While diet fads come and go, the low-carb diet seems to have mainstream staying power. Though it was first introduced in the 1860s, it became popularized by the controversial Atkins diet in the 1970s. And it continues to be popular with the paleo and keto diets. Low-carbohydrate diets are generally promoted for weight loss, among other claims, such as lowering the risk of diabetes and metabolic syndrome, per StatPearls.
Related: 9 Outdated Dieting Trends That Need to Be Retired ASAP, According to Dietitians
So what exactly are carbs, and do we need them?
Carbohydrates are one of the three macronutrients the body needs: carbohydrates, protein and fat. This means they are needed in large amounts and are essential for human health and proper body functioning. In fact, healthy adults should consume 45% to 65% of their daily caloric intake from carbohydrates, according to the 2020-2025 Dietary Guidelines for Americans. Additionally, they must be consumed through diet since the body cannot produce them. So what happens in your body when you avoid carbs, such as with a low-carb diet?
Carbohydrates are made up of sugars, starches and fibers. Sugars primarily offer sweetness. They also function to improve how food tastes, its texture and color, such as helping ice cream stay smooth and soft or creating a browning effect for baked goods. Sugar also helps to preserve foods, such as with jams and jellies. Starches are made up of many sugar molecules linked together. Starches are digested or broken down in the body into units of the sugar molecule glucose, which can be absorbed and utilized for energy immediately or stored for later as glycogen. Fiber is another type of carb which is not digestible but can be fermented in the colon or pass through the digestive system, absorbing water and adding bulk to stool. Carbohydrates combined with protein and fat in a meal or snack help you feel full and stay fuller longer.
The primary function of carbohydrates in the body is to provide quick energy, particularly for muscles and the brain. Our brains use the largest proportion of glucose, accounting for about 20% of the body's energy needs at any time to function (thinking, memory and learning) and for maintenance, per the National Library of Medicine. In addition to being an energy source, carbohydrates play a role in managing blood glucose and insulin, regulating cholesterol and promoting healthy bacteria growth in the gut. Carbohydrates can also help with regular bowel movements and make stool easier to pass, per StatPearls. Lastly, many carbohydrates, which primarily come from plant foods, also contain beneficial vitamins, minerals, phytonutrients and water.
Carbohydrates are mainly found in plant foods and dairy. Food sources for starches include fruits, vegetables and grains, such as apples, broccoli, lentils, spinach, unrefined whole grains, brown rice, potatoes, chickpeas, pasta and wheat. Food sources of fiber include brans, seeds, vegetables, brown rice, potato skins, oatmeal and beans. Sugar is also found naturally in fruit, vegetables, milk, yogurt and cheese, and as added sugar in candy, sugary drinks and processed foods.
Related: 10 Carbs You Should Eat Every Week, According to a Dietitian
Since carbohydrates are an essential macronutrient, it is generally not recommended that healthy adults completely cut out carbs (unless they are on a medically supervised diet). So what happens if you avoid carbohydrates?
As carbohydrates provide most of the fuel for daily activity and functioning, you'll probably have less energy if you stop eating carbohydrates. You may feel more tired and hungry and find concentrating more difficult. You may feel incredibly sluggish and find it difficult to perform athletically if you're used to being active and exercising regularly.
Our brains prefer glucose, and carbohydrates supply a steady and easily available source. When you stop eating them, your body is forced to find an alternative energy source for the brain. It will switch to breaking down fat into ketones, an alternative to glucose. This puts the body into ketosis, or a state with elevated ketones, which have side effects such as dizziness, weakness, fatigue and headaches.
Since carbs are rich in starch, fiber and water, you may find eating the recommended amounts of fiber more challenging when you stop eating carbs. As a reminder, fiber helps add bulk to stool and absorb water in the colon, making stool easier to pass, so cutting carbohydrates may make going to the bathroom a more unpleasant experience. In addition, you may experience changes in digestion or experience other digestive issues when you stop eating carbs, as fiber promotes the growth of healthy bacteria in the gut.
Related: 8 Foods to Help You Poop
Some population studies have linked low-carb diets to increased mortality. Other research, such as a 2022 study published in Current Developments in Nutrition, suggests that low-carbohydrate diets may increase LDL ("bad cholesterol") in some people. Other complications that have been linked to long-term carbohydrate-restricted diets include kidney damage, osteoporosis and impaired physical activity. However, many of the research findings are contradictory, and more rigorous studies are needed to determine the long-term safety of cutting carbohydrates from your diet.
Avoiding an entire food group is an extremely restrictive eating pattern that is difficult to sustain long-term. In addition, research shows that most people with restrictive eating patterns tend to eat fewer foods with phytochemicals, which offer cancer-protective factors. Furthermore, according to a 2020 review published in Nutrition Reviews, the quality of carb foods in your diet is more important than their quantity—essentially, choosing whole grains, fruits and vegetables over sugary drinks, candies and baked goods.
Related: How Disordered Eating Can Impact Brain Health, According to Science
Carbohydrates are essential to human health and functioning. Unless you have a medical reason to limit your carb intake, such as epilepsy or type 2 diabetes, avoiding carbs may have more downsides than health benefits.
Related: 7 Things You Shouldn't Do on a Low-Carb Diet, According to Dietitians
