EFFECTS OF DEHYDRATION

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Published: 2nd April, 2010

Diagram supplied courtesy of TORQ Australia. Adapted from Wilmore & Costill (1999) 'Physiology of Sport & Exercise'. Human Kinetics.

The diagram demonstrates the potentially catastrophic effects of dehydration. For every 1% of bodyweight you lose through dehydration, you get a corresponding 5% drop in performance. This is a huge performance loss and by way of putting some figures to it, a slightly dehydrated individual who usually kicks out 300 watts at threshold (time trial effort) will drop off to around 285 watts. Suffice to say that races are won or lost by much smaller differences in power than this. If dehydration reaches 4-5% of bodyweight, performance drops a whopping 20-30% and a fluid loss of 9-12% can be fatal.

The physiological effects of dehydration are interesting. The fluid losses cause blood volume to drop and as your blood plasma loses water, it becomes thicker. This decreases blood pressure, which then reduces blood flow to the muscles and skin. As less blood reaches the skin, thermoregulatory efficiency (the control of body temperature) is reduced and heat is retained within the body. The worse the dehydration gets, the more pronounced this cycle becomes.

So if that's what happens when you dehydrate, what's the best way to prevent it? Drinking fluid seems like the obvious answer, but it's a bit more complex than that, because hydration isn't the only variable you need to consider when you're exercising, there's also fuelling. During prolonged endurance exercise, an incorrect fuelling strategy WILL spell disaster. As your carbohydrate stores start to run low, you'll start to feel faint and you'll have trouble concentrating. Once your carbohydrate stores are empty, you will rapidly and spectacularly lose power and it'll be one of those unique times in your adult life that you'll be crying for your mummy, because it really is rather unpleasant.

Although some sports nutrition products are better than others at delivering energy, the basic rule of thumb is that you need to consume 1 gram of carbohydrate per Kg of bodyweight per hour. Any more than this and you won't use it. Any less and you're selling yourself short (and you'll run out of stored carbohydrate more quickly). So, a 70Kg individual needs to feed on 70 grams of carbohydrate per hour.

So, let's get back to hydration. In order to prevent dehydration, you'll need to consume fluid. How much fluid you take on board will depend entirely on the environmental conditions that you're exercising in. If you're exercising indoors or in dry or hot conditions, you're going to lose more fluid than in cooler or more humid conditions. The paradox is that you actually feel like you're losing more fluid in humid conditions when actually you're not. Sweat drips off you, but because evaporation rates are lower, you won't actually perspire as much and your thermoregulation systems will be much less efficient at driving heat away from your body. So as not to confuse the matter though, let's make this assumption: You will perspire more and have greater fluid losses in hot than cold environmental conditions: You will perspire more at higher than lower exercise intensities and finally: You will perspire more in dry than humid environmental conditions.

When perspiration rates are high, you should aim to consume as much fluid as possible. Pure unadulterated water will not hydrate you as quickly as an energy drink mixed at a 6% concentration though because of the osmolarity (potential to diffuse) in the gut. Sports drinks that are marketed as 'Isotonic' are designed for this use, but providing your energy drink is mixed at 6% (60 grams of carbohydrate per litre) it will be in balance with your body fluids and will hydrate you fairly rapidly. This 6% carbohydrate content also has another benefit though, because it supplies you with 60 grams of carbohydrate per litre, so based on my fuelling comments earlier on, our 70Kg individual would need to consume approximately a litre and a quarter of 6% energy drink per hour to satisfy his/her fuelling requirements. At the same time, this is probably about the limit in terms of fluid intake that a person of this body weight would be able to handle. In short, the key to hydration and fuelling in hot environmental conditions is to drink as much 6% solution as you possibly can and if this means exceeding your fuelling needs, well done for drinking so much.

When perspiration rates are low, you will need to drink less to remain hydrated. It is still important to remain hydrated however, because the same basic rules apply. If you don't drink enough, you will dehydrate and you WILL suffer a performance loss. In this situation however, a 6% carbohydrate drink on its own isn't going to be your best solution, because you'll just end up filling your bladder if you try to drink enough to satisfy your fuelling requirements. You therefore have a couple of cooler weather hydration/fuelling options:

1) Mix a stronger energy drink. If you mix your energy drink at 9%, you'll get 90 grams of carbohydrate per litre from it. For the 70Kg rider, this would mean drinking just over 750ml of drink per hour. A stronger mix of energy drink would mean that that the rider would need to drink even less. Effectively you're satisfying your energy needs without the need to consume so much fluid.

