Offsetting fluid loss during exercise is believed to be an important recommendation to maintain athletic performance.
There’s much debate as to whether an athlete should drink to thirst or whether a prescribed hydration strategy is warranted to negate dehydration.
To add further insight to this question (drinking to thirst vs prescribed), de Melo-Marins et al (2018) examined the impact of performance, cardiovascular and thermoregulatory strain during cycling to exhaustion in the heat (34°C, 40% RH).
These cyclists then performed this test under 3 conditions:
1) Prescribed fluid (PL) intake based on sweat rates.
2) Ad libitum (ADL) – i.e. drink whenever you want.
3) Control (C) – no fluid at all.
To assess hydration requirements, they first measured sweat loss. This was achieved by measuring pre & post cycling body mass. The difference in body mass would equate to litres lost via sweat (1kg weight loss = 1kg fluid loss).
During the trial, the cyclists drank ~167ml of fluid in the ADL condition and 1,080mL in PL which is a very significant difference when relying solely on your thirst receptors to initiate fluid intake – body mass was unchanged in the PL group, and dropped 0.8kg in the ADL and 0.9kg C group.
In the sports nutrition literature, a 2% drop in body mass is generally considered to be negative for performance. As the riders in this study were 75kg, they only experienced a modest ~1% loss in weight. Considering they were within this threshold, performance outcomes were the same.
That being said, skin temperature and heart rate were elevated in the ADL and C group. Thus, indicating more cardiovascular and thermoregulatory strain – indicative of increased physiological ‘load’ at a given intensity.
Although performance was maintain throughout the conditions, I would not expect this outcome to be the same if the cycling test was (1) longer, or (2) if they had multiple sessions within close proximity as they would go beyond the 2% threshold.
Therefore, ad libitum strategies are equally as beneficial as prescriptive strategies when training 60-90 mins. But if duration is longer (>90 mins) and sweat rates continue to increase, I’d highly recommend being prescriptive with your strategy.

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