Low-Carbohydrate Diets: An Update
Chester J. Zelasko, Ph.D. | June 1, 2004

The media are ablaze again with the publication of two studies on the effects of a low-carbohydrate diet. Scientists and physicians are again arguing about the benefits or hazards of this approach to losing weight. The problem seems to be that the pro-low-carb group is viewing the new studies as justification for this dietary approach, while the anti-low-carb group criticizes the research. This Newsletter will review the papers and give you a perspective that seems to be missing in the media: common sense.

Study 1: Low-Carb Versus Low-Fat
In the first study, 120 subjects were divided randomly into two groups. One group ate a low-fat diet--less than 30% of their daily caloric intake as fat; the other group followed a low-carbohydrate diet--less than 20 grams of carbohydrate per day. Both groups followed the assigned diet until they were halfway to their weight goal. Subjects in the low-carb group could eat as much as they wanted of meat, fish, poultry, eggs, and shellfish. Those in the low-fat group were limited to about 1,000 calories less than they needed to sustain their weight. The study lasted 24 weeks. The people on the low-carb diet lost more weight, more fat, and had a better improvement in lowering triglycerides than their low-fat counterparts.

Study 2: Low-Carb Versus Balanced
The second paper was a follow-up of a study that compared a low-carbohydrate diet (less than 30 grams of carbohydrate per day) with a conventional and more balanced diet that reduced fat to less than 30% and reduced calories by 500 calories per day. The original study lasted six months; the follow-up study six months later examined whether people sustained, re-gained, or lost more weight.

While not statistically significant, the low-carb group lost three more pounds over the year than the low-fat group: eleven pounds versus eight pounds. But look at the results in the time frame of the study: during the second six months, the low-fat group continued to lose weight, while the subjects in the low-carb group lost twelve pounds in the first six-month study and gained a pound back during the second six months. Although the initial weight loss on the low-fat diet was only four pounds in the first six months, the subjects lost another four pounds in the second six months. Blood lipids--cholesterol, triglycerides, etc.--were not significantly different between the two groups.

Who Wins?
The low-carb advocates view these studies as justification for their approach. The healthcare professionals who advocate a balanced or low-fat approach say the studies didn't last long enough to prove the safety of the low-carb approach. Here's what was reported in the journals but didn't get reported in the headlines:

  • Both studies reported significant loss of subjects. Whether low-carb or low-fat, almost half the subjects quit. Although a few more people completed the low-carb diet, it means that neither diet was satisfactory for the participants to sustain for a long time. That's problematic for both approaches to weight loss.
  • With the exception of triglyceride levels, neither diet did a good job of clinically improving serum lipids although that seemed to get a lot of attention in the press. Reporting reductions (or gains) in percentages always seems better than looking at the actual numbers. Part of the reason may be that cholesterol levels, on average, were not that high to begin with. What would have been interesting is to see how individual results improved for people with total cholesterol higher than 250 mg/dl over the course of the study.
  • There were significant side effects to the low-carb approach versus the low-fat diet. Constipation, bad breath, muscle cramps, diarrhea, and weakness all occurred at much higher rates in the low-carb dietary groups. The negative side effects may explain the lack of adherence to the low-carb approach.

Who's right? The second study seems to indicate that a low-carb diet is better for losing weight fast but less effective for keeping it off, while a low-fat diet is slower but leads to a more lasting weight-loss effect. I'm sure more studies are underway to confirm or refute those conclusions. But neither approach seems to be satisfactory when one looks at the benefits versus the ability to sustain the diet: a weight loss of eight or eleven pounds a year isn't enough to satisfy most dieters. It's an oversimplification to say that weight loss is simply eating less and exercising more, and yet until a way is found that will help people do exactly that--permanently--no program can be deemed successful.

The key to weight loss and better health is a change in lifestyle. As such, whichever way a person chooses to eat, it must be something that is palatable, provides enough energy to sustain activities, has few side effects, and still helps the individual lose weight and sustain it. Low-fat, low-carb, or balanced: it's up to each individual to find the way that permanently fits into his or her lifestyle to achieve the desired results.


  1. Yancy WS, et al. A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia: a randomized, controlled trial. Ann Intern Med. 2004;140(10):769-77.

  2. Stern L, et al. The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: one-year follow-up of a randomized trial. Ann Intern Med. 2004;140(10):778-85.
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