Pregnancy Hydration for Athletes: Why Sweat Rate and Sodium Loss Change Over Time

A real-world athlete case study shows that hydration and sweat composition is a moving target, not a fixed number. A blog post about pregnancy hydration for athletes that explains why sweat rate and sodium loss change over time.

Most hydration advice still assumes your sweat profile is stable: test once, get a number, build a plan, and repeat it forever. But sweat rate and sodium losses can shift meaningfully with intensity, weather, duration, heat acclimation, and physiological state, including pregnancy.

Pregnancy does not create a brand-new hydration problem. It exposes one that was already there: human hydration physiology is dynamic. Sweat rate, sodium losses, and thermal load shift with context, and those shifts can be large enough to change what “good” fueling looks like from one training block to the next. In other words, pregnancy is not an exception to hydration science.

This article uses an ongoing field case study from Nutritional Revolution, led by Kyla Channell, with professional gravel cyclist Sarah Sturm. The key point is that one-time hydration testing cannot fully describe a physiology that keeps moving.

1) The case study: real coaching, real sessions, real change

Sarah Sturm approached Kyla Channell and Nutritional Revolution to document her pregnancy while continuing to train and compete at a high level. The resulting work is longitudinal by design: repeated coaching sessions, repeated observations, and repeated adjustment decisions over time. That matters because pregnancy physiology is phase-dependent, not static.

This is also why the project is strategically important for hydration science. Most sweat data in sport are collected in single lab sessions or short windows. Those datasets are useful, but they miss transitions. Pregnancy gives us a rare opportunity to examine transitions directly in a real training environment where temperature, duration, workload, and recovery stress all interact.

With hDrop-based field monitoring plus prior baseline testing, Sarah’s observed ranges shifted:

*Interpretation is limited by differences in testing methods used across measurements.

Scientific nuance is essential: this project is still in progress. The increase in sweat rate appears clear in field observations. The increase in total sodium loss per hour is likely, given concurrent changes in sweat rate and concentration, but the team is still separating true physiological shift from normal day-to-day variability and measurement noise.

2) Why pregnancy changes the hydration equation

Pregnancy brings large systemic adaptations that directly affect fluid and heat regulation. Plasma volume expands substantially across gestation, commonly in the ~40-50% range by late pregnancy in healthy cohorts.^[1] Cardiac output also rises significantly during normal pregnancy, increasing circulatory and thermal transport demands.^[2] At the same time, endocrine and renal regulation supports sodium and water retention to maintain maternal-fetal homeostasis.^[3]

Put simply, the system is being re-tuned while the athlete is still training. The output you see at the skin level (sweat rate and sweat sodium concentration) should be expected to move as the internal system moves.

3) Thermoregulation is the real frame, not just “drinking enough”

Hydration discussions often get reduced to bottles per hour. That is too narrow. During exercise, evaporative sweat loss is the primary cooling pathway in many environments, and this is especially relevant when heat production rises.^[4] In pregnancy, where physiology is already adapting to support fetal development and maternal circulation, thermoregulation is not a side detail. It is central.

This framing also clarifies why hydration is a physiological constraint rather than a lifestyle preference. If heat dissipation cannot match heat production, thermal strain increases. Multiple evidence syntheses on exercise and passive heat stress in pregnancy suggest that controlling environmental load, exercise intensity, and total heat exposure is crucial for maternal-fetal safety.^[5] That is a heat-balance problem first. Hydration is one of the key levers that influences it.

So when an athlete asks, “How much should I drink?”, the stronger scientific question is: “What intake strategy supports heat dissipation and sodium balance for this session, in this environment, at this point in pregnancy?”

4) Sweat is output, not identity

One of the most persistent myths in sports hydration is that sweat sodium is a fixed personal trait. It is not. Even in non-pregnant athletes tested under controlled conditions, sweat rate and sweat sodium show meaningful within-person variability across days and conditions.^[4] The same review also highlights major between-person differences and methodological factors that affect measurements, reinforcing the need for context-aware interpretation.

In practice, your sweat response can shift with:

• Training intensity and duration
• Ambient temperature and humidity
• Heat acclimation status
• Clothing/airflow and terrain profile
• Hormonal state (including menstrual cycle phase and pregnancy stage)

That is why one number from one day is useful but incomplete. Sarah’s data trend is a good example: the important signal is not just the absolute value. It is the direction and consistency of change across repeated sessions tied to known training context.

5) Why one-time sweat testing fails in dynamic physiology

Single-session tests can establish a baseline. They cannot define a moving target by themselves. If your physiology is shifting and your training environment is shifting, static prescriptions will eventually misalign with real needs.

