Milk vs. Water: A Closer Look at Hydration
A recent billboard claim stating “Milk hydrates better than water” drew the attention of a physician from Harvard Medical School, who then took a deeper dive into the science behind it. Sponsored by the dairy industry, this statement has some backing, as research from as early as 2007 suggests that milk may help maintain fluid balance in the body longer than just water. However, experts emphasize some important considerations for everyday hydration.
The effects are most noticeable shortly after drinking. A 2016 study in The American Journal of Clinical Nutrition introduced a Beverage Hydration Index to evaluate various beverages. Over a four-hour duration, the study observed that both skim and whole milk resulted in less urine output compared to water, leading to a higher hydration score. Interestingly, these milks showcased hydration properties similar to specific oral rehydration solutions that treat fluid loss from diarrhea.
This advantage comes from milk’s unique nutritional makeup. Unlike water, which moves rapidly through the stomach, milk includes lactose, protein, and fat. These macronutrients slow down gastric emptying, allowing fluids to be absorbed gradually. So, instead of being quickly expelled, the liquid stays in the body for a longer period.
How Sodium Functions as a Fluid Retainer
The role of electrolytes is another key factor. Milk naturally contains sodium and potassium, with sodium behaving like a sponge in body tissues and blood, helping retain water and decrease urine output. Registered dietitian Melissa Majumdar elaborated to CBS News, saying, “Electrolytes like sodium and potassium enhance hydration, and the calories in beverages slow down gastric emptying, which delays urination.”
This physiological response is backed by several small studies. In a 2007 trial by Susan Shirreffs published in the British Journal of Nutrition, participants who rehydrated with low-fat or sodium-enriched milk maintained a positive fluid balance for five hours after exercising. In contrast, those consuming water or sports drinks ended up in a negative balance within an hour.
Another study from 2016 conducted at the University of Limerick found comparable results when young men drank milk post-exercise and thermal dehydration. It showed that milk intake restored fluid balance more effectively than carbohydrate electrolyte solutions or plain water, suggesting synergistic effects of milk’s nutrients enhance fluid retention.
A 2020 study in the journal Nutrients examined a milk permeate solution, derived from ultrafiltration, which has the carbohydrate and mineral content of milk but lacks protein and fat. It outperformed plain water and traditional sports drinks in terms of hydration over four hours, indicating that the electrolyte profile plays a significant role in fluid retention.
Challenges with Daily Use
Despite the promising data, experts linked with Harvard Health and other institutions recommend a cautious interpretation of these findings for daily hydration. The studies typically involve small numbers of participants and focus on hydration over just a few hours. It’s still unclear whether milk’s advantages would last throughout the day or provide tangible health benefits for individuals not experiencing dehydration from intense activities.
Another major drawback is the calorie content. In the studies, the amount of milk needed for enhanced fluid retention added up to more than 1,000 calories. For elite athletes consuming thousands of calories in rigorous training, that might be worthwhile. However, for an average person trying to stay hydrated while working at a desk or exercising moderately, opting for milk instead of water brings a significant calorie load that might not align with weight management goals.
Considering these factors, health professionals commonly endorse water as the primary source for routine hydration. Simply drinking when thirsty works well for most healthy individuals. The body’s thirst signals are usually reliable, except during extreme situations like intense heat or severe illness causing fluid loss.
For athletes needing to replenish both fluids and nutrients after rigorous exercise, milk can be an effective recovery drink. For everyone else, water remains the ideal, calorie-free choice, promoting kidney function, joint lubrication, and cellular health while avoiding unnecessary sugars or fats. As noted by Harvard Health, tap water is not only free but also doesn’t carry any caloric implications associated with using milk as a main fluid source.
