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How does the liver convert the food you eat into fuels?
The protein, complex carbohydrates, and fat that you eat -- and the protein, carbohydrate, and fat in your body -- can all be used to fuel your body functions. But first, the liver converts them into chemicals the body's tissues can use as fuel, as follows:
The liver converts the carbohydrates into a simple sugar, glucose, which can fuel most body functions.
It converts any excess protein you eat, over and above what you need to replace the proteins in your body, into glucose as well.
It converts the fats into chemicals called ketone bodies.
Once these conversions are made, the liver exports the glucose and ketone bodies into the blood stream for distribution to the tissues for use as fuel.
How do you develop ketosis on a low carbohydrate diet?
The amount of ketone bodies you make depends on how much glucose is available to your system. If you starve yourself, or if you eat a low to no carbohydrate diet, you will make large amounts of ketone bodies to compensate for the lack of carbohydrates. As a result, you will start to accumulate these ketone bodies in your blood stream. This accumulation is called ketosis, and the diet that causes this accumulation is called a ketogenic diet. Ketosis will give a distinctive taste in your mouth, and a distinct (and not very pleasant) breath odor -- it smells like a cross between apples and nail polish remover -- because some of these ketones can be exhaled from your lungs as as gas.
Can all tissues use ketone bodies as fuel?
Most tissues can use ketone bodies as fuel, so when little glucose is available, they switch to using ketone bodies. But there are several tissues of the body which don't do very well when you have ketosis. These include your brain, the muscles you use for rapid movement, and your red blood cells.
Ketosis and your brain
One tissue that cannot use ketone bodies very well as a fuel is the brain. The brain needs a certain amount of carbohydrate per day to function properly -- about 150 grams a day, or the equivalent of about 12 slices of bread, or 3 baked potatoes. The brain's need for carbohydrate is one of the reasons why the Food Guide Pyramid recommends that you eat 6-11 servings of the Bread, Cereal, Rice and Pasta Group a day.
If you eat a low carbohydrate diet for a while, or if you starve, you brain can start to use ketones for about 50% of its fuel needs. But when your brain uses ketones, it cannot function at its best -- thinking and reaction times tend to slow down. People tend not to notice these changes when they are on a low to no carbohydrate diet, because the brain changes make them slower to react to any stimulus, including the stimuli that come from self-awareness. So you may not be aware of this slowing down, but it may make a difference if you use machinery, such as a car, which requires quick thinking and quick reflexes.
Note that physicians have used ketogenic diets for people with epilepsy. Epilepsy is a kind of electric storm in the brain. Ketogenic diets slow down all brain functions, so if you are epileptic, a ketogenic diet will make your brain less likely to react to the trigger that sets off electric storms in your brain.
Ketosis and your muscles
Another tissue that cannot use ketones very well are the muscles that you need to use for fast movement. For example, the muscles in your hands cannot use ketone bodies very well, so on a low carbohydrate diet, your fast repeated hand movements will slow down and your hands' endurance will decrease -- for example you will become slower and more tired when you type for a long time.
Furthermore, on a low to no carbohydrate diet, the protein you eat, and protein from muscles is converted to glucose to supply the brain's needs. So if you go on this diet, you may feel tired because you have lost muscle mas as well.
Water weight loss with ketosis
When you begin a low to no carbohydrate diet, you will experience water weight loss, even when you don't change the total anount of calories that you eat. This water weight loss is due to the changes in kidney function, including loss of body sodium, that accompany the need to excrete the excess ketone bodies. As a result you will reach a new, slightly dehydrated weight.
You will also need to excrete the urea that is produced when you convert protein into fuels and into glucose. The urea is excreted along with water, so that you will continue to have higher than normal water loss.
A note about ketosis and diabetes
People who have Type I diabetes mellitus can develop ketosis as well. People who have Type I diabetes mellitus require insulin injections in order to use the glucose that the liver produces. When they don't have enough insulin in circulation, the body can't use the glucose, and acts as if there was no glucose in the blood stream. As a result, fat cells break up the fat they contain into free fatty acids, and send them via the blood stream to the liver. There the liver converts the fats into ketone bodies.
Because the fat cells release their fats at a very high rate, the production of ketones also occurs at a very high rate, a rate much greater than the rate at which the body can use the ketones. Under these circumstances, ketosis can become severe. The ketosis is coupled with dehydration and loss of sodium and potassium, which occurs because the body tries to excrete the excess glucose via the urine. The dehydration, ketosis, loss of sodium and potassium, and the development of high blood acid levels, together lead to a condition called diabetic ketoacidosis. Diabetic ketoacidosis can be fatal if the person is not treated with insulin. The ketosis you develop with a low to no carbohydrate diet is a very mild condition when compared with diabetic ketoacidosis.
Why can people lose weight on a low carbohydrate diet?
As noted above, when you start a low carbohydrate diet, you will have some amount of water weight loss. As you continue on the diet, you may experience a decreased appetite, and these diets lack variety, so you will eat less. As a result you will lose weight -- in fact any diet where you take in fewer calories than your body needs will result in weight loss.
The Great Nutrition Debate
On Thursday, February 24th, 2000, the US Department of Agriculture's Center for Nutrition Policy and Promotion (USDA-CNPP) hosted "The Great Nutrition Debate" concerning weight, weight loss, and low carbohydrate diets, as part of their Millenium Lecture Series. The Debate was attended by several advocates for low to no carbohydrate diets, including Dr. Robert Atkins and Dr. Barry Sears, and also by advocates of higher carbohydrate diets, including Dr. Dean Ornish. To hear or read a transcript of the debate, you may want to visit the USDA-CNPP homepage.
In conclusion...
Because a low carbohydrate diet is unbalanced, it can cause ketosis -- with the undesirable consequences we have noted. If you are considering a weight loss diet, or any special diet for that matter, please consult with your physician or a registered dietitian first, to determine what approach is appropriate for you.
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