Training in the Heat: How to Hydrate, Perform and Recover Better

If you’ve been training during the recent heatwave, you’ve probably noticed that your usual hydration strategy might not be enough.

The bottle that normally gets you through a long ride suddenly runs dry. Your kit is soaked. And although the intensity is the same as always, the effort feels much greater.

That's because when temperatures climb above 30°C, your body has to work much harder to stay cool. The main way it does this is through sweating, which means you lose more fluid and more electrolytes than you would during the same session in cooler conditions (Sawka et al., 2007; Shirreffs & Sawka, 2011).

The challenge is that these losses can vary enormously between athletes and even between sessions. Understanding what happens to your sweat losses in the heat can help you prepare better, hydrate more effectively, and recover faster.

WHY THE HEAT CHANGES EVERYTHING?

Sweating is one of the body's most important cooling mechanisms. As your core temperature rises during exercise, sweat is released onto the skin where it evaporates and removes heat from the body (Baker, 2019).

When environmental temperatures increase, your body responds by producing more sweat in an attempt to maintain a safe core temperature. Research consistently shows that sweat rate rises as ambient temperature increases, even when exercise intensity remains unchanged (Baker, 2017). For endurance athletes, this can lead to substantial fluid losses. In hot conditions, some athletes can lose more than 2–3 litres of sweat per hour.

However, producing more sweat doesn't always mean cooling more effectively. In humid conditions, a greater proportion of sweat drips from the body instead of evaporating, meaning you lose fluid without gaining the full cooling benefit (Sawka et al., 2007).

The result? Greater dehydration, increased cardiovascular strain, and a higher perception of effort during exercise (Cheuvront et al., 2010).

YOU DON’T JUST SWEAT MORE, YOU LOSE MORE SODIUM TOO.

When athletes think about hydration, they often focus on fluid intake. But sodium losses become increasingly important as sweat rates rise.

Sodium is the main electrolyte lost through sweat, and the amount lost varies dramatically between individuals. Some athletes lose less than 500 mg of sodium per litre of sweat, while others can lose more than 2,000 mg/L (Baker, 2017).

Even if your sweat sodium concentration remains unchanged, producing more sweat means losing more sodium overall. And over the course of a long ride, run, or race, these differences can become significant and may affect both performance and recovery (Shirreffs & Sawka, 2011).

HOW TO PREPARE BEFORE TRAINING IN THE HEAT

One of the biggest mistakes athletes make during a heatwave is assuming that their hydration strategy can remain unchanged. Sweat rate and sodium losses can increase substantially as environmental temperature rises, meaning a strategy that works in spring may be insufficient during a training session in the heat (Sawka et al., 2007; Shirreffs & Sawka, 2011).

Before heading out, it is worth considering:

• The expected temperature and humidity

• The duration and intensity of the session

• Your personal sweat rate and sodium concentration

• How much fluid and sodium you are likely to lose

This is where personalised planning becomes valuable. While generic recommendations can provide a starting point, sweat responses vary considerably between athletes (Baker, 2017).

Features such as the FLOWBIO Planner Tool use your historical sweat profile and the forecast conditions to estimate expected fluid and sodium losses before a session. This can help athletes prepare an appropriate hydration strategy in advance, particularly for long rides, races, or key training sessions in the heat.

WHAT SHOULD YOU DRINK DURING EXERCISE?

Once exercise begins, the challenge becomes replacing enough fluid and sodium to minimise excessive dehydration without over-drinking.

Research consistently shows that losing more than approximately 2% of body mass through dehydration can negatively affect endurance performance, cardiovascular function, and perceived effort, particularly in hot environments (Cheuvront et al., 2005; Sawka et al., 2007).

The difficulty is that sweat losses are not constant.

As conditions become hotter, intensity changes, or fatigue develops, sweat rate and sodium losses can fluctuate throughout a session (Baker et al., 2022). This means that a hydration strategy based solely on a single sweat test or previous experience may not always reflect what is happening on a given day.

Continuous sweat monitoring offers an alternative approach by tracking sweat losses as they occur. The FLOWBIO sensor, for example, provides real-time estimates of fluid and sodium loss during training, enabling athletes to adjust their hydration strategy based on their actual physiological response, rather than relying solely on pre-set targets or guesswork.

For long endurance sessions in hot conditions, this can provide a more accurate picture of hydration requirements as environmental and exercise demands change.

RECOVERY: THE PART MOST ATHLETES UNDERESTIMATE

Recovery does not begin with the next training session; it begins as soon as the current one ends.

Replacing the fluid and sodium lost during exercise is important for restoring plasma volume, supporting recovery, and ensuring athletes start their next session adequately hydrated.

