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 dietsin 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
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.
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:
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.
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' 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
Published 12 times a year, The University of California, Berkeley Wellness Letter is the only health letter to be top-rated by Money magazine and U.S. News & World Report as well as the Baltimore Sun and the Washington Post. To subscribe to the printed version click here.