Low-carbohydrate diets may be appealing to patients because of their simplicity and lack of calorie restriction in most. Physicians should be familiar with the popular names of LCDs and their key features, because patients often refer to their LCD by its popular name. Physicians seeking to advise patients can conclude that short-term (eg, 6 months) adherence to an LCD with high-quality protein may be associated with weight reduction, but the amount of weight loss is small and of questionable clinical significance. These diets may lead to improved blood pressure reduction and glycemic control and are not inferior to comparators, such as LFDs and calorie-restricted diets. Low-carbohydrate diets were often favorable over comparators in the reduction of triglyceride levels, but they were not consistently favorable in their effects on HDL, LDL, or total cholesterol levels (
Table 2).
Physicians must keep in mind that the literature is surprisingly limited, considering the popularity of these diets and the claims of health benefits in the public press. The variable definition of LCDs and VLCKDs makes it difficult to interpret results, and studies do not consistently address the source or quality of the protein and fat that are consumed in LCDs. To the authors’ knowledge, no long-term epidemiologic studies have evaluated primary cardiovascular outcomes or all-cause mortality with LCDs as there are for other diets, such as the vegetarian diet.
8,29-31 One main concern is that certain meats have been implicated in worsened all-cause mortality
8-10 and increased cancer risk,
11 and strict LCDs may be difficult to follow without intake of substantial amounts of meat. Our review found no safety issues identified in the current literature, but patients considering LCDs should be advised of the paucity of data on long-term safety and efficacy.
The RCTs assessing LCDs are small and of short duration, making it difficult to draw conclusions about overall health effects. Other limitations include several sources of heterogeneity, such as diversity of participants (nondiabetic, prediabetic, and diabetic participants; normal weight, overweight, and obese participants) and variety of interventions (macronutrient content, calorie restriction, LCD, VLCKD, counselling, medication). The studies did not consistently address the source or quality of the protein and fat, nor did they address the type of weight lost (ie, muscle, water, fat). Furthermore, the phase of evaluation (eg, induction, maintenance) in LCD studies convolutes the evidence. Another limitation common to most nutrition research is the reliance on dietary recall, which is highly susceptible to error.
Low-carbohydrate diets seem to have short-term efficacy in weight loss without negatively affecting blood pressure, glucose, and cholesterol compared with other diets. Conclusions about long-term efficacy and safety cannot be made, however.