Hydration for Triathletes: Hidden Gains

William Horkoff
Sep 1
3 min read

This is a topic that I have found extreamly interested in this past year as I think when most triathletes think about hydration, they think about race day aid stations, bottles on the bike, or gels on the run during a race/training. But the truth is this... hydration is a 24/7 performance variable. This is something you need to be priotizing not just around racing, but training as well. How well you manage your fluids and electrolytes throughout the week will allow you to get more out of yourself in training.

In this post, i'll break down why proper hydration matters for triathletes, what the science says, and how you can dial in your daily and training hydration strategy.

The Cost of Getting Dehydrated

It doesn’t take much. Research shows that losing as little as 1-

2% of body mass through sweat can impair endurance, power output, and cognitive sharpness. At 3-4%, performance falls off a cliff your cardiovascular system strains harder, body temperature spikes, and your ability to push through efforts nosedives. I feel like we have all had those sessions where you cramp, feel geniunley cooked & wish you carried more fluids/sodium.

For triathletes training 10–20+ hours per week, dehydration isn’t just about comfort. It’s about

Power on the bike - Plasma volume drops, your heart rate runs higher for the same watts.
Run economy - Dehydrated muscles lose elasticity, stride feels heavy.
Mental sharpness - Focus, pacing decisions, and reaction times all decline.
Recovery - Nutrient transport slows, glycogen replenishment suffers, inflammation stays higher.

If you’re under-hydrated, you’re leaving performance on the table.

(Sources: PMC 2021, Physio-pedia)

Why Daily Hydration Matters as Much as Training Hydration

Here’s the trap many triathletes fall into. Nailing bottles during the long ride on Saturday, but ignoring hydration Monday through Friday entirely.

Hydration is cumulative. If you show up to a session already hypohydrated, the stress stacks: higher HR, lower watts, slower paces. Over time, this compounds into poor adaptation. The same 60-minute tempo run that should be building your aerobic engine instead becomes a struggle session.

Elite dietitian's emphasize this point talk about this all the time, but hydration is a day-long, week-long practice not a single event before or during training. Even outside of sport, things like desk work, travel, altitude, and hot environments drive fluid losses that most athletes underestimate.

(Sources: Ironman.com, UCHealth)

Sweat, Sodium, and the Individual Factor

Hydration is not one-size-fits-all. Some triathletes lose 700 mg of sodium per liter of sweat, others lose 1,500+ mg. Add in differences in sweat volume (0.5 L/hour vs. 2+ L/hour) and the spread gets huge. For me personally this is something that I got tested before IRONMAN New Zealand in my (9:39 Performance) and found I loose around 1200mg sodium per L/Fluid lost.

Knowing your numbers is a performance advantage.

Practical ways to do this:

Weigh yourself pre and post-training to estimate sweat rate.
Book yourself into a PF&H Sweat Test Center These are offered all over the place. Highly recommend. This would be your best bet.
Look for salt stains on clothing or sting in the eyes signs of higher sodium loss.
Test in training don’t wait until race day to find out if your gut can handle 1L/hour or 800 mg sodium/hour.

(Source: JISSN 2018)

Practical Hydration Guidelines for Triathletes

While your exact numbers will vary, here are evidence-based starting points.

Daily Hydration

Aim for ~30-40 mL per kg of body weight across the day.
Spread fluids evenly, with ~80% consumed before 4 p.m. to avoid disrupted sleep.
Prioritize fluids with electrolytes (especially sodium) if you train more than once per day or live in hot conditions.

Before Training

(500–1500 mL) of fluid 2-3 hours before.
Add electrolytes if you’re a heavy/salty sweater.

During Training

400–2000mL per hour (adjust to sweat rate, conditions, and gut tolerance). Highly individual here
Include 500–1,500 mg sodium per hour in hot or long sessions. Equally as indivdual here

After Training

Replace 16–24 oz (500–750 mL) for every pound (0.5 kg) lost in sweat.
Include sodium to lock fluids in water alone won’t cut it.

(Sources: Nationwide Children’s, Stittsville Physio)

The Takeaway

Hydration is the quiet performance multiplier in triathlon. Get it wrong, and training feels harder, recovery drags, and race execution crumbles. Personally for me, my days start with drinking 1-1.5l of fluids with 800-1000mg sodium to kick start my day.

As a triathletes, it's not lie we already obsess over watts, pace, and nutrition. It’s time to put hydration in the same category as its equally as important to make sure its measured, personalized, and practiced daily.

As always, appreciate you for reading!