Low-carbohydrate Diet - Practices and Theories

Practices and Theories

Today, the term "low-carbohydrate diet" is most strongly associated with the Atkins Diet and other diets that share similar principles. The American Academy of Family Physicians defines low-carbohydrate diets as diets that restrict carbohydrate intake to 20 to 60 grams per day, typically less than 20 percent of caloric intake. Some low-carbohydrate diets may exceed one or more of these definitions, notably the maintenance phase of the Atkins Diet. There is no consensus definition of what precisely constitutes a low-carbohydrate diet. Medical researchers and diet advocates may define different levels of carbohydrate intake when specifying low-carbohydrate diets. For the purposes of this discussion, this article focuses on diets that reduce (nutritive) carbohydrate intake sufficiently to significantly reduce insulin production and to encourage ketosis (production of ketones to be used as energy in place of glucose).

The body of research underpinning low-carbohydrate diets has grown significantly in the decades of the 1990s and 2000s. Most of this research centers on the relationship between carbohydrate intake and blood sugar levels (i.e. blood glucose), as well as some related hormone levels. Some evidence suggests blood sugar levels in the human body should be maintained in a fairly narrow range to maintain good health. The two primary hormones that regulate blood sugar levels are insulin, which lowers blood sugar levels, and glucagon, which raises blood sugar levels. These are both produced in the pancreas: insulin from beta cells and glucagon from alpha cells.

In western diets (and many others), most meals are sufficiently high in nutritive carbohydrates to evoke insulin secretion. The primary control for this insulin secretion is glucose in the blood stream, typically from digested carbohydrate. Insulin also controls ketosis; in the non-ketotic state, the human body stores dietary fat in fat cells (i.e., adipose tissue) and preferentially uses glucose as cellular fuel. Diets low in nutritive carbohydrates introduce less glucose into the blood stream and thus evoke less insulin secretion, which leads to longer and more frequent episodes of ketosis. Some research suggests that this causes body fat to be eliminated from the body, although this theory remains controversial, insofar as it refers to excretion of lipids (i.e., fat and oil) and not to fat metabolism during ketosis.

Low-carbohydrate diet advocates in general recommend reducing nutritive carbohydrates (commonly referred to as "net carbs," i.e. grams of total carbohydrates reduced by the non-nutritive carbohydrates) to very low levels. This means sharply reducing consumption of desserts, breads, pastas, potatoes, rice, and other sweet or starchy foods. Some recommend levels less than 20 grams of "net carbs" per day, at least in the early stages of dieting (for comparison, a single slice of white bread typically contains 15 grams of carbohydrate, almost entirely starch). By contrast, the U.S. Institute of Medicine recommends a minimum intake of 130 grams of carbohydrate per day (the FAO and WHO similarly recommend that the majority of dietary energy come from carbohydrates).

Low-carbohydrate diets often differ in the specific amount of carbohydrate intake allowed, whether certain types of foods are preferred, whether occasional exceptions are allowed, etc. Generally they all agree that processed sugar should be eliminated, or at the very least greatly reduced, and similarly generally discourage heavily processed grains (white bread, etc.). Low-carbohydrate diets vary greatly in their recommendations as to the amount of fat allowed in the diet. The Atkins Diet does not limit fat. Others recommend a moderate fat intake.

Although low-carbohydrate diets are most commonly discussed as a weight-loss approach, some experts have proposed using low-carbohydrate diets to mitigate or prevent diseases including diabetes, metabolic disease and epilepsy. Some low-carbohydrate proponents and others argue that the rise in carbohydrate consumption, especially refined carbohydrates, caused the epidemic levels of many diseases in modern society, including metabolic disease and type 2 diabetes.

There is also a category of diets known as low-glycemic-index diets (low-GI diets) or low-glycemic-load diets (low-GL diets), in particular the Low GI Diet by Brand-Miller et al. In reality, low-carbohydrate diets can also be low-GL diets (and vice versa) depending on the carbohydrates in a particular diet. In practice, though, "low-GI"/"low-GL" diets differ from "low-carb" diets in the following ways. First, low-carbohydrate diets treat all nutritive carbohydrates as having the same effect on metabolism, and generally assume that their effect is predictable. Low-GI/low-GL diets are based on the measured change in blood glucose levels in various carbohydrates - these vary markedly in laboratory studies. The differences are due to poorly understood digestive differences between foods. However, as foods influence digestion in complex ways (e.g., both protein and fat delay absorption of glucose from carbohydrates eaten at the same time) it is difficult to even approximate the glycemic effect (e.g., over time or even in total in some cases) of a particular meal.

Another related diet type, the low-insulin-index diet, is similar except that it is based on measurements of direct insulemic responses (i.e., the amount of insulin in the bloodstream) to food rather than glycemic response (the amount of glucose in the bloodstream). Although such diet recommendations mostly involve lowering nutritive carbohydrates, there are some low-carbohydrate foods that are discouraged as well (e.g., beef). Insulin secretion is stimulated (though less strongly) by other dietary intake. Like glycemic index diets, there is difficulty predicting the insulin secretion from any particular meal, due to assorted digestive interactions and so differing effects on insulin release.