ABSTRACT

The persistence of an epidemic of obesity and type 2 diabetes suggests that new nutritional strategies are needed if the epidemic is to be overcome. A promising nutritional approach suggested by this thematic review is carbohydrate restriction. Recent studies show that, under conditions of carbohydrate restriction, fuel sources shift from glucose and fatty acids to fatty acids and ketones, and that ad libitum–fed carbohydrate-restricted diets lead to appetite reduction, weight loss, and improvement in surrogate markers of cardiovascular disease.

INTRODUCTION

The persistence of an epidemic of obesity and type 2 diabetes suggests that new nutritional strategies are needed if the epidemic is to be overcome. A historical perspective and recent research point to some form of carbohydrate restriction as a likely candidate for a new nutritional approach, and we present a thematic review regarding carbohydrate restriction.

The examination of diets before modernization can remind us of the remarkable ability of humans to adapt to their environment and can provide a context within which to view current diets. In contrast to current Western diets, the traditional diets of many preagricultural peoples were relatively low in carbohydrate (1, 2). In North America, for example, the traditional diet of many First Nations peoples of Canada before European migration comprised fish, meat, wild plants, and berries. The change in lifestyle of several North American aboriginal populations occurred as recently as the late 1800s, and the numerous ensuing health problems were extensively documented (3–5). Whereas many aspects of lifestyle were altered with modernization, these researchers suspected that the health problems came from the change in nutrition—specifically, the introduction of sugar and flour.

In a similar manner, before the discovery of insulin, the removal of high-glycemic carbohydrates such as sugar and flour from the diets of diabetics was found to be a successful method of controlling glycosuria. An analysis of the pattern of food consumption during the more recent obesity and diabetes epidemic found that the increase in calories was almost entirely due to an increase in carbohydrate (6). Given this context, it is reasonable to postulate that diets low in carbohydrate may be as healthy as, or even healthier than, the higher-carbohydrate diets introduced into modern society only recently.

This thematic review summarizes studies involving low-carbohydrate diets (LCDs) published over the 4 y since the last comprehensive reviews of the topic (7, 8). Articles were identified by us through attendance at scientific meetings, reading of publications, reference searching, manuscript reviews, and weekly Medline searches from January 2002 to December 2006 with the use of the terms "diet," "carbohydrate," and "fat."

THEMATIC REVIEW

Definition of low-carbohydrate diet

Much of the controversy in the study of LCDs stems from a lack of a clear definition. The rationale of carbohydrate restriction is that, in response to lower glucose availability, changes in insulin and glucagon concentrations will direct the body away from fat storage and toward fat oxidation. There is a suggestion of a threshold effect, which has led to the clinical recommendation of very low concentrations of carbohydrate (<20–50 g/d) in the early stages of popular diets. This typically leads to the presence of measurable ketones in the urine and has been referred to as a very-low-carbohydrate ketogenic diet (VLCKD) or a low-carbohydrate ketogenic diet (LCKD). Potent metabolic effects are seen with such diets but, beyond the threshold response, there appears to be a continuous response to carbohydrate reduction. The nutritional intake of <200 g carbohydrate/d has been called an LCD, but most experts would not consider that to provide the metabolic changes associated with an LCKD. We suggest that LCD refers to a carbohydrate intake in the range of 50–150 g/d, which is above the level of generation of urinary ketones for most people.

Other macronutrients

Because an instruction only to restrict carbohydrate intake could theoretically create a diet containing any level of daily energy intake from protein and fat, confusion exists among researchers and the lay public about what constitutes an LCD. As early as 1980, LaRosa found that subjects following an LCD do not necessarily replace the carbohydrate with either protein or fat, but that they, rather, reduce starch and sugar intake (9). Under such conditions, even though the absolute amounts of fat and protein do not increase, the percentage of fat and protein will increase. Recent research reviewed below has determined that the reduction in calorie intake is a result of appetite and hunger reduction. In this way, LCDs are also low-calorie diets that include an increase in the percentage of calories from fat and protein but not necessarily an increase in absolute amounts of fat and protein.

General physiologic principles in carbohydrate restriction

A review outlined the way in which a marked reduction in carbohydrate intake leads to a general change in metabolism from a "glucocentric" (glucose) to an "adipocentric" (ketone bodies, fatty acids) metabolism (