Tuesday, January 12, 2010
The most obvious characteristic of a low-carb diet is that it is low in carbohydrates. The original Atkins diet recommends that dieters start its Induction phase with essentially zero grams of carbohydrates. The 2002 version of the Atkins diet allows dieters to do Induction with up to twenty grams of carbohydrates. The Protein Power diet begins its Phase I Intervention stage at thirty grams of carbohydrates. As all of these diets progress, additional carbohydrates are introduced in a controlled manner, but even at maintenance, most low-carbers eat no more than 100 grams of carbohydrate per day.
By contrast, the US Department of Agriculture recommends that both children and adults eat 45-65% of their daily calories as carbohydrates. That can mean well over 300 grams of carbohydrates per day for a person consuming a 2000 calorie diet.
What happens if we ignore the USDA guidelines and don't eat enough carbs every day? Carbohydrates are popularly thought to be essential for providing energy. Specifically they are thought to be necessary to provide fuel for the brain and to refill stores of glycogen in muscles and in the liver.
The American Diabetes Association tells us that the brain and central nervous system normally have a daily requirement of about 130 grams of carbohydrate in the form of glucose. However, after a period of adaptation, most of these tissues are also able to use ketones as an energy source. This reduces the carbohydrate requirement to about 30 grams of glucose per day. As low-carbers with Ketostix already know, ketones are produced in abundance from the fats and amino acids consumed on a low-carb diet. The remaining need for thirty grams of glucose can easily be met through a metabolic pathway called gluconeogenesis, which allows the body to use amino acids from proteins and the glycerol backbones from fats to synthesize glucose in the absence of any carbohydrate intake.
Glycogen, which is a storage form of glucose, can similarly be replenished by the glucose made through gluconeogenesis. As far as the general energy requirements of the body, these can be met very efficiently both by the utilization of dietary fat and by the mobilization of stored fat.
Carbohydrates, therefore, are not an essential element of a healthy diet. There are essential fats, which include the omega-3 and omega-6 fatty acids. Because they are not produced by the body, omega-3 and omega-6 fatty acids must be consumed in order to ensure the normal function of the nervous system, heart and immune system. There are essential amino acids, including isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Although some amino acids can be synthesized by the body, these eight cannot. Unless they are ingested, and ingested in the proper amounts, the body is unable to assemble all of the structural and enzymatic proteins that are needed to sustain life.
By contrast, there is no disease state associated with an insufficient intake of carbohydrates. It is true that the body needs carbohydrates for energy within certain types of tissues, for synthesis of the backbones of DNA and RNA, and for signaling purposes, but it is well able to synthesize all of these from the raw materials provided by the amino acids in the proteins we eat.
For those of us raised on the dogma of eating low-fat and high-carb, this is hard to believe. But if we think about our caveman ancestors, we realize that they didn't have access to pasta, potatoes or rice, or even high-carb fruits and vegetables on a regular basis. They were able to survive and reproduce without a high carbohydrate intake because, amazingly enough, there is no such thing as an essential carbohydrate.