Influence of estrous and circadian cycles on calcium intake of the rat

• We measured relative calcium intake over the estrous and circadian cycles in the rat.
• Circadian fluctuations in CaCl2 solution intake paralleled changes in water intake.
• The preference scores for CaCl2 did not change over the circadian cycle.
• CaCl2 solution intake or preference scores did not change over the estrous cycle.

The food, water and sodium intake of laboratory rats fluctuates over the circadian and estrous cycles. Blood calcium and calcium-regulating hormones also wax and wane in response to these cycles, raising the possibility that the same might be true of calcium intake. To investigate this, we monitored the fluid intakes of female Long-Evans rats given a choice between water and 10 mM CaCl2 solution for two consecutive estrous cycles. We found that calcium solution intake changed over the circadian cycle in a similar manner to water intake; the preference scores for CaCl2 solution remained stable. We did not detect any changes in calcium solution intake or preference scores during the estrous cycle despite a decrease in fluid intake at estrus. Thus, fluctuations in intake of calcium solution during the circadian cycle appear to be nonspecific and probably the result of changes in fluid balance. Estrous changes either do not influence calcium intake or their effects are masked by other factors, resulting in stable levels of calcium intake.