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The Lowdown on Low Carb

It's estimated that 7 to 10% of Americans are now on low-carbohydrate diets, but that in recent years more than half of those who have tried such diets have given up. Reality is setting in, perhaps. Only recently have some well-designed studies begun to show how these diets work and how they don’t. Yes, people do tend to lose more weight, and lose it faster, on low-carb diets than on conventional low-fat diets—in the short term at least. But low-carb diets don’t live up to all the hype. And their long-term safety remains the big unanswered question.

There are many types of low-carb diets, which replace high-carb foods (grains, fruits, vegetables, and sweets) with high-protein and/or high-fat foods. Atkins-type diets are the most extreme, forbidding or severely limiting carb-rich foods. Diets like the Zone and South Beach are more moderate, allowing healthy (high-fiber) carbs and lean protein. Many people make up their own low-carb diets, choosing from the endless array of foods now labeled "low-carb." This label means little, however, since it has no legal definition (see Wellness Letter, January 2004).

The new studies have used the popular Atkins-type diets, not least because the Atkins Foundation has funded some of them. In five studies, obese people were randomly assigned to low-carb diets (ranging from 25 to 40% of calories from carbs) or low-fat diets (25 to 30% of calories from fat, 55 to 60% from carbs). The low-carb groups lost more weight in six months. But in the two studies that continued longer, the weight loss on the two diets was comparable after a year. In one, the low-carb group did better during the first six months, but regained some weight during the next six, so it ended up a statistical tie. In the other study, the low-fat group caught up with the low-carb group by losing more weight during the second half of the year.

Normally, eating lots of highly saturated animal fat boosts total and LDL ("bad") blood cholesterol. But, to the surprise of many researchers, these studies found that in Atkins-type diets the weight loss usually overrides this effect of the saturated fat. However, some of the studies did find that low-carb diets raise LDL by at least 10% in about one-third of people.

The low-carb groups did better at reducing triglycerides (fats in the blood) and maintaining or slightly raising HDL ("good") cholesterol. HDL often drops on low-fat diets. But the low-fat groups generally did better at reducing LDL.

The studies showed that even when people are allowed to eat as much as they want, those on low-carb diets reduce their calorie intake (usually to 1,200 to 1,800 a day)—the ultimate goal of all diets. That may be because low-carb diets suppress appetite, are satiating, and/or limit food choices so greatly. But the same was true of the low-fat groups in most of the studies. During the first week or two the weight loss on low-carb diets can be especially dramatic, since most of it is water.

More food for thought

• People in these studies lost, on average, 5 to 26 pounds over six months—not a lot, considering that most were obese. And the individual responses varied tremendously. In one study, published in the Annals of Internal Medicine, people on a low-carb diet lost 11 pounds on average after a year, but the range was from a loss of 30 pounds to a gain of 8 pounds. The low-fat group lost 7 pounds on average, but ranged from a loss of 26 pounds to a gain of 11 pounds.

• Many people in the studies—one-quarter to one-half— dropped out, even though they had the support of being in clinical trials. Dropout rates tended to be higher for low-fat diets. That usually means that the dieters weren’t getting the results they wanted, or that the diets were too hard to stick to.

• None of the studies lasted more than a year. When it comes to diets, even a year doesn’t mean much. The hard part is to keep the weight off permanently.

• These studies were done before the boom in low-carb processed foods in supermarkets and restaurants. With so many more "low-carb" options today—from brownies to chips—dieters may have a harder time losing weight.

• Many other studies have shown that high-carb diets (especially those containing lots of vegetables, fruits, and whole grains) can help people lose weight safely and effectively, and also reduce the risk of heart disease, diabetes, and certain cancers.

Bottom line: No one diet is best for everyone. Some people lose more on low-carb diets, others do better with low-fat diets. Some fare best with structured meal plans, like Jenny Craig or Weight Watchers, which focus on portion control. Choose a plan that you can stick to for years and that’s safe over the long haul (the long-term safety of low-carb diets is still unknown—see below). You’re more likely to keep off weight that you lose gradually—a pound or two a week—than weight lost quickly on a crash diet. Exercise improves the chance of permanent weight loss.

And by the way: If you do want to try a low-carb diet, the South Beach Diet is probably the best choice. Even though some of the science behind it is iffy, it does include a wide variety of healthy foods and emphasizes "good" fats and better carbs (once you get past the restrictive first two-week phase). It limits or eliminates most foods high in saturated fat, such as whole milk, bacon, and fatty meats, focusing instead on lean protein sources, low-fat or nonfat dairy products, and healthy fats such as those in olive oil, nuts, and fish. Instead of sugary or refined carbs, it emphasizes high-fiber whole grains.

Low-carb risks

Recent six-month or one-year studies have found few, if any, risks from low-carb diets, other than minor complaints such as constipation, headaches, bad breath, muscle weakness, and nausea. But no one knows what happens after years, or even decades. Here are some concerns:

• Foods promoted on Atkins-type diets—notably red meat, butter, and cream—are high in saturated fat and thus may increase the risk of heart disease (regardless of the effect on cholesterol) and many types of cancer. In contrast, foods restricted on these diets—whole grains, fruits, and vegetables—are known to lower the risk of heart disease, diabetes, hypertension, stroke, cancer, and other chronic disorders. Atkins-type diets also usually lack important vitamins (including folate and vitamin C), minerals (such as calcium, potassium, and magnesium), healthful phytochemicals, and fiber.

• A high intake of animal protein over long periods can in-crease the risk of kidney disorders and possibly bone loss. For instance, a 2003 study of female nurses found that in those with mild kidney problems (about one-quarter of the women) such a diet can accelerate the loss of kidney function. Millions of Americans have mild kidney problems but are unaware of it. At highest risk for kidney disease are those over 65 and those with high blood pressure or diabetes.

Words to the wise: If you go on an Atkins-style diet, you should first be checked by your doctor and then be monitored periodically, particularly for kidney function and LDL cholesterol. Even Atkins’s books recommend a medical checkup and monitoring. Above all, people with medical problems, such as diabetes or high blood pressure, should first consult a doctor.

UC Berkeley Wellness Letter, November 2004