Context 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 (eg,
number of participants, design of dietary evaluation), participant variables
(eg, age, sex, baseline weight, fasting serum glucose level), diet variables
(eg, carbohydrate content, caloric content, duration) were abstracted from
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
(eg, 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 = .002), restriction of calorie
intake (P = .03), but not with reduced carbohydrate
content (P = .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.