I found this from 1950:

Perspiration as a Factor Influencing the Requirement for Calcium and Iron
Frances A. Johnston, Thelma J. McMillan and Erica R. Evans

The College of Home Economics and the School of Nutrition, Cornell University, Ithaca, New York

Perspiration was collected from 4 young women, each of whom served as a subject for 4 periods of approximately an hour each. The temperatures used were 97°F. dry bulb and 93°F. wet bulb. The collection was made in enamel tubs in which the subjects sat during the sweating period. Perspiration from all parts of the body except the scalp was collected.

The mean loss of body-weight per hour during the collection periods was 10.87 ounces (308 gm). The mean loss of iron per kilogram of body-weight loss for each of the 4 subjects was 0.24, 0.26, 0.26, and 0.28 mg; the mean loss per hour was 0.48, 0.063, 0.080 and 0.108 mg. The mean loss of calcium per kilogram of body-weight loss for each of the 4 subjects was 13.42, 30.42, 32.52 and 40.10 mg; the mean loss per hour was 4.17, 6.80, 8.84 and 14.33 mg.

The administration of 25 mg of iron with breakfast on the day of the collection did not increase the loss of iron, nor did a subject who was taking iron therapy daily secrete more iron than the other subjects.


okay, they were losing almost 11 ounces of body weight AN HOUR and losing about 30 milligrams of calcium an hour.
If you are taking your 1000 mg of Calcium a day; you are COVERED. and ALL exercise will do this to you, but since Bicycling doesn't "weight bear" it doesn't rebuild what you lose