Minerals are lost in sweat. Prolonged exercise, especially in the heat, has the potential to create mineral deficiencies. Is during-exercise replacement necessary? To answer this question we need to know:
Many minerals have several regulatory mechanisms. For example:
Low blood levels of calcium may be corrected by the release of calcium from bone.
Of the 21 minerals possibly important in human nutrition, we will eliminate from discussion the trace minerals arsenic, boron, chromium, cobalt, copper, molybdenum, nickel, selenium, silicon, and vanadium. Too little is known about these minerals. Reliable data is not available about their concentration in sweat. Functions and toxicities are uncertain. We will eliminate fluorine/fluoride because water is generally fluoridated, and sweat levels are uncertain. Finally, we will eliminate iodine and chlorine/chloride. These minerals are consumed along with sodium.
This leaves calcium, iron, magnesium, manganese, phosphorus, potassium, and sodium as possible candidates for concern and replacement.
Let us discuss these remaining minerals. Mineral intakes, absorption efficiency, body content, and sweat levels are listed in Table 2.
Intakes are based on US averages. Soil content can considerably change the mineral content of some foods and therefore intake. Sweat concentrations vary considerably, and reliable data is often not available. Fitter athletes may have different concentrations than those less fit. Concentrations may change as a workout progresses.
Let us look at sodium to see how the table works. (We will discuss details about sodium below.) The typical daily intake is 4,000 milligrams (4 grams). Absorption efficiency is greater than 90%. Therefore, more than 3,600 milligrams are absorbed daily.
The sodium body content of a 70-kilogram (154 pound) person is about 90,000 milligrams (90 grams).
Sweat contains between 230 and 1,700 milligrams of sodium per liter (quart). Assuming an intermediate sweat concentration of 1,000 milligrams, a gallon (4 quarts, 4 liters) of sweat could contain 4,000 milligrams. Sodium losses in a gallon of sweat can exceed daily intakes and lead to deficiencies.
The Intake X AE / Sweat multiplies typical intakes by absorption efficiency and divides by typical concentrations in one liter (quart) of sweat. This gives an estimate how many liters (quarts) it takes to use up a typical day’s intake of the mineral.
Manganese concentrations in sweat are so relatively low that it would take about 300 liters (about 75 gallons) of sweat to use up a typical daily intake.
As you can see from the table, sodium is critical. Just four liters (quarts) of sweat might result in a loss equivalent to a typical daily intake. Calcium and iron are also of concern; as you will read, acute symptoms are not generally a problem.