"What people can tolerate varies widely based on age, metabolism, activity level, body size and gender," says Dr. Stephen Phinney, nutritional biochemist and an emeritus professor of UC Davis. For healthy adults the number can be higher, he says, while others will feel and function better if they stay between 50 and 100 grams a day. "I've seen some people get in trouble when they eat over 25 grams."
If you're lean and active, you can tolerate a higher carb intake than if you're fat and sedentary, says Dr. Walter Willett, chairman of the department of nutrition at the Harvard School of Public Health. But genetic predisposition, he adds, will also play a role.
Good carb or bad? How to choose wisely
Food scientists divide carbohydrates into two categories: good and bad. A good carb is one that doesn't raise your blood sugar quickly. (Some people call these complex carbs.) Examples are whole grains, brown rice and legumes. Bad, or simple, carbs trigger a fast rise in blood sugar. Some examples are white bread, refined pasta, processed cereals, cookies, candy and sugary sodas.
When evaluating carbs, look at both the fiber content — it should be high — and glycemic index, which should be low, says Dr. Walter Willett of the Harvard School of Public Health.
One rule to use when buying bread (the words "whole grain" on a package can often mislead) is the 6 to 1 rule, he says: Look for a ratio of 6 grams of carbs to 1 gram of fiber to determine whether the product is truly whole grain. An example: If the bread has 24 grams of carbs per serving and 4 grams of fiber, the ratio is 6 to 1 — that's good. If it has 44 grams of carbs and 2 grams of fiber, it's 22 to 1 — not so good.
The glycemic index ranks food on a scale of 1 to 100 based on a measure of how fast blood sugar rises after a food is consumed. Foods with a glycemic index below 55 are considered low glycemic.
As a general rule, the more processed a food, the higher the glycemic levels and the lower the fiber levels. In addition, when flour gets refined, many minerals and vitamins get lost or depleted along with the fiber.
Unfortunately, the vast majority of carbs that Americans eat are the bad kind. In the typical American diet, 55% of calories come from carbohydrates, according to Dr. Frank Hu, professor of nutrition and epidemiology at the Harvard School of Public Health. More specifically, Hu says, the carb breakdown in our daily diet goes like this:
Sugary sodas, sweet beverages and fruit juice: 10% of total calories.
Refined starches, including white bread, cakes, bagels, cookies and muffins: 20% to 25%
Potatoes, white rice, tortillas and refined-grain cereals: 10% to 15%
Healthful sources, including nonstarchy vegetables, whole grains and legumes: 5% to 10%.
'Net' is key when counting
Counting carbs is easier than counting calories, if you know where to look. Start by being aware of what foods are naturally high in carbs. Those include anything with flour and sugar, starchy vegetables (corn, peas and potatoes), rice, pasta, cereals and sweets. You can find carb counters online and tables in low-carb diet books.
On packaged foods, look at labels. Then you'll want to calculate "net carbs," the number that counts. First find the total grams of carbohydrates per serving, then subtract grams of fiber and sugar alcohols. For example, if one serving of canned beans has 18 grams of carbohydrates and 6 grams of fiber, net carbs equal 12 grams.
Why do this? Fiber is a nondigestible carbohydrate, so it's not absorbed by the body. Sugar alcohols, found in certain foods labeled "sugar-free" — including gum, candies, cookies and some sodas — are lower in calories, absorbed only slowly and don't affect blood sugar levels much.
Tips to help cut them from your diet