Carbs, low-carbs: A Mayo Clinic specialist cuts through the confusion
By Mayo Clinic staff
With carbohydrate claims covering everything from cereal boxes to restaurant menus, you're likely wondering what the terms mean — net carb, total carb, carb wise, carb fit, just to name a few. Low-carb options are prominent on grocery store shelves, but does that mean these foods fit into a healthy diet? And can a low-carbohydrate diet help you lose weight safely and permanently?
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.
Many food products claim to be low carb. What does this mean and are these foods healthier?
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, but they're working on it.
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-carbohydrate 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.
Is there a downside to eating many low-carb food products?
Low-carb food products have only been on the market a short time, so it's too soon to tell what the potential pitfalls may be. We do know, however, that these 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-carbohydrate candy bars — they often replace the carbohydrates with substances such as the sweeteners sorbitol or maltitol. Sorbitol and maltitol can act as laxatives when consumed in large quantities and may cause diarrhea, cramping or other digestive discomfort.
What's the difference between the terms total carbohydrates and net carbohydrates?
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 carbohydrates."
Net carbohydrates — 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, and these substances do contribute calories.
Are there "good" carbs and "bad" carbs?
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, may provide excess calories. And excess calories from any source leads to weight gain.
What's the theory behind low-carbohydrate diets?
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-carbohydrate 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. The weight loss is more likely the result of eating fewer total calories, whether they're from carbohydrate, fat or protein.
So why do low-carbohydrate diets work?
Three factors contribute to weight loss with low-carbohydrate diets:
Loss of water weight.
The initial weight loss from low-carb diets is water weight. By eating fewer carbohydrates, your body burns its stored carbohydrates (glycogen) and fat for energy. When your body burns glycogen, water is released, and you lose weight.
Burning fat without carbohydrates creates byproducts called ketones that build up in your bloodstream (ketosis). When they're in a state of ketosis, many people find they have a decreased appetite or less drive to eat. But prolonged ketosis may deplete mineral stores in the bones, causing them to become porous and brittle.
Most low-carbohydrate diets reduce your overall calorie intake because the diet limits a whole group of foods. And when you consume fewer calories than you need, you lose weight.
If you eat certain combinations of foods — for example high-fat, high-protein foods with carbohydrates — will you lose weight faster?
There's nothing special about certain foods or combinations of foods. A calorie is a calorie no matter when or how it's consumed.
What are the long-term health risks of low-carb diets?
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.
Do you have to stay on this diet throughout your entire life?
Theoretically, in order to maintain weight loss if you do lose weight, you need stay on the program. But a low-carb diet doesn't appear to be easier to maintain than any other diet. A study published in the May 2003 issue of the New England Journal of Medicine found that after one year, four in 10 people on one low-carb diet — the Atkins diet — dropped out, and four in 10 people on the traditional calorie-control diet dropped out. This may suggest that the low-carbohydrate diet, like so many diets, is no easier to stick to long term.
What's the difference between glycemic index and glycemic load?
Glycemic index is a measure of the degree to which a specific food — enough to total 50 grams of carbohydrates — raises your blood sugar. Potatoes raise blood sugar higher and faster than apples, for example. So potatoes earn a high-glycemic-index rating and apples get a low-glycemic-index rating. But glycemic index doesn't account for the amount of food you typically eat in a serving.
Glycemic load is a measure of how much a typical serving size of a particular food raises blood sugar. For example, the glycemic index for carrots is pretty high. But the amount of carbohydrates in a serving size of carrots — about a 1/2 cup — is low. So carrots have a high glycemic index but a low glycemic load.
Comparison of glycemic index and glycemic load of certain foods
Food Glycemic index Glycemic load
Apple 40 6
Baked potato 85 26
Brown rice 50 16
Carrots 92 5
Corn flakes 92 24
Orange juice 50 13
Plain bagel 72 25
Potato chips 54 11
Pound cake 54 15
Table sugar (sucrose) 58 6
Adapted from International Table of Glycemic Index, 2002
Ranges for glycemic index (GI) and glycemic load (GL)
High 70 or more 20 or more
Medium 56 to 69 11 to 19
Low 55 or less 10 or less
Can the glycemic index help you lose weight?
The theory behind low-glycemic diets is similar to that behind low-carb diets: high-glycemic-index foods raise blood sugar and insulin levels and cause weight gain. So if you eat low-glycemic-index foods, you'll lower your blood sugar and insulin levels and you'll lose weight. Though some people do lose weight on these diets, this theory hasn't been scientifically proved.
You may find potential problems with a diet that emphasizes eating only foods with a low glycemic index. Many factors play a role in how much your blood sugar rises, including your age and weight. And people typically don't eat single foods, but instead eat a combination of foods as part of a meal that affect blood sugar differently. Also, how much you eat of a certain food and how that food is prepared has varying affects on blood sugar levels.
If you're at a healthy weight, do you need to distinguish between various types and amounts of carbohydrates?
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.