Could a low-carb diet give you an edge in losing weight? Help you keep weight off permanently? Here's what you need to know about the low-carb diet. By Mayo Clinic Staff

A low-carb diet limits carbohydrates — such as grains, starchy vegetables and fruit — and emphasizes dietary protein and fat. Many types of low-carb diets exist, each with varying restrictions on the types and amounts of carbohydrates you can eat.

A low-carb diet is generally used to lose weight. Some low-carb diets say that they have health benefits beyond weight loss, such as reducing risk factors associated with heart disease, certain cancers, diabetes and metabolic syndrome.

Why you might follow a low-carb diet
You might choose to follow a low-carb diet because you:

  • Want to change your overall eating habits
  • Enjoy the types and amounts of foods featured in low-carb diets
  • Want a diet that restricts certain carbs to help you lose weight

Check with your doctor or health care provider before starting any weight-loss diet, especially if you have any health conditions, including diabetes.

As the name says, a low-carb diet restricts the amount of carbohydrates you can eat. Carbohydrates are a type of macronutrient found in many foods and beverages. Most carbohydrates occur naturally in plant-based foods, such as grains. Food manufacturers also add carbohydrates to processed foods in the form of starch or added sugar.

Common food sources of naturally occurring carbohydrates include:

  • Fruits
  • Vegetables
  • Milk
  • Nuts
  • Grains
  • Seeds
  • Legumes

Your body uses carbohydrates as its main fuel source. Sugars and starches are broken down into simple sugars during digestion. They're then absorbed into your bloodstream, where they're known as blood sugar (glucose). From there, the glucose enters your body's cells with the help of insulin. Some of this glucose is used by your body for energy, fueling all of your activities, whether it's going for a jog or simply breathing. Extra glucose is stored in your liver, muscles and other cells for later use or is converted to fat.

The theory behind the low-carb diet is that insulin prevents fat breakdown in the body by allowing sugar to be used for energy. Proponents of the low-carb diet believe that decreasing carbs results in lower insulin levels, which causes the body to burn stored fat for energy and ultimately helps you shed excess weight and reduce risk factors for a variety of health conditions.

Typical menu for a low-carb diet
In general, a low-carb diet focuses on proteins, including meat, poultry, fish and eggs, and some nonstarchy vegetables. A low-carb diet generally excludes or limits most grains, beans, fruits, breads, sweets, pastas and starchy vegetables, and sometimes nuts and seeds. Some low-carb diet plans allow certain fruits, vegetables and whole grains. A daily limit of 50 to 150 grams of carbohydrates is typical with a low-carb diet. Some low-carb diets greatly restrict carbs during the initial phase of the diet and then gradually increase the number of allowed carbs.

In contrast, the 2010 Dietary Guidelines for Americans recommends that carbohydrates make up 45 to 65 percent of your total daily calorie intake. So, if you consume 2,000 calories a day, you would need to eat between 900 and 1,300 calories a day from carbohydrates, or between 225 and 325 grams of carbohydrates a day.

Weight loss
Most people can lose weight on almost any diet plan that restricts calories and what you can eat — at least in the short term. Over the long term, though, studies show that it's common to regain the lost weight, regardless of the diet plan you follow. Some studies also have shown that people who continued to follow certain low-carb diet plans for two years lost an average of nearly 9 pounds (4.1 kilograms) overall, which is similar to the amount of weight lost on higher carbohydrate diets. And it may not be just cutting carbs that leads to weight loss. Some studies show that you may shed some weight because you eat less on low-carb diets because the extra protein and fat keep you feeling full longer.

Other health benefits
Some low-carb diets, including the Atkins Diet, say that their eating plans can prevent or improve serious health conditions, such as metabolic syndrome, diabetes, high blood pressure and cardiovascular disease. In fact, almost any diet that helps you shed excess weight can reduce or even reverse risks factors for cardiovascular disease and diabetes. Most weight-loss diets — not just low-carb diets — may improve blood cholesterol or blood sugar levels, at least temporarily. Low-carb diets may improve HDL cholesterol and triglyceride values slightly more than do moderate-carb diets. And it may not only be how many carbs you eat but also the kinds of carbs you eat that are important to your health. Whole grains, legumes, vegetables, fruits and low-fat dairy products, for instance, are generally healthier than are carbs from sweets and processed and refined grains, such as potato chips, white bread and white rice.

If you suddenly and drastically cut carbs, you may experience a variety of temporary health effects, including:

  • Headache
  • Dizziness
  • Weakness
  • Fatigue
  • Constipation

In addition, some diets restrict carbohydrate intake so much that they can result in nutritional deficiencies or insufficient fiber. This can cause such health problems as constipation, diarrhea and nausea. Eating carbs that are high fiber, whole grain and nutrient dense can improve the health profile of some low-carb diet programs. And some low-carb diets, such as Atkins, now recommend taking small amounts of extra salt, along with vitamins or supplements, to help prevent diet side effects.

It's also possible that severely restricting carbohydrates to less than 20 grams a day can result in ketosis. Ketosis occurs when you don't have enough sugar (glucose) for energy, so your body breaks down stored fat, causing ketones to build up in your body. Side effects from ketosis can include nausea, headache, mental fatigue and bad breath.

It's not clear what kind of possible long-term health risks a low-carb diet may pose because most research studies have lasted less than a year. Some health experts believe that if you eat large amounts of fat and protein from animal sources your risk of heart disease or certain cancers may increase.

Oct. 11, 2011