Cycling when it is hot and humid

July 2015 has been a month of two halves. One hot, dry and lovely cloudless skies while the other has been stormy, cloudy and rainy. This pretty much sums up the British summertime which is why you see so many cyclists out on the road when the sun finally shines.

Riding in the heat needs special attention. That’s especially true somewhere like Britain where true heat is rare. We’re not geared up for it, we’re not used to it and too often, we cyclists don’t take it into proper consideration when out riding. Add humidity into the mix and cycling begins to get complicated.

How our body manages heat

Anyone who managed to stay awake in Biology knows that the body is an exceptionally adaptable organism but not a very efficient one. As our body performs its daily duties, it produces heat as a by-product. That’s why we have a warm body temperature.

As we add physical activity into the mix, our muscles also produce heat. When this heat raises our temperature above normal our body seeks to lose it. Blood takes the heat away from the muscles to the skin where it is radiated out. The technical term is conduction for the blood taking the heat and radiation for when the body lets it out the skin.

This method works well when the exterior temperature is lower than our core temperature. Radiation doesn’t work if it’s warmer outside than you are inside. That’s when evaporative cooling comes in. That’s sweat to those of us without doctorates. Liquid is a far better conductor than air, which is why we have evolved to sweat.

When our core temperature reaches a certain level, the body triggers our sweat glands. These glands produce liquid which takes the heat from the skin and uses the higher external temperature to evaporate it off the body. This is a very efficient way to cool the body as long as the outside temperature is relatively dry.

Add humidity into the mix and things begin to get problematic. Humid air is already laden with moisture. That means it doesn’t always have room for more, i.e. your sweat. So in humid conditions, neither radiation nor evaporative cooling is very effective.

The importance of hydration

When you begin sweating, your body begins losing liquid. As most of you will know, dehydration not only harms the body, but more importantly it harms physical performance. Dehydration occurs when the body loses more fluid than you’re taking on and can be a serious issue.

Of all the challenges facing a cyclist, dehydration is one of the most serious.

Water makes up a significant portion of our body mass. It’s everywhere, but most importantly it is in our blood. As a cyclists relies on blood to transport energy and oxygen to our muscles, blood is very important to us. As we dehydrate, our blood gets thicker, meaning our heart has to work harder to pump it around the body and it doesn’t flow anywhere as quickly or as smoothly as we need it to.

This reduces the energy and oxygen getting to our muscles and reduces the conduction and radiation effect of cooling. All bad news. Add in that extra pressure on the heart and you begin to see how important hydration is. It’s serious enough without even mentioning extra stress on the other organs, the effect it has on our sodium levels and everything else that happens.

So, dehydration lowers athletic performance, thickens the blood, adds stress to the heart and impairs our ability to lose heat.

Symptoms of dehydration

So we know that dehydration is bad but how do we recognise it? As usual when talking about our bodies, there is no hard and fast rule. While the body is amazing at many things, it’s not too good at letting us know how thirsty we are. Hunger and thirst signals are not prioritised by our nervous system so can take a while to get through.

So it’s up to us to pre-empt those signals and take on enough water to prevent dehydration as much as possible.

Symptoms of dehydration usually include:

  • Thirst
  • Dizziness or light-headedness
  • Dry mouth and lips
  • Headache
  • Sluggishness or lethargy
  • Heart palpitations

How liquid much is enough?

Deciding how much liquid to take on is very subjective. Much depends on your level of fitness, how hard you’re working, how much you’re sweating, how hot it is and how thirsty you are. While there is such a thing as over hydration, in the grand scheme of things, it’s better to tackle dehydration than over hydration.

Geraint Thomas did a piece for the BBC regarding the 2014 Tour de France in which he said “I had been drinking around three bottles of fluid every hour – 1.5 litres – to keep myself hydrated and to ride at that threshold.” (Source)

Now us mere mortals won’t need quite that much and the conventional wisdom is to drink a 500ml bottle of liquid every hour. As mentioned, this depends on you, the temperature, your work rate and so on. It’s going to be trial and error until you figure it out for yourself I’m afraid.

All I would say is that while over hydration is a risk, it’s far better to drink too much than not enough!

What to drink

What to drink while on a ride again depends on you, how far you’re going and your work rate. A mix of water, squash and an electrolyte drink to replenish stocks is suggested. If you’re only out for an hour or two in the cool, you may not need electrolytes. If you’re out for longer or if it’s warmer, you might.

I tend to have a 750nl bottle of squash and another bottle of isotonic drink or a High5 tablet on longer rides. I will then drink regularly out of each throughout the ride. Sip little and often and begin before you start getting thirsty.

This combination of squash and something else works well for me, but might not suit you. As with how much to drink, deciding what to drink is a personal choice. As long as you mix water and isotonic drink for longer rides, you should be fine. Just make sure you drink something that will rehydrate you every time you go out riding.


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