Carbohydrates and weight loss: what the research shows

These days, carbs seem to be enemy, likely to strike fear into anyone who's dieting for weight loss. Nothing could be further from the truth: Carbohydrates are the body's main source of energy. Sure, there are certain kinds of carbs you're better off avoiding for weight loss and overall health. To understand why, it's helpful to look at what carbohydrates actually are and what they do in the human body.

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To throw it back to the basic science: Carbohydrates are molecules that have carbon, hydrogen, and oxygen atoms. They're one of three macronutrients, the others being protein and fat. 

Carbohydrates fall into three categories: sugars, starches, and fiber. Their role is to provide energy to the body, and they're the body's primary source of it.

When carbohydrates are ingested, the body converts them into glucose, a sugar. The hormone insulin ushers that glucose out of the bloodstream and into cells, where they're used for immediate energy or stored as glycogen in the liver or muscles or fat throughout the body. The body eventually burns that glycogen for energy; when glycogen stores are depleted, the body starts burning fat instead. 

There are two main kinds of carbohydrates. Simple carbs (also known as simple sugars or simple starches) are quickly processed by the body and converted into blood sugar. They include the carbs contained in cakes, cookies, candy, ice cream, and many processed foods. They provide quick energy but are low in nutrients and fiber. 

Complex carbs take more time to digest. Found largely in vegetables and whole grains, they give the body a longer-lasting form of energy. 

When it comes to losing weight, it's more important to create a calorie deficit—to burn more calories than you consume—than to eliminate carbs.

Low-carb diets can result in weight loss—and in some cases, pretty rapid weight loss—but they can be difficult to sustain and can cause some physical side effects. But one potential benefit of low-carb diets is that they can be more satiating than low-fat diets, making you less hungry and potentially consume fewer calories.

When dieting, it's more important to select whole-food sources of carbohydrates, such as whole grains, fruits, and vegetables. Many processed foods contain ingredients that encourage you to consume more calories; that can keep you from losing weight.

Studies have found that low-carb diets can help with weight loss, mostly in the short term. Low-carb diets also may improve health conditions such as type 2 diabetes, high blood pressure, and heart disease (OSU, 2019). Low-carb diets might also help you burn more calories (Ebbeling, 2018). 

But some low-carb, high-fat diets can cause an increase in LDL ("bad") cholesterol levels.

Several variations on low-carbohydrate diets have been popular for decades, including Atkins, paleo, and keto. Each restricts carbs and starchy foods and encourages high-protein foods and healthy fats.  

• Keto—Ketogenic diets restrict carbohydrate intake to between 20 and 50 grams per day and encourage the consumption of dietary fats. The theory is that by restricting carbs and protein, you force your body into a state of ketosis, in which it begins burning body fat for fuel.

• Paleo—The paleo diet attempts to recreate the eating patterns of our hunter-gatherer ancestors. That includes lean meat, fish, eggs, fruits, vegetables, and nuts. Dairy products, cereals, and added sugars are not allowed. 

• Atkins—The Atkins diet emphasizes protein and fats and restricts carbs. It's essentially the grandfather of the now-trendy keto diet. It has four phases, in which you gradually add carbs back to your diet.

In the eyes of science, there isn't one diet plan that's the magic bullet for long-term weight loss. Studies have shown that low-carb diets can help you lose weight early on, but the advantages are usually short-term. Studies have found that at 12 and 24 months, the benefits of a low-carb diet aren't significant compared to other eating plans. (It bears repeating: To lose weight and keep it off, it's important to consume fewer calories than you burn, creating a calorie deficit). 

Because carbohydrates are the body's main source of energy, when you start a low-carb diet, you might feel some initial side effects: including headache, fatigue, weakness, or constipation.

Some low-carb diets restrict healthy food groups like fruit, at least at first. Keep in mind that too much restriction can deprive you of nutrients your body needs, potentially leading to vitamin deficiencies and gastrointestinal issues.

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Current version

February 05, 2020

Written by

Michael Martin

Fact checked by

Tzvi Doron, DO

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About the medical reviewer

Tzvi Doron, DO

Dr. Tzvi Doron is Board Certified in Family Medicine by the American Board of Family Medicine.