The common belief that milk is dehydrating is a widespread misconception that overlooks the biological mechanisms of fluid balance. While plain water is often considered the standard for rehydration, scientific studies show that some beverages are retained more effectively by the body than water alone. The question of whether milk helps or hinders hydration depends heavily on its unique nutritional makeup and, for some people, their individual digestive response to it.
Milk’s Performance as a Hydrating Beverage
Milk is consistently ranked as one of the most effective beverages for rehydration, frequently outperforming plain water over a sustained period. This performance is measured using the Beverage Hydration Index (BHI), a metric that compares fluid retention after consumption relative to water, which is scored 1.0. Studies show that both skimmed and full-fat milk achieve BHI scores significantly higher than water, with skimmed milk often scoring near 1.5. This indicates that milk leads to less urine output and better fluid retention in the hours following consumption. Milk’s complex composition slows the rate at which the fluid is excreted. Milk is also an effective fluid replacement after strenuous activity, sometimes restoring whole-body net fluid balance better than carbohydrate-electrolyte sports drinks.
Key Components Affecting Fluid Balance
Milk is approximately 87% water, providing substantial fluid volume. The other components, however, make it a better fluid retainer than water.
The presence of electrolytes, specifically sodium and potassium, helps the body maintain fluid balance. These minerals are lost through sweat and urine, and their replenishment helps to slow the rate of fluid excretion after drinking. The sodium in milk is important as it increases plasma volume and stimulates water absorption in the small intestine.
Additionally, the protein and natural sugars (lactose) found in milk contribute to a slower rate of gastric emptying compared to plain water. By keeping the fluid in the stomach and small intestine longer, the body absorbs the water and nutrients over a more extended period. This sustained absorption prevents the rapid drop in the blood’s osmotic pressure that occurs after drinking large amounts of plain water, which can trigger a diuretic effect and increase urination.
When Milk May Cause Digestive Distress
While milk is generally hydrating, it can lead to symptoms that mimic dehydration for individuals with lactose intolerance. Lactose intolerance results from a deficiency in the enzyme lactase, which is required to break down lactose in the small intestine.
When undigested lactose reaches the large intestine, it becomes an osmotically active substance. This draws water into the bowel lumen from the surrounding tissues to achieve osmotic equilibrium. This influx of water, combined with gas produced by the fermentation of lactose by gut bacteria, results in symptoms like bloating, gas, and diarrhea.
Severe diarrhea is a form of fluid loss that can result in actual dehydration if not replaced. This secondary effect of digestive distress is the reason milk may be perceived as dehydrating by those who are intolerant, but it is a reaction to malabsorption, not an inherent property of the beverage itself.