some people can eat fast food every day and not gain weight? Scientists may have discovered why.

New research published in the journal Scientific Reports found that adding a lot of salt to a high-fat diet actually prevented weight gain in mice — the exact opposite of the experts’ hypothesis.

For their study, scientists from the University of Iowa fed groups of mice different foods — some that were high fat — with various levels of salt. Surprisingly, mice that ate a high-fat, high-salt diet gained the same amount of weight as mice that were fed regular food. The mice on the high-fat diet with the least added salt gained the most weight.

Related: Related: 5 Teeny Tiny Tweaks That Can Help You Lose Serious Weight

When researchers explored why this might have happened, they discovered that salt had a significant effect on how well the mice were able to digest their food — specifically how well their bodies were able to absorb fat. They theorized that this could explain why people don’t always feel great after eating fast foods that are high in fat and salt.

Of course, plenty of people regularly eat fast food that’s high in salt and fat and gain weight — a lot of it. That’s because sodium doesn’t affect people the same way, says Keri Gans, author of The Small Change Diet. “It’s totally different for each person,” she tells Yahoo Health.

Gans stresses that this new research should not be seen as an excuse to eat more fast food, even if it doesn’t make you gain weight. “We always associate poor health with high weight, but you can definitely be a thin person and still have a lot of health complications that that have nothing to do with your weight,” she says. She lists high cholesterol, high blood pressure, and diabetes among them.

Science backs that up: A recent study published in Archives of Internal Medicine found that people who ate a high-salt diet had a 20 percent higher risk of dying from any cause during the 15-year study, and a significantly elevated risk of dying from a heart attack.

Related: The Scary Impact Salt May Have on Reproductive Health

While we need sodium for our bodies to function properly, Gans says we shouldn’t necessarily seek out it out because we already get plenty of sodium from our diet. Aside from the usual salty suspects, there’s sodium in foods we don’t normally associate with it, such as tomato juice and cottage cheese. 

And, of course, there’s a lot of salt in many fast foods. Gans points out that a quarter-pound burger with cheese from Wendy’s contains 1,220 milligrams of sodium. “That’s half of the amount of sodium you should be consuming for the day,” she says — and that burger contains far less sodium that other fast-food options that may contain bacon, added condiments, or more meat.

So, if you’re planning to have fast food or any other sodium-packed meal, Gans says to just be aware that you’re getting a serious helping of salt along with it. Her advice? Go for low-sodium options for the rest of the day.

Read This Next: 8 Foods That Should Have Warning Labels

Let’s keep in touch! Follow Yahoo Health on Facebook, Twitter, Instagram, and Pinterest. Have a personal health story to share? We want to hear it. Tell us at YHTrueStories@yahoo.com.