Though the buzz over low-carb diets has subsided, you can still find low-carb products and claims on grocery store shelves and restaurant menus. But can a low-carb diet help you lose weight safely and permanently? And do these foods fit into a healthy diet?

Donald Hensrud, M.D., a preventive medicine and nutrition specialist at Mayo Clinic, Rochester, Minn., answers these and other common questions regarding carbohydrates, low-carb diets and why you need carbohydrates in your diet.

There's no legal definition for the term "low carb." The Food and Drug Administration (FDA), which regulates health claims on food labels in the United States, hasn't yet defined what low carb means.

Low carb — and similar claims such as "carb wise" or "carb fit" — are actually marketing terms created by manufacturers to sell food products. People may buy low-carb foods believing that they're healthier. But that isn't necessarily the case. For example, you can buy low-carb cakes and cookies, but that doesn't mean these foods, which may be high in fat and calories, are healthy. One low-carb nutrition bar, for example, can have 12 grams of total fat, 6 grams saturated fat and 240 calories.

Low-carb food products can be high in fat and calories, and some can cause digestive symptoms. When food companies make low-carb products — for example, low-carb candy bars — they often replace the carbohydrates with substances such as the sweeteners sorbitol or maltitol, which are sugar alcohols. Sorbitol and maltitol can act as laxatives when consumed in large quantities and may cause diarrhea, cramping or other digestive discomfort.

The FDA calculates total carbohydrates by subtracting grams of protein, fat, water and ash — a scientific term for the nonburnable part of a food that includes minerals such as calcium and phosphates — from the total weight of the food. The resulting number is listed on the food label as "total carbs."

Net carbs — a term not approved or defined by the FDA — is the total number of carbohydrates minus fiber, glycerin and sugar alcohols. Net carbs, like low carb, is a marketing phrase used by proponents of low-carb diets to show a reduced carbohydrate amount on their products. Their theory is that fiber, glycerin and sugar alcohols — which are all forms of carbohydrates — don't raise blood sugar, so they shouldn't be tallied when counting carbs. But in reality, glycerin and sugar alcohols can raise blood sugar to some extent, and these substances do contribute calories.

Historically, nutritionists and dietitians have supported the notion that there's no bad food — everything can be eaten in moderation. But some foods offer no nutritional benefit beyond calories. Sugar is an example. Apart from the calories, there's no nutritional reason to consume sugar, so you could label that a "bad" carb. On the other hand, whole grains — such as whole-wheat pasta, brown rice or oatmeal — provide many vitamins and minerals, fiber, and other substances that promote health. This puts them in the "good" carb category.

But you have to watch how much you eat and keep your portions in check. Too much of any food, including whole grains, provide excess calories. And excess calories from any source leads to weight gain.

The theory is that carbohydrates raise blood sugar levels, which then kicks in insulin. Insulin drives blood sugar into the cells and prevents fat breakdown in the body, which means you won't burn excess fat and lose weight.

Proponents of low-carb diets take this one step further. They say that if carbohydrates raise blood sugar and insulin levels and cause weight gain, a decrease in carbs will result in lower blood sugar and insulin levels, leading to weight loss. And because you're not eating the carbs, your body breaks down fat to provide needed energy. Some people do lose weight on low-carb diets, but the weight loss probably isn't related to blood sugar and insulin levels to any extent. The weight loss is more likely the result of eating fewer total calories, whether they're from carbohydrate, fat or protein.

Four factors contribute to weight loss with low-carb diets:

• Loss of water weight. When you initially decrease your carbohydrate intake, your body burns glycogen. Glycogen contains large amounts of water, so burning glycogen leads to the release of water and increased urination, causing weight loss.
• Decreased appetite. Burning fat without carbohydrates creates byproducts called ketones that build up in your bloodstream. When this happens, you may not feel as hungry.
• Increased feeling of fullness. Low-carb diets are usually higher in fat. And fat takes longer to digest, which makes you feel fuller longer.
• Reduced calories. Most low-carb diets reduce your overall calorie intake because they strictly limit the variety of foods you can eat. Carbohydrates — including bread, pasta, rice, cereals, milk, most fruit and any sweets — usually provide over half of people's daily calories. On a low-carb diet, however, carbohydrates are limited or avoided, thus leading to a significant reduction in calorie intake.

There's nothing special about certain foods or combinations of foods. A calorie is a calorie no matter when or how it's consumed.

No one knows the long-term health effects of low-carb diets. Though a few studies have looked at the benefits and risks, none has been conducted over a long enough period to show whether these diets increase the risk of health conditions that develop over many years, such as heart disease, cancer, and kidney or bone problems.

Theoretically, in order to maintain weight loss if you do lose weight, you need to stay on the program. But a low-carb diet doesn't appear to be easier to maintain than any other diet. Studies comparing low-carb diets and low-fat diets found that after a year, people dropped out of both diets at similar rates. This suggests that the low-carb diet, like so many diets, is no easier to stick to long term.

Yes — but not the way people are doing it in terms of net carbs and total carbs. You need to look at the health effects of food and make wise food choices. Many carbohydrate-containing foods — such as whole grains and fruits — are loaded with essential vitamins, minerals, fiber and other substances that promote health. They form the foundation of a healthy diet, along with other plant-based foods, including vegetables, legumes, nuts and seeds. Variety and portion control are keys to a healthy diet. And excluding or severely limiting one food group — such as carbohydrates or fat — isn't a proven answer to long-term health.