22.4–22.7: Nutrients are grouped into six categories.

A jaguar takes a drink at a watering hole in Belize.

All animals need water to survive. In the early 1900s, an Italian man sentenced to death reportedly volunteered to determine how long he could live without food or water. The fatal “experiment” lasted only 17 days before he died. Water, which is considered an essential nutrient, constitutes about 60% to 65% of the body weight in most mammals. This water plays important roles in both the intracellular fluid and the extracellular fluids, including blood, in humans and other animals (FIGURE 22-6). Water in body fluids serves a variety of critical purposes, all of which can be impaired when the animal becomes dehydrated.

Figure 22.6: Water is an essential component of the diet.

• Water transports nutrients and waste materials throughout the body.
• It takes part in chemical reactions.
• It serves as a solvent for many vitamins and minerals, amino acids, and sugars.
• It lubricates many joints, the spinal cord, and the eyes.
• It helps regulate body temperature.

A person expending about 2,000 kcal/day needs about 2–3 liters of water each day. This can come directly from drinking water, but there are many other sources of water in our diet, including milk, which is about 90% water, juices, and food, and the water released as a by-product of many chemical reactions. Water intake must offset the water lost in urine, feces, respiration, and sweating. It is important to remember, too, as we saw in the Chapter 3 StreetBio, that drinking too much water too quickly can also be dangerous. The consumption of too much water can lead to water intoxication—which sometimes happens to marathon runners during a race. When coupled with the loss of salt through sweating, over-consumption of water can lead to severe sodium imbalance. This imbalance causes dizziness, nausea, confusion, and swelling of the extremities. And in serious cases, it can lead to swelling of the brain, which can cause death.

Water usage varies among animal species. Tremendous water efficiency has evolved in some desert mammals—the kangaroo rat, for example, gets all of the water it needs, not from drinking, but from food and metabolic processes. Among the marine birds and reptiles, most are able to drink salt water. They can do this with the aid of salt glands that remove and excrete the excess salt they consume.