New Studies on Low Carbohydrate Diets

Efficacy and safety of low-carbohydrate diets: a systematic review 

ABSTRACT

Bravata DM, Sanders L, Huang J, Krumholz HM, Olkin I, Gardner CD, Bravata DM.

Center for Primary Care and Outcomes Research, Stanfod University School of Medicine, Stanford, CA 94305-6109, USA. bravata@healthpolicy.stanford.edu

Journal of the American Medical Association, Apr 2003; 289(14):1837-50.

Low-carbohydrate diets have been popularized without detailed evidence of their efficacy or safety. The literature has no clear consensus as to what amount of carbohydrates per day constitutes a low-carbohydrate diet. 

Objective: To evaluate changes in weight, serum lipids, fasting serum glucose, and fasting serum insulin levels, and blood pressure among adults using low-carbohydrate diets in the outpatient setting. 

Data sources: We performed MEDLINE and bibliographic searches for English-language studies published between January 1, 1966, and February 15, 2003, with key words such as low carbohydrate, ketogenic, and diet. 

Study selection: We included articles describing adult, outpatient recipients of low-carbohydrate diets of 4 days or more in duration and 500 kcal/d or more, and which reported both carbohydrate content and total calories consumed. Literature searches identified 2609 potentially relevant articles of low-carbohydrate diets. We included 107 articles describing 94 dietary interventions reporting data for 3268 participants; 663 participants received diets of 60 g/d or less of carbohydrates - of whom only 71 received 20 g/d or less of carbohydrates. Study variables (e.g. number of participants, design of dietary evaluation), participant variables (e.g. age, sex, baseline weight, fasting serum glucose level), diet variables (e.g. carbohydrate content, caloric content, duration) were abstracted from each study. 

Data extraction: Two authors independently reviewed articles meeting inclusion criteria and abstracted data onto pretested abstraction forms. 

Data synthesis: The included studies were highly heterogeneous with respect to design, carbohydrate content (range, 0-901 g/d), total caloric content (range, 525-4629 kcal/d), diet duration (range, 4-365 days), and participant characteristics (e.g. baseline weight range, 57-217 kg). No study evaluated diets of 60 g/d or less of carbohydrates in participants with a mean age older than 53.1 years. Only 5 studies (nonrandomized and no comparison groups) evaluated these diets for more than 90 days. Among obese patients, weight loss was associated with longer diet duration (P = 0.002), restriction of calorie intake (P = 0.3), but not with reduced carbohydrate content (P = 0.90). Low-carbohydrate diets had no significant adverse effect on serum lipid, fasting serum glucose, and fasting serum insulin levels, or blood pressure. 

Conclusions: There is insufficient evidence to make recommendations for or against the use of low-carbohydrate diets, particularly among participants older than age 50 years, for use longer than 90 days, or for diets of 20 g/d or less of carbohydrates. Among the published studies, participant weight loss while using low-carbohydrate diets was principally associated with decreased caloric intake and increased diet duration but not with reduced carbohydrate content. 

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June 23, 2003

"Nutrition news is important. We help you understand it!"

Today's Topic: Low-Carbohydrate Diet and Weight Loss

Review of "A Low-Carbohydrate as Compared with a Low-Fat Diet in Severe Obesity"

There is continuing controversy about the utility and safety of diets for weight loss that severely restrict carbohydrates. Two studies appeared in the same edition of the New England Journal of Medicine (NEJM). The first compared a low-fat diet with a low-carbohydrate diet in 132 subjects for six months. 

The subjects were severely obese with an average BMI of almost 43. More than 80 percent had diabetes or the prediabetic condition called metabolic syndrome. Those on the low-carbohydrate diet, restricted to 30 grams or less per day, lost significantly more weight (5.9 kg or 13 lbs) than people on the low-fat diet (1.9 kg or 4 lbs). In addition, serum triglycerides and insulin sensitivity improved more with the low-carbohydrate diet. The study was published in the May 22, 2003 NEJM. 

HERE'S WHAT YOU NEED TO KNOW: The authors pointed out that 40 percent of the participants dropped out of the study, weight loss differences between the two groups were not great, and the study was not long enough to reach a conclusion about effectiveness. Although this diet works in the short term, it is difficult to follow (but what diet is easy?) and it does not provide adequate nutrients without supplementation. An accompanying editorial pointed out that both studies suffered from high dropout rates, the weight loss was relatively small, and in the study we describe tomorrow, there was no difference at one year. 

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NUTRITION NEWS FOCUS

June 24, 2003

"Nutrition News is important. We help you understand it." 

Review of "A Randomized Trial of Low-Carbohydrate Diet for Obesity"

The low-carbohydrate diet popularized by Dr. Robert Atkins was tested for one year in 63 obese adults. It was compared with a low-fat diet and professional contact was minimized, according to the authors, to mimic how the average person approaches weight loss diets.

The subjects had an average BMI of 34 and half were told to follow one of the two diets. At 3 and 6 months, those on the low-carbohydrate diet weighed significantly less with the greatest loss being 7 kg (15 lbs) versus 3.2 kg (7 lbs). However, at 12 months both groups had regained about half the lost weight and there was no longer any statistical difference between the two groups. The study was published in the May 22, 2003 New England Journal of Medicine.

Here's what you need to know: A major failing of this study was that almost no advice on how to restrict calories was given to the low-fat diet group, but it was fairly easy for those restricting carbohydrates to identify target foods. In this study, serum total and LDL cholesterol dropped in those on the low-fat group but not in the low-carbohydrate group. However, HDL cholesterol increased in that former group. By the end of the 5 months, only 20 percent were in ketosis, indicating most who remained in the study were no longer restricting carbohydrates to a large extent. Also, 40 percent of the subjects dropped out. This study shows relatively short-term benefit of a low-carbohydrate diet on weight loss with no benefit on insulin. Mixed results that cannot recommend or advise against such a diet.

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