Keep freediving your best: hydration and nutrition

Keep freediving your best: hydration and nutrition

We lose a lot of fluids during a freediving session. With those fluids go important salts and electrolytes. Adequate hydration ensures the fluids and salts are replaced, but it can be a challenge!

I recently ran my first trail running race. A 55 km (35 miles) mountain race with 2.8 km (9000 feet) elevation gain. Apart from learning (again) that I’m not built for running, I did learn many things that I’ll incorporate into my freediving. I also learned that running an ultra-marathon as a first race is a questionable idea. Regardless, in this article we’ll look at how to keep your body functioning with adequate hydration and nutrition.

Image by Free-Photos from Pixabay

Many freedivers have contacted me in the past year about cross-training. A common question is whether cross training on an empty stomach, slightly dehydrated, or even after a long fast helps to adapt. The short answer is no, the long answer is the rest of this article.

Salt, water, fuel

Depending on the weather, I can run for about 2-3 hours on an empty stomach, drinking only water (on a normal, non-keto diet). One particular dry and sunny day, I did a 25 km run that I thought would take me just about that amount of time. I packed a few bars, took 3 liters of water and set out.

5 hours later, I walked into building and started to shiver uncontrollably. I managed to eat and drink, but couldn’t keep it in. It was the most uncomfortable running experience ever.

So what went wrong? Hydration.

The trail was much more difficult than I expected, and I could not run a large part of it. Furthermore, as a bigger person (6’2″ and 190 lbs) I have a large volume to surface area ratio. This means I thermo-regulate less effectively than smaller people. Although I ran later in the day, I sweat much more than I thought I would.

Losing sweat means losing salt. And losing salt increases the risk of cramping. Although I had 3 liters of water with me, I did not bring any salt pills or electrolyte solution. I had no hydration protocol, and didn’t know I needed it.

Unfortunately, I did need it. The last 6 km of the trail, even though it was fairly flat and simple, took me 2 hours. Muscles I didn’t know existed cramped with every step I took.

I was at about 1100 meters of elevation, and evening was around the corner as I reached my end point. Because I was running slowly, I generated minimal heat, and my body was cooling down. It didn’t bother me. Like many cold-water freedivers I am well adapted to the cold. But being so low on energy after 5 hours of exercise, the after-drop was horrendous. I simply did not have the energy to heat myself up.

After-drop is the effect that occurs ‘after’ cold exposure. When vasoconstriction reduces the cold blood near the skin mixes with the warm blood in the core. The result is a sudden drop in core temperature. The after-drop is the reason we often shiver after, rather than during a dive.

It took about an hour from the time I stopped running until I could drink and eat again. Thermo-regulation doesn’t work very well when the body runs out of fuel. I went to the nearest pizza shop wearing a down jacket in the middle of summer. Halfway through my meal I suddenly felt like I was on fire.

Implications for cross-training

Cross-training does not work very well if you are not well-hydrated. Not because you won’t be able to do the training. After all, you can probably still do a cycling interval session relatively dehydrated. The real problem is recovery and adaptation.

Training dehydrated your body will have to work harder to get nutrients where they need to be. It is also more difficult to remove waste metabolites from working muscles. You won’t be able to push as hard as when you are not hydrated and nourished, and your recovery will be longer. Training adaptation will be poorer because:

  1. You couldn’t push as hard in the first place and,
  2. You will need to rest longer before you can exercise again.

Remember that intense muscle soreness is not usually the result of a good training session. Adaptation is a result from training stress and recovery.

If you are well hydrated and nourished you should feel good after you complete your exercise. Sure you might be tired, thirsty and hungry, but you should start recovering after you drink water and eat.

As an extreme example, the terrible run I just described caused me to take 3 days of rest. I used Heart Rate Variability (HRV) and resting heart rate to determine when I was recovered. During that four day period I could have done two interval sessions and a recovery run. It would have been much more effective and taken me less time overall!

Remember that Over Training Syndrome (OTS) is an issue for many high level athletes. From competitive cycling to swimming, OTS can have detrimental physical and psychological setbacks. In, Longer and Deeper, I discuss the importance of “tapering” in high-intensity training.

Implications for longer diving sessions


Staying hydrated during a freediving session is somewhat of a challenge. You end up urinating (diuresis) a lot simply because of the immersion and dive response, and drinking is not always first priority. An additional challenge is freediving in cold water during the summertime. As soon as you have your wetsuit on, the sweating starts and you have to rush into the water simply to cool down! Thermoclines will add another level of difficulty – you might be sweating at the surface but freezing cold at depth.

Hydration is not just about water. It is about maintaining a perfect electrolyte balance. The most important electrolytes are sodium, chlorite and magnesium. You can find these in gatorade and salt pills.

