In my previous post I left you with this chart:



















This chart is an estimation of the cardiac output distribution in four different scenarios. Cardiac output is the flow rate through the heart, which is the product of the heart rate and the stroke volume (CO=HR*SV). Heart rate, as you know, increases with exercise, but also increases with other stressors such as elevated core temperature. Stroke volume also increases with exercise (in a healthy individual such as an athlete). The height of each column in the chart depicts the cardiac output whether cool or hot, and whether at rest or exercising. When cool at rest, cardiac output is about 5 liters per minute, and the majority of that blood flow is sent to your organs that require a fairly consistent amount of blood flow to function. When hot at rest cardiac output increases to send more blood to the skin for cooling. The viscera and muscle blood flow does not change much. Exercise drastically increases cardiac output to near your physiologic maximum to meet the demand to the muscles, and also sends more blood to the skin for cooling. If you are hot while exercising, your body is forced to send even more blood to the skin for cooling, and must steal that blood from your muscles.

When you are hot you cannot exercise as effectively because you cannot deliver the amount of blood to your muscles as you can when you are cool. But what about acclimatization? Acclimatization is a progressively improving physiologic response to exercising in heat. An unacclimatized person can produce about 1 liter of sweat per hour whereas an acclimatized person initiates sweating earlier and can sweat 2-3 liters per hour. An acclimatized person also produces much more aldosterone which conserves sodium from the sweat. An unacclimatized person sweating profusely can lose 15-30 grams of sodium in a day. An acclimatized person on the other hand will only lose 3-5 grams of sodium in a day, even though they are sweating more!

There is still a lot of talk about high level of salt intake in our sport. Usually people are concerned about “cramping” from low sodium. This remains a controversial topic among coaches and athletes, but to contribute to the controversy I will give my opinion here (and in my next post). Unfortunately most of the opinions you read are stated as fact by people with no scientific background.

The typical logic goes like this: There is salt in my sweat. I sweat more when I exercise in the heat. I need to replace that salt. If I don’t replace that salt I will cramp. When that person experiences cramping they fall back on that they should have eaten more salt. The same logic happens when people feel tired and they assume they should have eaten more food. We like to look for the easy fix, in this case salt tabs. What about training?

Studies have been done on Ironman athletes comparing those that take in high levels of salt, and those that do not. Their plasma sodium levels are not appreciably different, and either are their cramping experiences. The difference is the amount of sodium in their sweat. The more salt they take in, the more they excrete in their sweat. An acclimatized person however should have a high plasma aldosterone and be retaining their sodium. What happens with a high salt diet? You could probably guess it. Plasma aldosterone levels are reduced. Your body is trying to get rid of that extra salt load in your sweat and urine. Doesn’t it make sense to utilize your training and acclimatization rather than fight it?

The unacclimatized individual will lose more salt, and when forcing fluids is at greater risk of having problems such as hyponatremia so may consider increasing salt intake during the acclimatization period if it is hot and the training or event is long (>3 hours). This is not your typical well trained athlete however. What about the 3-4 grams of sodium an acclimatized athlete loses? Well, a modest amount of sodium intake is fine, which occurs anyway when drinking sports drink and eating food.

And what about cramping? I will address that in the next part.

How do you go about becoming acclimatized? Simply by being aerobically fit you are ahead of the game. Especially if you have years of aerobic exercise experience, your heart is likely more efficient, and able to deliver a higher cardiac output (due to a larger stroke volume). Even when exercising in cool weather your body has to adapt to cooling, so just by being aerobically fit you are somewhat more acclimatized than an inactive individual. Larger people have more trouble cooling because of a lower surface area to body mass ratio, and fat in insulating. Losing excess body weight can also assist in cooling. The acclimatization that we are primarily looking for though occurs when the body is exercised in heat. This is best done progressively. Acclimatization actually begins early (the first week of heat training), but the full effect can take up to six weeks, and if done cautiously and progressively can actually take longer. During particularly hot periods (heat waves) there can be progressive sodium loss and progressive dehydration, so taking some easier days and avoiding exercising in the hottest part of the day for several consecutive days is prudent.

What is the fate of the athletes at Ironman St George? It is so early in the season that most of us have not had the opportunity to acclimatize well. There will still be people that race well, but I anticipate a large attrition from the heat.

In the next part I will address heat injuries.