The American College of Sports Medicine recommends consuming approximately 150% of the fluid deficit accumulated during exercise to achieve full rehydration (Sawka et al., 2007). For example, if an athlete loses 2 litres of fluid during a session and drinks 1 litre, they may need to drink a further 2 litres or so during the recovery period to fully replenish those losses.

Sodium replacement is equally important, particularly for athletes with high sweat sodium concentrations, as sodium helps retain ingested fluid and supports restoration of fluid balance (Shirreffs & Sawka, 2011).

Using measured sweat losses rather than estimates can make recovery planning considerably more precise. The FLOWBIO Recovery Plan uses data recorded on fluid and sodium loss during each session, as well as the intake of any fluids and/or supplements, to provide personalised recovery targets with estimated timings. This helps athletes know exactly how much fluid and sodium they need to replenish before their next training session.

EVERY ATHLETE RESPONDS DIFFERENTLY

Perhaps the most important point is that there is no single hydration strategy that works for everyone.

Two athletes completing the same session in the same weather can lose dramatically different amounts of fluid and sodium through sweat (Baker, 2017). Likewise, an individual athlete's sweat profile can change significantly between winter and summer, or between a cool morning ride and a hot afternoon race.

This is why personalised monitoring becomes increasingly valuable as environmental temperatures rise. Understanding your own sweat response, rather than relying on generic guidelines, allows hydration strategies to evolve alongside changing conditions.

FLOWBIO's validation study demonstrated strong agreement with established laboratory reference methods for both sweat sodium concentration and whole-body sweat loss across hot-dry and hot-humid environments, including conditions reaching 40°C (Bandiera et al., 2026). While no wearable sensor can eliminate the physiological challenges of training in extreme heat, having access to personalised sweat data can help athletes make more informed hydration decisions before, during, and after exercise.

WHY YOUR HYDRATION STRATEGY IN APRIL MAY NOT WORK IN JULY

One of the biggest mistakes athletes make is assuming their sweat profile stays the same throughout the year.

Research shows that sweat rate and sodium losses can change substantially with environmental temperature, humidity, exercise intensity, clothing, and heat acclimation status (Baker et al., 2022).

In other words, a hydration strategy that works perfectly during a cool spring ride may be completely inadequate during a summer heatwave.

Sweat losses are highly individual and can change considerably as environmental conditions change. Continuous monitoring, therefore, offers a practical way to track how hydration needs evolve over time. The FLOWBIO Sensor has been validated against laboratory reference methods and shown to provide reliable estimates of sweat sodium concentration and whole-body sweat loss across both hot-dry and hot-humid conditions (Bandiera et al., 2026).

Understanding how your sweat profile changes from session to session can help you make more informed decisions about hydration, fuelling, and recovery when conditions become challenging.

REFERENCES

American College of Sports Medicine, Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 39(2), 377–390. 

Baker L. B. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports medicine (Auckland, N.Z.), 47(Suppl 1), 111–128.

Baker L. B. (2019). Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature (Austin, Tex.), 6(3), 211–259. 

Baker, L. B., De Chavez, P. J. D., Nuccio, R. P., Brown, S. D., King, M. A., Sopeña, B. C., & Barnes, K. A. (2022). Explaining variation in sweat sodium concentration: effect of individual characteristics and exercise, environmental, and dietary factors. Journal of Applied Physiology (Bethesda, Md.: 1985), 133(6), 1250–1259. 

Bandiera, D., de Bardonnèche, J., Margout-Jantac, D., Dubois, L., El Allaoui, N., Rubio, J. S. E., Aubin, J. C., Racinais, S., Tessitore, A., & Pitsiladis, Y. (2026). Sweat sodium composition and sweat loss estimation through wearable sensors and predictive equations in dry and humid hot conditions. Frontiers in Physiology, 16, 1717275.

Cheuvront, S. N., Carter, R., 3rd, Castellani, J. W., & Sawka, M. N. (2005). Hypohydration impairs endurance exercise performance in temperate but not cold air. Journal of Applied Physiology (Bethesda, Md.: 1985), 99(5), 1972–1976. 

Cheuvront, S. N., Kenefick, R. W., Montain, S. J., & Sawka, M. N. (2010). Mechanisms of aerobic performance impairment with heat stress and dehydration. Journal of Applied Physiology (Bethesda, Md.: 1985), 109(6), 1989–1995. 

Kenefick R. W. (2018). Drinking Strategies: Planned Drinking Versus Drinking to Thirst. Sports Medicine (Auckland, N.Z.), 48(Suppl 1), 31–37. 

Shirreffs, S. M., & Sawka, M. N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of Sports Sciences, 29 Suppl 1, S39–S46.