Is beer hydrating? What a 500 ml glass counts for

• Default serving500 mlAlcohol
• Counts toward goal500 mlhydration factor 1.00
• Calories215 kcal43 kcal per 100 ml
• Alcohol5% ABV20 g per serving

Weak and wet beats strong and small

A pint of lager is about 95% water. The controlled trial behind the hydration factors in this app included a 4% lager among its thirteen drinks. Volunteers drank a full litre of it. The researchers then weighed everything they produced for four hours. The lager finished level with still water. It did not beat water, and it did not lose to it.

That is why the hydration factor here is 1.00 rather than a penalty. The volume of fluid arriving is large enough to cover the extra urine the alcohol causes. Concentrate the same alcohol into a small glass and the arithmetic flips. That is why the catalog discounts wine and spirits but not beer.

What the diuretic effect actually depends on

The follow-up detail is the one almost nobody quotes. The same research group later gave men a litre of beer with 4% alcohol or with none. The difference in urine output depended on how hydrated the men already were. In well-hydrated volunteers the alcoholic beer produced about 1279 ml of urine. The alcohol-free version produced 1121 ml. That is a real difference, but a small one against a litre of intake. Other volunteers had exercised in the heat and were already dehydrated. In them there was no measurable difference at all. The body's own water-saving hormones overruled the alcohol.

Read it as a dose effect, not a switch. One beer in an evening is a net gain of fluid. Six beers is a large alcohol dose spread over hours. The hangover that follows is not mainly a water problem anyway.

Source for that second study: Hobson RM, Maughan RJ. "Hydration status and the diuretic action of a small dose of alcohol." Alcohol and Alcoholism, 2010;45(4):366-373. PubMed.

The number that matters more than hydration

500 ml at 5% ABV is 20 g of pure alcohol and 215 calories. That is more than a can and a half of cola. The app keeps that alcohol on its own track instead of hiding it inside the water total. A week of "just a beer with dinner" adds up somewhere you can see it. On iPhone the app also writes those grams to Apple Health. On Android it syncs the water to Health Connect, but the alcohol figure stays inside the app.

Hydration factors follow the Beverage Hydration Index: Maughan RJ, Watson P, Cordery PA, et al. “A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index.” American Journal of Clinical Nutrition, 2016;103(3):717-723. Read the paper on PubMed. In WaterChamp a beer counts the same as plain water.