A low-carb diet limits carbohydrates — such as those found in grains, starchy vegetables and fruit — and emphasizes foods high in protein and fat. Many types of low-carb diets exist. Each diet has varying restrictions on the types and amounts of carbohydrates you can eat.
As the name says, a low-carb diet restricts the type and amount of carbohydrates you eat. Carbohydrates are a type of calorie-providing macronutrient found in many foods and beverages.
Carbohydrates can be simple or complex. They can further be classified as simple refined (table sugar), simple natural (lactose in milk and fructose in fruit), complex refined (white flour) and complex natural (whole grains or beans).
Common sources of naturally occurring carbohydrates include:
· Legumes (beans, lentils, peas)
Food manufacturers also add refined carbohydrates to processed foods in the form of sugar or white flour. Examples of foods that contain refined carbohydrates are white breads and pasta, cookies, cake, candy, and sugar-sweetened sodas and drinks.
Your body uses carbohydrates as its main fuel source. Complex carbohydrates (starches) are broken down into simple sugars during digestion. They're then absorbed into your bloodstream, where they're known as blood sugar (glucose). In general, natural complex carbohydrates are digested more slowly and they have less effect on blood sugar. Natural complex carbohydrates provide bulk and serve other body functions beyond fuel.
Rising levels of blood sugar trigger the body to release insulin. Insulin helps glucose enter your body's cells. Some glucose is used by your body for energy, fueling all of your activities, whether it's going for a jog or simply breathing. Extra glucose is usually stored in your liver, muscles and other cells for later use or is converted to fat.
The idea behind the low-carb diet is that decreasing carbs lowers insulin levels, which causes the body to burn stored fat for energy and ultimately leads to weight loss.
Since 1860, and more recently, in 1972, low carbohydrate (low-carb) diets have been a strategy for weight loss.
Today, there continues to be an interest in low-carb approaches. While all low carbohydrate approaches reduce the overall intake of carbohydrates, there is no clear consensus on what defines a low-carb diet. There are three macronutrients—carbohydrates (4 kcal/gm), fat (9 kcal/gm), and protein (4 kcal/gm) found in food. Therefore, studies have defined low carbohydrate as a percent of daily macronutrient intake or total daily carbohydrate load. We will define it here as:
Very low-carbohydrate (< 10% carbohydrates) or 20-50 gm/day
Low-carbohydrate (<26% carbohydrates) or less than < 130 gm/day
High-carbohydrate (45% or greater)
For reference, the Institute of Medicine proposes Americans obtain 45%-65% of calories from carbohydrates.
Low-carb approaches stem primarily from the hypothesis that lowering insulin, a critical hormone that produces an anabolic, fat-storing state, improves cardiometabolic function, and induces weight loss. This approach has been recently called the carbohydrate-insulin model. Studies have shown low-carb approaches to be superior to other dietary approaches in producing rapid weight loss for the first 6-12 months. While diets inducing weight-loss produce a caloric deficit, the mechanism of low-carb diets remains in debate. When lowering carbohydrates from the diet, the macronutrient intake of fat and protein generally increases to compensate for the reduction of carbohydrates. One hypothesis of why low-carb approaches produce rapid weight loss compared to other diets is that fats and protein increase satiety and produce less concomitant hypoglycemia. This increase in satiety and less rebound hypoglycemia then reduces hunger and overall food intake and produces a caloric deficit. Additionally, another hypothesis contends that low-carb diets can produce a higher metabolic burn than high-carb diets. In recent studies, there appears to be a metabolic advantage of approximately 200 to 300 more calories burned compared to an iso-caloric high-carb diet. However, these theories remain controversial.
The ketogenic (keto) diet, a specific version of low-carb, deserves mention. Keto diets restrict carbohydrate to induce nutritional ketosis and typically limits carbs to 20-50 gms daily. Restricting carbs to under 50 gms induces glycogen depletion and ketone production from the mobilization of fat stored in adipose tissue. Nutritional ketosis produces ketone bodies (acetoacetate, acetone, and beta-hydroxybutyrate) and can is measurable as serum or urinary ketones. Nutritional ketosis generally increases serum ketones to 1 mmol/L to 7 mmol/L but does not produce metabolic acidosis. Diabetic ketoacidosis, by definition, includes metabolic acidosis, hyperglycemia, and serum ketones (generally over 20 mmol/L).
Despite the debate, it is clear from numerous systematic reviews that low-carb diets are as effective, if not more effective for weight loss compared to other diets. The evidence for benefits and concerns for low-carb will be further delineated below.