Hydration in the heat: sodium, ice and the pre-cool

Heat slows every cyclist, and no drink undoes that. What hydration can do is stop a hot day becoming a dangerous one and keep the slowdown at the small end of the range. Three things matter in roughly this order: arrive at the start cool, drink enough that you do not lose much more than 2% of your body weight, and put sodium back in proportion to how salty you sweat. Ice does more than most riders expect; drinking to a rigid schedule does less.

Why heat costs watts in the first place

Périard's review of exercise under heat stress lays out the mechanism: the body diverts blood to the skin to shed heat, which competes with the muscles for cardiac output, and as core temperature climbs the brain also reduces how hard it lets you push. Dehydration makes both worse because it lowers blood volume. The ACSM position stand on fluid replacement puts the threshold for measurable performance loss at around 2% body-mass loss in warm conditions, and notes that losses above that also raise core temperature further for the same work. So the target is not "never lose any weight"; it is "keep the loss small".

The other thing to know is that heat acclimation changes everything. Racinais's consensus recommendations describe how one to two weeks of training in the heat lower core temperature, raise sweat rate and plasma volume, and reduce sweat sodium. If you have an event in the heat, the hydration advice below helps, but heat training helps more.

How much you sweat, and how salty it is

Baker's review of sweat testing gives the range: whole-body sweat rates in athletes span roughly 0.5 to 2 litres an hour depending on intensity, conditions and the individual, and sweat sodium runs from around 200 mg to 2,000 mg per litre, with an average near 1,000 mg per litre. That ten-fold spread in sodium is the reason generic "electrolyte" advice fits so few people. A light, low-sodium sweater and a heavy, salty one can lose a gram of sodium an hour between them at one end and eight at the other.

The cheap way to find your own number is to weigh yourself before and after a ride of known duration, nude or in dry kit, and add back what you drank: a kilo lost is roughly a litre of sweat. Sweat sodium needs a test, and our post on sweat testing goes through the options; failing that, white crust on black shorts and a stinging face are the usual tells for a salty sweater.

Sodium: what to put back and how

The Sports Dietitians Australia position statement on exercise in hot environments recommends that riders with large sweat and sodium losses, especially over more than two hours, include sodium in their drinks and food rather than relying on water alone, because plain water replaces volume without replacing the salt that holds it in the blood. The ACSM position stand suggests drinks containing roughly 460 to 690 mg of sodium per litre (20 to 30 mEq) during prolonged exercise. In practice:

• Moderate sweaters. A standard sports drink or electrolyte tablet in each bottle covers you. Most sit in or near the ACSM range.
• Salty sweaters on long rides. You will likely need more sodium per litre than a standard tablet provides. Salted food, salt capsules or a higher-sodium drink mix bring the total up. The Australian statement is explicit that this is individual and worth working out in training.
• Short rides. Under about ninety minutes, sodium replacement during the ride matters little; a salted meal afterwards does the job.

Cramp is the reason many riders reach for salt, and the evidence there is less clear than the marketing. Our post on electrolytes and cramp covers what is and is not known.

Pre-cooling: the part most riders skip

Starting with a lower core temperature gives you more room before you reach the point where the brain starts pulling the plug. Bongers's meta-analysis of cooling studies found that pre-cooling before exercise in the heat improved performance by a meaningful margin, that cooling during exercise ("per-cooling") helped by a similar amount, and that the methods that worked best mixed internal cooling (cold or ice drinks) with external (ice vests, cold towels).

The simplest internal method is ice slurry. Siegel's trial had runners drink about 7.5 g per kilo of ice slurry (just over half a litre for a 70 kg athlete) in the half hour before running in the heat; rectal temperature at the start was about 0.66°C lower than after a cold drink of the same volume, and time to exhaustion increased by nearly a fifth. That was running in a lab at 34°C, so treat the size of the gain as a lab number, but the direction is consistent across studies.

• How to do it. A slushy of crushed ice and sports drink, sipped over the last 20 to 30 minutes before the start. A domestic blender and a freezer are enough. Cold water works, just less well than slurry.
• What it does not do. It buys you perhaps half an hour of extra margin at the start. It does nothing for hour four. For long events you need to keep cooling going.
• Ice on the bike. Ice in the bottle keeps drinks cold for an hour or two; ice in a stocking down the back of the jersey, the classic Grand Tour trick, is per-cooling and the meta-analysis supports it. Neither is elegant. Both work.

Drinking on the ride: enough, not as much as possible

The ACSM position stand recommends a personalised plan that keeps body-mass loss under about 2% and avoids gaining weight, which for most riders in the heat lands somewhere between 0.4 and 0.8 litres an hour. Two bottles an hour is a reasonable starting point on a very hot day for a heavy sweater; one for a light sweater. Plan where the refills are: a hot day without a tap for three hours is a logistics failure, not a physiology one.

Over-drinking is a real risk and it is worth stating plainly. The international consensus statement on exercise-associated hyponatremia identifies drinking beyond thirst, and especially drinking more than sweat losses over several hours, as the main cause of dangerously low blood sodium; slower participants in long events are the most affected because they have the most hours to drink in. If you finish a hot ride heavier than you started, you drank too much. Our post on hyponatremia covers the warning signs.

A hot-day checklist

1. Two days before. Normal salted eating, normal drinking. No need to "pre-hydrate" beyond a glass or two of water with meals.
2. Morning. Half a litre of water or sports drink with breakfast, then nothing forced. Check urine is pale, not clear.
3. Last 30 minutes. Ice slurry or a very cold drink, around 500 ml. Stay in the shade. Warm up less than usual; you will be warm enough.
4. On the bike. A bottle every 45 to 60 minutes, sodium in it, more if you are a known salty sweater. Pour water over your head and neck at stops; evaporation is cooling too.
5. After. Weigh yourself. Replace about 1.5 times what you lost over the following hours, with salt, which is what the ACSM stand recommends for rapid rehydration.

If you are pacing a hot event by power, remember that the number you can hold will be lower than on a cool day. Our post on racing in the heat deals with adjusting targets; the point of everything above is to make that adjustment as small as your physiology allows.