Epidemiological evidence shows an inverse association between dairy intake and body weight. While a number of substances may explain the possible weight loss effects of dairy, research attention has focussed mainly on the calcium content. Calcium too shows an inverse association with body weight and data from clinical trials have shown that administration of high calcium diets to overweight subjects causes weight loss. The reason for the ability of calcium to cause weight loss are not fully understood. Some evidence suggests that low plasma calcium may increase cellular levels of calcium ions, a condition known as the calcium paradox. The high levels of calcium in the cell may then interfere with calcium signalling, disrupt the insulin receptor cascade and thereby contribute to cellular insulin resistance. However, some research suggests that calcium may form soluble soaps with fat in the gut, and this may lead to increases in fat excretion.
For example, researchers1 have investigated the effects of low fat dairy products on faecal fat excretion in a randomised crossover study. Subjects consumed 2300 mg per day calcium from low fat dairy products or 700 mg per day from a low calcium diet for 7 days before crossing over to the other treatment. Faecal fat and calcium content was monitored for the last 5 days of each diet. Following consumption of the low fat dairy products, total faecal fat content increased significantly from 5.4 grams per day on the low calcium diet to 11.5 grams per day on the high calcium diet. This was reflected in an increased faecal energy excretion in the high calcium group. Calcium caused a larger increase in the excretion on monounsaturated fatty acids compared to saturated fatty acids, suggesting that the effect was selective, and may be magnified by dietary fatty acid composition.
These results may explain the inverse association between dairy intake and body weight in humans. Increasing the calcium intake by 1600 mg per day caused a doubling of the excretion of fat in the faeces, which amounted to 54.9 kcal per day. This is a small amount and it is unclear as to whether the resting metabolic rate or the thermic effect of food would be adjusted by the hypothalamus to compensate for the reduced calorie intake. It is likely that it was the calcium content of the dairy that caused the effects as it is known that calcium can form soluble soaps with fatty acids which may prevent their absorption. However, it cannot be discounted that some other factor within the dairy products were responsible for the effect on faecal fat excretion. It is also possible that components within the dairy products may be acting synergistically with the calcium to increase fat excretion.
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