“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

Many well-known diets, including the Zone diet and the South Beach diet, focus on cutting and counting carbs to varying degrees. The most famous is the Atkins diet, which starts with an induction phase, a very-low-carb diet of fewer than 20 grams daily, and ramps up the carb allotment later in the diet. Other low-carb diets are less strict. The Zone diet, also known as the 40-30-30 diet, is a calorically restricted diet that recommends that 40% of calories come from carbs, 30% from protein and 30% from healthful fats (ones from plants and fish). The South Beach diet more closely resembles the Atkins regimen but does not restrict carbohydrates as much in the early phase.

Whether you’re ready for a whole new way of eating or just want to cut back on carbs, here are some ways to do so:

Substitute sugar-free beverages for sugary soft drinks, sports drinks and juice.

Look for low-carb and sugar-free products in stores. Low-carb tortillas, bread, pasta and ice cream are in many grocery stores.

Instead of a starchy vegetable, such as potatoes, corn or rice, serve two green vegetables and a nonstarchy soup or salad.

Skip the bread basket at restaurants.

Have olives or cheese on high-fiber wafers as an appetizer.

Boost your intake of most green vegetables, nuts and berries.

At lunch, order an entree salad instead of a sandwich. Ask for your burger bunless, served on top of extra lettuce and tomato, with cheese.

Order your burrito naked and your tostada without the tortilla but with guacamole.

Add portions of fish, poultry, cheese, meat and eggs to your diet: These are virtually carb-free. Add peanut butter (the kind without added sugar), which is relatively low in carbs.

Get a low-carb cookbook or search for low-carb recipes online.

What the nutrition experts say

Dr. Frank Hu, professor of nutrition and epidemiology, Harvard School of Public Health:

“Almost everyone could improve his or her health by cutting back and paying more attention to carbs. Reduce refined carbs in the diet and replace them with lean protein, whole grains, fruits, vegetables, legumes and fats from vegetable sources. Reduce the overall amount of carbs from 55% of calories to below 40%, and make as many of those good carbs as you can.”

Dr. Ronald Krauss, senior scientist at Children’s Hospital Oakland Research Institute and founder and past chair of the American Heart Assn. Council on Nutrition, Physical Activity and Metabolism:

“Avoid white starches, sugars and trans fat; look for whole kernel (not just whole wheat) grain products; load up on vegetables, limit red meats (especially processed); and don’t agonize about saturated fat. Even better, burn up calories by getting plenty of exercise; then you won’t have to worry as much about choosing between fats and carbs.”

Dr. Stephen Phinney, nutritional biochemist and emeritus professor of medicine at UC Davis:

“A person’s carbohydrate intake should match his tolerance. In my case, since I am carbohydrate intolerant, I eat less than 50 grams of net carbs a day from vegetables, berries and fermented dairy, including sour yogurt, cheese and buttermilk. I’d rather eat a diet higher in fat, rich in protein and lower in carbs than take two drugs a day with side effects, which I used to have to do to control my blood pressure.”

Dr. Edward Saltzman, associate professor of nutrition and medicine at Tufts University:

“A very-low-carb diet is likely healthier for the long term, but it’s difficult to consume given the food environment in which we live. I’ve never recommended a very-low-carbohydrate diet, one under 20 grams a day, for my patients, though I have suggested patients stay between 100 to 120 grams. You can eat a lot of vegetables, lean meat and some dairy and have a healthy diet not high in carbs.”

Joanne Slavin, professor of nutrition at the University of Minnesota, member of the 2010 Dietary Guidelines Advisory Committee:

“Americans have to eat fewer calories. But I see no value in making a hit list for carbs. There are many healthy eating patterns, and potatoes, pasta, white bread and rice surely fit into many of these.”

Dr. Eric Westman, director of the Lifestyle Medicine Clinic at Duke University Medical Center:

“If we were to design a one-size-fits-all diet, it should probably be a low-carb diet. We should go back to the days of hunter-gatherers. The secret to maintaining a low-carb diet is to increase fat intake, but only natural fats, not man-made fats. I can keep patients on a low-carb diet forever if they can have cream, butter and bacon.”

Dr. Walter Willet, chairman of the department of nutrition, Harvard School of Public Health:

“Eating moderate carbohydrates can be healthy if they’re comprised of high fiber and whole grains. Personally, I avoid refined starches and sugars, and limit my carbohydrates to what I get from vegetables and whole grains. If I only eat healthy carbs, I feel so full, I really can’t consume more than 40% of my calories from carbs per day, so I tend to stay well under that.”

health@latimes.com