White Paper:
Drinking to Thirst

written by Meg Garvey, PhD

Drinking to Thirst: Abstract

It has been determined that if an athlete drinks according to their innate thirst mechanism during exercise they will under-replenish compared to the fluids that have been lost due to thermoregulation. In fact, seminal research has determined that individuals practicing this hydration strategy consumed approximately half of the fluids lost during exercise in both hot as well as cool environments (1). The purpose of this paper was to investigate the efficiency of ad libitum fluid consumption within our dense validation data set and compare those results to other researchers in the field. 

Defining "Ad Libitum"

As mentioned in previous White Papers, thermoregulation is a series of mechanisms that keep the body at a consistent core temperature and replenishing fluids is a method to assist in thermoregulation. The formal definition of ad libitum indicates as much as needed.  When it is applied to a drinking strategy it indicates that fluid consumption is at the level of whenever and whatever volume is desired (2). While this strategy has been deemed adequate at rest, the same mechanism has been found to be less sensitive during exercise. Historically this term has been used synonymously with “drinking to thirst” and will be continued throughout this paper as well.

What Our Dataset Indicates

For the purposes of this paper secondary data analysis was conducted on Nix Biosensors ground truth validation data set (data collected between October 2021 and August 2022). This data set included 1,157 workout observations among 257 athletes (86.5% female, 77% non-Hispanic white, with ages ranging from 18-67 years) demographics of this population are summarized in Table 1. 

Table 1.
Population Demographics.

Source: Nix proprietary studies.

In order to compare to the findings of other studies a categorical variable was created to denote what percentage of fluids lost were replenished while utilizing an ad libitum hydration strategy. This variable allows for the visualization of workouts that replenished less than 50% of what was lost versus those that replenished 50-100% and those that consumed over the necessary amount (defined as fluids lost per weigh-in and weigh-out protocol). It was found that 45.5% of the 1,157 workouts observed replenished less than 50% whereas 30.9% were over-hydrated (Graph 1). 

Graph 1.
Drinking to Thirst Replacement Breakdown.

Source: Nix proprietary studies.

In addition to these findings, it was further broken down into categories by the length of the workout. For workouts lasting longer than 60 minutes, the results increased to find that 83.2% of the workouts observed replenished less than 50% of the fluids lost to sweat.

Relation to Other Research Findings

In a review by Cheuvront et al (2007) they indicate that predictive modeling for hydration recommendations for marathon runners is intensively futile as the needs vary between elite and recreational runners, and between different body sizes. Due to these limitations, many marathon runners in their review relied on ad libitum drinking strategies that commonly resulted in fluid deficits of >2% body mass (3). Within the current study’s dataset, although under-hydration was occurring, the mean percent body mass loss remained <2%; however, the range, particularly for workouts over 60 minutes, includes individuals whose loss was >2% body mass. It is also important to note that only a small percentage of the data analyzed was from running mileage over 18 miles.

Kenefick et al (2018) indicated results similar to those of the current study, that for activities of <60 minutes and performed at a lower intensity (RPE ~5) ad libitum drinking may not necessarily exceed 2% body mass loss (4). However, performance deficits can vary between sports. 

It was noted with our analysis that almost 31% of the workouts observed resulted in over-hydrating when compared to the fluids being lost, while only 8.4% of workouts over one hour over-hydrated. In a study by Belval et al (2019), it has been noted that fluid accessibility during certain competitions may also exacerbate this impulse to overconsume. While the International Association of Athletics Federations recommends feed zones/water stations to be placed approximately 3 miles away from each other, many races include more frequent stops (5). Athletes participating in such races, especially those who fall into a more “recreational racer” group, should educate themselves prior to race day on their typical hydration requirements and the importance of fluid balance to prevent over-hydrating, which comes with its own set of side effects.

Drinking to thirst works when at rest,
but not during exercising


While drinking to thirst is a reasonable hydration strategy at rest, this strategy has been found to lead to dehydration during exercise. For activities lasting longer than 60 minutes, athletes could benefit from having a tailored hydration plan in place to reduce the risk of dehydration, subsequent performance impairments, and other health-related side effects. Tracking actual hydration requirements during training sessions not only improves the training effectiveness of each of those sessions but also allows for more data to be considered when creating competition hydration planning for optimal performance.