What in milk, (Calcium, fat, ...) causes it to be an antidiuretic? In an experiment I conducted, the subjects drank the same amount of water as milk on separate days. There was a significant difference in the amount of fluid excreted after each drink. Why?

The antidiuretic effect of milk can be attributed to several components present in it. One of the main factors is the electrolyte content, specifically calcium. Milk contains a significant amount of calcium, which plays a role in regulating fluid balance in the body.

When you consume milk, the calcium present in it helps to stimulate the release of a hormone called calcitonin. Calcitonin acts in the kidneys to reduce the amount of water excreted in urine. It does this by reducing the reabsorption of water from the filtrate, thereby decreasing urine output.

Furthermore, the protein and fat content of milk can also contribute to its antidiuretic effect. The presence of these components slows down the absorption of water through the gastrointestinal tract. This means that the water from milk is absorbed at a slower rate compared to plain water. As a result, more water remains in the body, leading to a decrease in urine production.

In your experiment, you observed a significant difference in fluid excretion between milk and water. This discrepancy can be explained by the factors mentioned above. Drinking milk, with its calcium, protein, and fat content, likely triggered the release of calcitonin and slowed down the water absorption, resulting in lower urine output compared to drinking the same amount of water alone.

It's important to note that individual variations, such as sensitivity to certain components in milk or differences in metabolism, can also influence the observed effects.