Pregnancy makes this visible quickly, but the same logic applies outside pregnancy as well. Athletes in heavy heat blocks, altitude camps, race-season build phases, and hormonal transition windows can all outgrow fixed hydration scripts.

The traditional workflow often looks like this:

1. Take one sweat test.
2. Assign one fluid target and one sodium target.
3. Repeat indefinitely.

The longitudinal workflow is different:

1. Start with baseline ranges.
2. Collect repeated field measurements under real training conditions.
3. Interpret trends alongside session context.
4. Adjust intake strategy as physiology and environment evolve.

That second workflow is what this case study is modeling in real time.

6) The shift: from fixed hydration plans to longitudinal hydration monitoring

Longitudinal hydration is a practical category shift. Instead of treating hydration data as a static identity label, it treats hydration as a time-series signal tied to workload and environment.

In this model, the key output is not a single “correct” sodium number. The key output is a decision framework:

• What is trending up or down?
• Under what session conditions?
• Are changes large enough to modify strategy now?
• How much uncertainty remains in the current estimate?

This is where hDrop is useful in a way static testing is not. It captures sweat rate and sweat sodium data during actual training sessions and allows repeated observations across weeks and phases. That supports dynamic decision-making rather than one-time prescription.

7) What the Sarah Sturm project enables right now

Through Nutritional Revolution’s ongoing coaching process with Sarah Sturm, the field setup enables several high-value outcomes:

• Tracking hydration-related changes across pregnancy phases instead of inferring them from one session
• Comparing like-with-like sessions to isolate probable physiology shifts
• Updating fluid and sodium strategy based on trend direction, not guesswork
• Building an individualized profile that includes uncertainty and context boundaries

This is also a stronger scientific communication model. Rather than over-claiming from a single dataset, the project explicitly treats findings as evolving evidence and keeps interpretation tethered to repeated measurement.

8) Practical takeaways for athletes and coaches

• Know your numbers, but do not freeze them. Baseline data are useful; trend data are better.
• Frame hydration as thermoregulation support. The target is stable heat balance under session-specific demands.
• Track context with every measurement. Intensity, environment, duration, and physiological phase must travel with the data.
• Adjust in ranges, not absolutes. Dynamic systems need adaptive plans with decision thresholds.
• Protect scientific honesty. Distinguish clear trends from preliminary signals and measurement noise.

The central point stands: pregnancy did not invent dynamic hydration. It revealed it.

Know your numbers with hDrop. More importantly, track how they change.

Sources

1. Aguree S, Gernand AD. Plasma volume expansion across healthy pregnancy: a systematic review and meta-analysis of longitudinal studies. BMC Pregnancy Childbirth. 2019;19:508. https://pubmed.ncbi.nlm.nih.gov/31856759/
2. Meah VL, et al. Cardiac output and related haemodynamics during pregnancy: a series of meta-analyses. Heart. 2016;102(7):518-526. https://pubmed.ncbi.nlm.nih.gov/26794234/
3. Soma-Pillay P, et al. Physiological changes in pregnancy. Cardiovasc J Afr. 2016;27(2):89-94. https://pubmed.ncbi.nlm.nih.gov/27213856/
4. Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med. 2017;47(Suppl 1):111-128. https://pmc.ncbi.nlm.nih.gov/articles/PMC5371639/
5. Ravanelli N, et al. Heat stress and fetal risk. Environmental limits for exercise and passive heat stress during pregnancy: a systematic review with best evidence synthesis. Br J Sports Med. 2019;53(13):799-805. https://pubmed.ncbi.nlm.nih.gov/29496695/
6. Capelli I, et al. Sodium balance in pregnancy and hypertensive disorders of pregnancy. J Nephrol. 2022;35(6):1755-1768. https://pubmed.ncbi.nlm.nih.gov/35713555/
7. Mottola MF, et al. 2019 Canadian guideline for physical activity throughout pregnancy. Br J Sports Med. 2018;52(21):1339-1346. https://pubmed.ncbi.nlm.nih.gov/30337444/
8. Mitchell HI, et al. Heat loss responses at rest and during exercise in pregnancy: A scoping review. Temp (Austin). 2022;9(1):61-79. https://pubmed.ncbi.nlm.nih.gov/34420641/
9. Nutritional Revolution case study context (Kyla Channell and Sarah Sturm). https://nutritional-revolution.com

Educational content only; not medical advice. Pregnancy training and hydration plans should be individualized with qualified clinical and coaching support.