Because people vary so much in terms of how much water they need each day, there is not an established Recommended Daily Allowance (RDA) or Estimated Average Requirements (EAR) for water. However, in 2004, the Institute of Medicine set an AI value of 3.7 liters (about 15½ cups) of total water intake per day for men living in temperate climates and 2.7 liters (about 11½ cups) per day for women living in temperate climates. (INFOGRAPHIC 13.13)

Question 13.8

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These totals include water from a combination of food, drinking water, and other beverages. Current consumption patterns indicate that fluids provide about 80% of our total water intake, with plain water making up about 45% of those fluids and other beverages making up the rest. Therefore, the AI for fluids is 3.0 liters for men and 2.2 liters for women. (Both tap and bottled water are considered safe to drink; the EPA regulates the former, while the FDA regulates the latter, and they have similar standards.) The body also produces a small amount of water as a byproduct of metabolic reactions, which also helps meet our total water needs.

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During his marathon, Robinson continued to drink even when he was not thirsty, causing a major water imbalance. But another factor, out of his control, may have contributed to his imbalance, too. Some athletes release excess ADH when exercising vigorously for long periods, making the kidneys retain water. This hormonal reaction likely worsened Robinson’s water imbalance. He then started to feel the common symptoms of hyponatremia, a condition in which sodium levels in the blood become low because of increased blood volume.

Unfortunately, the symptoms of hyponatremia sometimes mimic the symptoms of dehydration, or deficiency of water, which is in part why Robinson was given saline at the hospital—the doctors there assumed he was dehydrated from his run. There are a variety of symptoms associated with dehydration, including nausea, dizziness, elevated temper-ature, and concentrated urine. Dehydration is typically caused by excessive sweating, excessive urine output (because of diuretic use or uncontrolled diabetes), fever, vomiting, or diarrhea. It is also a widespread problem in developing countries, where safe, drinkable water can be hard to find. Although 70% of Earth is composed of water, only 3% of it is drinkable.

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To distinguish between hyponatremia and dehydration, some city marathons now test blood sodium concentrations in unwell athletes before deciding whether to treat for dehydration (which might involve saline) or for hyponatremia (which involves a hypertonic, or high-sodium solution). Other marathons have scales available so that runners can weigh themselves; weight gain from excess fluid during a marathon is a common sign of hyponatremia.

Water, one of the six essential classes of nutrients, is vital for life, but any time we overconsume a nutrient, even water, we may encounter trouble. The seven essential major minerals are also vital—in some ways they are just as important to our health as our own tissues; our cells and organs could not do their jobs properly without them. But as we will see in the next chapter, the nine trace minerals, though less abundant in our bodies, are just as important to our survival.

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