“If you eat less fat, you gain less fat.”

For years, the medical community has pushed this message, leading many would-be dieters to adopt a low-fat diet.

If getting lean were as simple as following a low-fat diet, though, then why are so many people still overweight? 

And if low-fat dieting is the path to a lean body, how can people claim that high-fat diets like the ketogenic diet also work? 

Here’s what science says.

What Is a Low-Fat Diet?

A low-fat diet plan is a dieting regimen that restricts dietary fat and prioritizes protein and carbs. In other words, it’s a moderate-to-high-carb, moderate-to-high-protein, low-fat diet.

Generally speaking, low-fat diet foods are any foods that contain less than 3 grams of dietary fat per 100 calories. 

Foods that commonly appear on low-fat diet food lists include vegetables, fruits, whole grain cereals, egg whites, skinless chicken and turkey breast, beans, lentils, peas, seafood, and low-fat dairy.

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Can Following a Low-Fat Diet Prevent Body Fat Gain?

This idea has been kicking around for decades, mostly because of a misunderstanding about how the body processes different macronutrients. “Don’t eat fat to lose fat” is a scrumptious soundbite, but it’s also wrong.

Protein and carbs are rarely converted directly into body fat. Fat is chemically very similar to body fat, however, which is why studies show that your body prefers to store dietary fat as body fat and burn carbs and protein for energy.

This has led some people to believe you can “hack” your metabolism by eating very little dietary fat and gorging on protein and carbs instead. This way, they claim, you can stay lean eating far more calories (and carbs, particularly) than with a balanced diet.

Unfortunately, this scheme doesn’t work. Overeating carbohydrate causes fat gain, just not in the same way as overeating fat. Whereas dietary fat is easily converted into body fat, carbs must undergo an energy-intensive process known as de novo lipogenesis (DNL).

Research shows that DNL rarely occurs under normal dietary conditions—carbohydrate intake has to be sky high (700-to-900+ grams per day for several days) for DNL to result in significant fat gain. And even then, about 15-to-25% of the energy in carbohydrate is lost during the process of converting it into fat, so 100 calories’ worth of carbs might only turn into 75 or 85 calories’ worth of body fat.

Overeating carbs can increase body fat levels in another way, though. As your body burns more carbs for energy, it burns proportionally less of the fat you eat, and thus more dietary fat is stored as body fat. In other words, the more carbs you eat, the more fat you’ll store and the less you burn throughout the day. This is why most studies show that people gain similar amounts of body fat after overeating carbs or fat.

And what about protein? While excess protein is never converted directly into body fat, it can increase the storage of dietary fat (just like carbs).

(And if you’d like specific advice about how much of each macronutrient, how many calories, and which foods you should eat to reach your fitness goals, take the Legion Diet Quiz, and in less than a minute, you’ll know exactly what diet is right for you. Click here to check it out.)

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