In later years, George Watson, Roger Williams, William Kelley, and others continued research in this area. They believed that people's metabolisms functioned differently when it came to two factors, which are largely determined by heredity:
- Autonomic nervous system dominance. There are two branches of this system. One of these branches, the sympathetic nervous system, is often referred to as the "fight or flight" branch. It helps you burn energy. The other branch is the parasympathetic nervous system. This branch helps you conserve energy. It also helps you digest food. Advocates of this diet believe that one branch tends to be stronger or more dominant than the other.
- Rate of cellular oxidation. This refers to the rate at which cells convert food into energy. Some people are fast oxidizers, because they rapidly convert food into energy. In order to balance their systems, fast oxidizers need to eat heavier proteins and fats to that burn slowly. In contrast, slow oxidizers convert food into energy at a slow rate. In order to balance their systems, it's recommended that they eat mainly carbohydrates rather than protein and fat.
In the book The Metabolic Typing Diet, researcher William Wolcott offers a simple home self-test for identifying your metabolic type. For an accurate diagnosis, a trained health practitioner can provide a thorough assessment that may include urine and blood tests. Wolcott provides three general metabolic types:
- Protein types -- Protein types are fast oxidizers of parasympathetic dominant. They tend to be frequently hungry, crave fatty, salty foods, fail with low-calorie diets, and tend towards fatigue, anxiety, and nervousness. They are often lethargic or feel "wired", "on edge", with superficial energy while being tired underneath.
- Carbo types -- Carbo types are slow oxidizers or sympathetic dominant. They generally have relatively weak appetites, a high tolerance for sweets, problems with weight management, "type A" personalities, and are often dependent on caffeine.
- Mixed types -- Mixed types are neither fast or slow oxidizers, and are neither parasympathetic or sympathetic dominant. They generally have average appetites, cravings for sweets and starchy foods, relatively little trouble with weight control, and tend towards fatigue, anxiety, and nervousness.
According to the metabolic typing diet, the three metabolic types should eat the following foods:
- Protein types should eat diets that are rich in protein, fats and oils, and high-purine proteins such as organ meats, pate, beef liver, chicken liver, and beef. Carbohydrate intake should be low.
- Carbo types should eat diets that are high in carbohydrates and low in protein, fats, and oils. They should eat light, low-purine proteins.
- Mixed types should eat a mixture of high-fat, high-purine proteins and low-fat, low-purine proteins such as cheese, eggs, yogurt, tofu, nuts. This type requires relatively equal ratios of proteins, fats, and carbohydrates.
Unlike other diets that recommend the same plan for everyone, the metabolic typing diet recognizes that we are individual. Our metabolisms differ, so our diets should as well.
The metabolic typing theory may help to explain why some people do better on a high protein, low carb diet, while others do better on a high carb diet. Wolcott explains that the effects of different diets on the metabolic types:
- High Carb, Low Fat Diet -- This diet is suitable for carbo types. However, in protein types and mixed types, a high carb, low fat diet can increase fat storage by increasing insulin, and lower metabolic rate by breaking down muscle tissue due to insufficient protein intake, and may disrupt adrenal and thyroid function.
- High Protein, High Fat Diet (e.g. Atkins Diet, Hamptons Diet) -- This diet is suitable for protein types. However, in carbo types and mixed types, a high protein, high fat diet can increase fat storage by disturbing cellular oxidation, and lower metabolic rate by creating a shortage of glucose caused by low carb intake. It may also disturb adrenal and thyroid function.
- 40-30-30 Diet (e.g. Zone diet) -- This diet is suitable for mixed types. However, a 40-30-30 diet can increase fat storage by disturbing oxidation. It can lower metabolic rate by creating a shortage of glucose in carbo types and a shortage of protein in protein types, both resulting in muscle breakdown.
Critics of the metabolic typing diet say that a diet rich in organ meats, pate, and saturated animal fats is unhealthy.
Wolcott W, Fahey T. The Metabolic Typing Diet. Broadway Books, New York. 2000.