It's important to replenish electrolytes as well as drink water.

When it’s hot and humid, water isn’t enough. Hyponatremia (low sodium) and hypokalemia (low potassium) can be serious, even fatal. A year ago, I experienced an electrolyte imbalance on a hot, humid ride in April. I drank a bottle of water every hour, which is a good volume. I sweated buckets. After 49 miles—just 1 mile from my house—I couldn’t pedal another inch. I couldn’t stand another swallow of water. I waited for the nausea to subside, but it didn’t, and finally I got a ride for that last mile. Then I ate a lot of salty food.

People sweat on average a liter per hour while working out. I was curious how much I sweat, so a 92 °F, humid afternoon gave me perfect opportunity. To measure how much you sweat, weigh yourself before and after your workout. The difference is the amount of water lost, and 1 pound = 0.45 liters. After 35 hot, brutal, and dehydrated minutes of running, I lost 1 pound, which works out to 0.8 liters per hour.

One liter of sweat contains 0.9 g sodium and 0.2 g potassium. You have to replenish these electrolytes as well as water.

A 5.5 oz can of V8 contains 0.45 g sodium and 0.32 g potassium. These fit neatly into a jersey pocket. Drink 2 per hour. These little cans are easy to drink in one gulp, like a shot!

Potatoes, particularly the skins, and bananas are rich in potassium. A serving of Kettle chips (9 chips) contains 0.18 g sodium and 0.42 g potassium. A medium banana has 0.42 g potassium. Pair these with something salty, like pickles or pretzels.

A Nuun tablet contains 0.36 g sodium and 0.1 g potassium. It also contains magnesium and calcium. These dissolve in a bottle of water. You need 3 an hour.

A 12 oz bottle of Gatorade contains 0.16 g sodium and 0.045 g potassium. You have to drink 5 ½ bottles every hour to get enough salt! Gatorade has a lot of sugar, so this is not really the best sports drink.

I did a lot of this research on the web, but I also consulted a doctor who specialized in sports medicine. He confirmed what I had learned, and said that he made his marathon runners drink a V8 every 20 minutes.

Since my electrolyte episode last year, I bike with V8. (Even so, it’s taken a while to get the dose right. I need more than I think I do!) I noticed right away that after a long, hot bike ride, I wasn’t wiped out for the rest of the day. I could take a little break after my ride and still have energy to mow the yard, clean the kitchen, and go for a walk in the evening. I also noticed that I didn’t need to eat nonstop when I got home. I used to get very hungry on a long bike ride, even if I had food with me. I would eat and eat and eat. It turns out that it wasn’t the calories my body was trying to replenish. It was the salt.

If you are worried about hypertension, recent research suggests that salt may not be the evil villain we have thought for so long, and exercise is far more important than salt intake in reducing health risks, particularly cardiovascular disease.

Sweat also contains magnesium, calcium, and trace minerals including iron. You probably don’t need to replenish these while exercising, but you might try to get a little extra in your diet on days that you sweat a lot.

An interesting fact about calcium is that vitamin D controls calcium uptake. You might have plenty of calcium in your diet, but without vitamin D you are unable to absorb it. It can be more important to increase your vitamin D. You don’t have to eat foods fortified or naturally rich in vitamin D, because our body makes its own. But it requires ultraviolet radiation, which is why vitamin D is known as the “sunshine vitamin”. Sunscreen blocks UV rays, but you don’t need much UV for vitamin D production. This only helps from early March to October. During the winter, the angle of the UV rays is too low to help us make vitamin D.

Therefore, don’t worry much about losing calcium to sweat if you are biking or running in the sun.

Similarly, vitamin C helps with iron absorption. V8 has a lot of vitamin C, and an orange is the best post-ride snack.