The newest index on the block… the hydration index!

Study Deep Dives has previously covered the glycemic index and the insulin index, but beverages now also have their own index: namely, the new “hydration index”. People usually think about hydration in the context of sports and exercise^[1], so it may be beneficial to know which beverages can best hydrate you when access to fluids (or bathrooms, see Figure 1 for a rough guide to bathroom-guided hydration statuses) is limited.

Figure 1: BHI for tested drinks

Reference: Maughan et al. Am J Clin Nutr. 2015.

Not all beverages are created equal from a hydration standpoint. Absorption is affected by the amount of fluid ingested, electrolyte and carbohydrate content, and the presence of diuretic agents (substances that promote urine production). For example, milk^[2] has been shown to be more effective than both water and sports drinks for rehydration after strenuous exercise.

The rehydration process is affected both by the volume^[3] of fluid ingested as well as the sodium content. It has long been known that the presence of carbohydrates and electrolytes^[4] in a drink increases the rate of fluid absorption after drinking. Coffee is often thought of as negatively affecting hydration status, but this is based on studies^[5] examining the acute effects of high levels (more than 300 mg) of caffeine on individuals who had been deprived of caffeine for a period of days or weeks. A tolerance to the diuretic actions of caffeine develops with regular intake, and the amounts of caffeine found in normal sized servings of tea, coffee, soda, etc., do not have diuretic effects.

This study is the first to develop a method for systematically quantifying values for hydration and fluid balance. Similar to how the glycemic index^[6] is intended to define the blood glucose response to the ingestion of foods compared to a white bread or glucose standard, a beverage hydration index (BHI) could serve to quantify water excretion from the kidneys in response to various beverages compared with still water. The cumulative volume of urine passed over a fixed period of time can be measured as a marker of fluid absorption and retention. The aim of this new study was to determine the fluid balance responses to the ingestion of a set amount of commonly consumed beverages ingested in a euhydrated state (a normal fluid balance).

The volume and amount of electrolytes and sugars of ingested fluids affect their absorption and retention, but various beverages have yet to be systematically compared. This study set out to determine if a beverage hydration index, similar in principle to the glycemic index, could be established.