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In a randomized crossover study, scientists administered 10 test subjects three isocaloric one-week diets consisting of low calcium and normal protein (LC/NP; 500 mg calcium and 15 percent of energy derived from protein), high calcium and normal protein (HC/NP; 1,800 mg calcium and 15 percent of energy from protein), and high calcium and high protein (HC/HP; 1,800 mg calcium and 23 percent of energy from protein).

Calcium intake had no effect on 24-hour energy expenditure or fat oxidation, but fecal fat excretion increased approximately two-and-a-half-fold with the HC/NP diet compared with the LC/NP and the HC/HP diets. The HC/NP diet also increased fecal energy excretion as compared with the LC/NP and HC/HP diets. No effects on blood cholesterol, free fatty acids, triacylglycerol, insulin, leptin or thyroid hormones were observed.

The researchers concluded a short-term increase in dietary calcium intake, paired with a normal protein intake, increased fecal fat and energy excretion by approximately 350 kJ/d, noting this observation may explain why a high-calcium diet produces weight loss. They also noted the interaction between dietary calcium and protein levels may be important.