2) Consume gels or bars. This would be my recommendation, because it gives you so much more flexibility. If it were possible to devise a reliable system whereby the strength of your energy drink was gauged by flinging open the window to your bedroom, sticking your finger outside and waving it briskly through the air, it would be splendid. "Ah, it's a 6% day, I shall fill my bottles like thus". Real life dictates that every situation is different and even if you did get it right for the first hour, the weather could change in the second hour and you're stuck with the decision you made during the earlier finger-waving thing. To this end, a much more sensible approach would be to mix your drinks at 6% anyway. If it turns out to be a high perspiration day, just drink as much of this solution as possible and you'll be fuelling/hydrating under the principles described earlier. If however it's cooler, you can drink enough of your 6% drink to keep hydrated and then take the shortfall of fuel in through bars or gels. For instance, if we take our 70 Kg individual and he/she consumes 500ml of 6% solution per hour to maintain hydration, the total fuel intake will have been 30 grams of carbs. This person is therefore fully hydrated, but is missing 40 grams of carbohydrate per hour. Work out how much carbohydrate your energy bars or gels contain in grams and work these into the equation. It might be that you need an energy bar per hour or two gels per hour in order to maintain your fuelling requirements? This shouldn't faze you. It's not a mobile maths exam, because with a little practice and logic, you should very quickly be able to figure out whether you're drinking enough to satisfy your fuelling requirements. Then you can decide roughly at what rate to consume your bars or gels.


As an aside, it's a little known fact that we are actually self-hydrating organisms. Through our metabolism (oxidative phosphorylation), we actually produce water as a bi-product and according to Wilmore & Costill, authors of 'Physiology of Sport & Exercise,' during rest we actually produce 150 to 250ml per day. In addition to this, our 70Kg cyclist will also produce about 150ml of water per hour during intense exercise. During very cool weather, this would help to explain why one has to get off the bike to have a pee every now and then. It's a combination of this canny self-hydrating mechanism and perhaps drinking a little too much for the environmental conditions.

Last and by no means least, there's the issue of electrolytes. These are dissolved salts that are capable of conducting electricity, so are vital for muscle and neural (nerve) function. They also play a major roll in maintaining fluid balance within the body. There are 5 electrolytes: Sodium, Chloride, Potassium, Magnesium and Calcium. The last one is less important than the other four and by far the most important are the first two. Having electrolytes in your energy drink has the following benefits:

1) They help to replace electrolytes lost through sweating (in case you hadn't noticed, sweat is salty). Sodium and Chloride help to maintain the volume of the blood and also help to transport nutrients into cells so that they can be used for energy production, tissue growth and repair. Potassium is present in much higher concentrations in the muscle cells than in the blood, so losses through sweating are much lower than with Sodium or Chloride. Potassium deficiency would typically be symbolised by muscle cramping. Low magnesium levels are linked to muscle fatigue and cramping too, but again losses through perspiration are less substantial than with Sodium and Chloride.

2) They prevent hyponatraemia. This is a rare condition that affects ultra endurance athletes and is also referred to as 'water intoxication'. If you consume water-only or an energy drink without electrolytes over a long period of time, the combination of sodium chloride loss through sweating and the dilution of the remaining salts in the blood steam with the fluid you're taking in can cause headaches, cramping, loss of strength and nausea. If left unchecked, this could become quite a serious condition.

To summarise, Ed Burkes's book 'Serious Cycling' makes the following recommendations with regard to the amounts of electrolytes that should be present per litre in an energy drink, so check yours:

Sodium: 400-1,100mg/l
Chloride: 500-1,500mg/l
Magnesium: 10-100mg/l
Potassium: 120-225mg/l

If you have found this subject interesting and would like some more comprehensive information on this subject and others relating to training and nutrition, TORQ have a special offer for XC Racer readers. We have a 52-page Performance Resource worth £5, which we will e-mail to you as an electronic copy free of charge if you send us an email to enquiries@torq.ltd.uk quoting "Free TORQ Performance Resource". If you would like a hard copy of this brochure, this can be ordered direct from the TORQ website www.torq.ltd.uk

So, in summary, when perspiration rates are high, do not consume bars or gels, just drink an electrolyte-containing energy drink mixed at 6% carbohydrate - and drink as much as you can. This is the quickest way to hydrate and you'll be fuelling yourself adequately by virtue of the fact that you're consuming significant quantities of this 6% solution. When perspiration rates are low, drink less or you'll be taking numerous comfort breaks and satisfy your energy needs through more concentrated 'dryer' forms of energy like gels and bars.

Most importantly - enjoy the ride…