11.4: Excess Dietary Protein

What happens to excess amino acids when you eat more protein than you need? Amino acids themselves aren’t stored in the body. The amino part (-NH₂) comes off, and what remains must be dispensed with. It can, for example, be used for ATP production or stored as fat (Figure 9.2, 9-3).

The amino (-NH₂) that’s taken off is a liability. It can be converted to ammonia (NH₃), which is toxic even in fairly low concentrations. To prevent this toxic effect, aminos are used to make urea* (NH₂ -CO-NH₂), which can be tolerated at much higher levels. The urea is excreted in the urine. (We, of course, make urea even without excessive protein intake, since body proteins are continually broken down and replenished. This replenishment is provided for in our protein requirement.)

Urea excretion requires water, so we make more urine (urinate more) when we eat excessive amounts of protein. So, eating lots of protein raises the body’s need for water (and we feel thirstier). Chronically excessive protein (or amino acid supplement) intake may possibly be damaging to the kidneys. This has been shown in animal experiments, but whether this is a cause for concern in humans is uncertain.

Not all animals use urea to get rid of aminos. Birds make uric acid instead. Unhatched chicks are stuck inside an egg, so they can’t get rid of urea. Mother hens can’t “urinate” for them (as our mothers do for us). Unlike urea, uric acid forms crystals at low concentrations.† This “removes” it from the liquid surrounding the developing chick. (Similarly, you can take sugar out of sugar-water by getting the sugar to crystallize.)

Freshwater fish don’t need to make urea or uric acid. They can get rid of their excess aminos directly as ammonia. It’s diluted to low, harmless concentrations by the vast amount of water that surrounds the fish.

^{*Urea is made mostly in the liver. A diseased liver (e.g., cirrhosis) can have trouble making urea. The resulting rise in ammonia is thought to play a role in the personality changes, impaired consciousness, and coma that can occur with severe liver disease. Alcohol abuse is the major cause of cirrhosis in this country. In parts of Asia and Africa, chronic infection with hepatitis-B virus is the major cause.
†Humans make some uric acid. In gout, uric acid crystals form in and near joints and tendons because of high uric acid. The crystals cause pain and disability. About 95% of people with gout are men. Uric acid levels are lower in premenopausal women; levels rise after menopause.}