As a rule of thumb;

  • If you are diving for less than one hour and you have eaten a meal in the past few hours, don’t worry about replacing electrolytes. You can get the electrolytes from food, so drink water to replace the fluids.
  • If you like to dive on an empty stomach, or if you dive for a long time, consider electrolyte drinks or pills. Electrolyte drinks commonly contain some sugar. This is not only for energy, the sugar actually helps increase water uptake.
  • And of course remember that if you go for the pills you still need to drink water for hydration. Here are the salt pills I use for exercise and to stave off the keto-flu (affiliate link: Roctane electrolyte pills)

How much you should take depends per individual. Sweat rates during exercise can range from practically nothing to over 3 litres per hour. That means if you are out of the water for 10 minutes to rig the line with an 8mm wetsuit on in summer, you could lose half a litre (a pint) of water. Signs of dehydration are thirst, little and strongly coloured pee, headaches, and cramps.

Urine colour chart infographic

Hydration infographic courtesy of Healthdirect Australia.


Eating during longer dive sessions can be equally challenging. Most of us like to have a fairly empty stomach during our dives. However, you will certainly have to eat to keep up consistent performance.

Anytime you exercise, you are likely burning some fats and some glucose. Whenever the caloric expenditure goes up, you burn more glucose, since it is more readily accessible.

During a dive you are likely losing at least 250 kcal per hour as heat, whether you dive in warm or cold water. Add to this your duck-dives, kicking, fighting that monster-tuna, and safety diving, and you can easily get to 750 to 1000 kcal per hour.

You simply cannot expect to keep up this type of energy expenditure for an entire day without eating. Maybe if you’re very well-adapted to a keto diet, but you will still have to ensure adequate hydration.

In ultra-running, the rule of thumb is to replace at least 40% of your calories lost with food (affiliate link: Training essentials for ultra-running) . Food here implies normal whole foods with carbs, fats and proteins, not just candy bars. Considering the energy expenditure may be roughly the same, I would suggest you try the same during a long day of diving. Take a break every 2-3 hours and eat something, and don’t forget to also drink in the meantime.

Many divers lose considerable weight during long bouts of training. If you are doing long sessions days or even weeks in a row, try to eat a bit more in the evening so that you don’t become weary and fatigued as time wears on.

Implications for competitive freediving

Whether drinking sports drinks or being well-hydrated affects your freediving has never clinically been studied. However, we know from many well-studied sports such as running that performance declines as hydration declines. This decline increases until the dehydration is an outright health risk.

Performance during endurance exercise increases when athletes drink sport drinks as opposed to water or a placebo. Whether performance during sprints increases is still debated. It seems to in soccer, but doesn’t in basketball (more in this article).

A large part of a competitive dive is anaerobic, and so resembles a sprint. So it is probably safe to assume that a sports drink is not going to make you a better diver. However, dehydration will almost certainly make you worse. Try to start your dive well-hydrated, and if you tend to sweat a lot consider some electrolyte supplements before your dive.


Jaap is a geologist by trade and a freediver by passion. Jaap wrote the book Longer and Deeper in 2018. His book teaches how to train for freediving and spearfishing on land.

This Post Has 4 Comments

  1. Connor Davis

    Good article! Thanks for some good insights.

    Very glad to see somebody confirm my observation of weight loss on diving trips. Mine tend to be 3 divers, 10 days, warm water, diving 3-5 hours a day. Divers that go full tilt often lose 5-10 lbs, sometimes more. I’ve had people argue with me that it must be water weight, too fast to be anything else, but I knew that wasn’t right. With a modest breakfast, modest lunch and big supper (our pattern) we just can’t replace the calories fast enough. I usually start a trip with just a little bit of fat around my waist. Its gone at the end of the trip and I’m pretty sure I lose a good bit of muscle mass as well. Performance begins to suffer about day 5. Not sure if that is lack of fuel or what. It takes a good six months or more to put it all back on.

    Hydration is clearly an issue with us(its hot) but we drink much less than it seems like we should, even when I’m focusing on drinking. Not sure why that is, but I keep pretty good track of water consumption and it is always a good bit less than 1 gallon per person per day, sometimes close to half that. Something I wondered about, do divers absorb water through the skin when submerged?

    Dry land exercise, I feel dehydration, it makes a big difference. Not so obvious when diving.

    Just an aside, urine color is all off when diving. Increased urine flow during water immersion means that you may be peeing clear and still getting dehydrated.

  2. Jaap

    Hi Connor, you’re totally right, urine color is off when diving because of immersion diuresis, but will ‘normalize’ shortly after. Regardless, it’s kind of hard to keep track off when diving.

    Perhaps a nearly constant state of mild dehydration also has something to do with the weight and muscle loss during multiple days of diving.

  3. Connor Davis

    ” hard to keep track off when diving.”?? Why would that be?, just look down. . . . Oh yeah, you guys have to wear wetsuits, never mind.

    We thought about water weight loss from dehydration, and a couple of pounds of water loss do come back quick once I get home, but that doesn’t account for near all of it.

    1. Jaap

      ha ha ha… Our visibility is so shoddy we wouldn’t notice without a wetsuit…

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