Sitting on your butt right now, reading this, your metabolic rate (the energy consumed by all cellular processes in your body) is not much over 1x that of your resting, or "basal", metabolic rate (BMR). If you're an endurance athlete (a special kind of deranged masochist) your daily output might be 3-4x your BMR. Tour de France athletes, during the race, achieve upwards of 5-7x BMR daily. It is thought that this is the upper limit for pretty much all warm-blooded animals-- this is the"metabolic ceiling", the highest caloric output our bodies can sustain. But why? What, specifically, limits an organism's metabolic ceiling?
Let's briefly back up and consider a simple equation that describes what must happen with all of the energy that an organism takes in.
energy input = energy output ± energy storage
Basically this equation says "an organism uses some of the energy it takes in, and it stores the rest for later use." The right side of this equation is comprised of energy output (the sum total of all metabolic processes happening in cells), and energy storage. The energy storage part, in animals, mostly means energy stored as fat (and a little bit as carbohydrate, in the form of glycogen).
Biologists have long thought that the left side of this equation limits the maximal metabolic rate of an organism, its metabolic ceiling. Put another way: an organism's ability get energy (how good it is at getting food, digesting food, and getting oxygen to cells to use that energy) limits how far and fast an animal can move, and how much energy it can devote to immune function, growth and reproduction. This idea applies to human exercise too. The amount of metabolic work that your muscles can do is affected by all sorts of factors, like glycogen and fat availability, enzymes, bloodflow, etc., but it is thought to be ultimately limited by oxygen availability. That is, if you could get more oxygen to your muscles, they'd be able to perform more work and you'd go faster/higher/stronger.
But in some conditions, organisms may be limited by their capacity to expend energy (the right side of the equation), not acquire it. Some predictions: the amount of milk a mammal can produce is limited not by how much energy she can take in, but in the processes that use this energy to make milk, such as processes in the mammary glands; a human runner is limited by her ability to break down carbohydrates and convert them into ATP (this probably isn't true but it's an example of this kind of thinking).
So, under what conditions might the metabolic ceiling--the total number of calories converted into work-- be limited by these sorts of factors? For one, maximal exercise. And of the factors that may limit total work performed during exercise, one stands out: Heat.
Heat is generated as a byproduct of metabolism. Endotherms (warm-blooded animals) rely on this principle to generate body heat. And we endotherms do produce a ton of heat-- not just on purpose to stay warm (explanation here ), but also any time our cells break down food molecules for energy. This heat probably has all kinds of effects on other cell processes and may inhibit some of them. In this way, some things that an animal must do- such as produce milk-- may be effectively limited by heat production. This idea, that heat produced from metabolism places an upper limit on metabolic rate, is called, appropriately, Heat Dissipation Limit Theory, or HDL.
Above we considered the dogma that oxygen delivery to muscles places an upper limit on how much work muscles can do, and therefore how far/fast you can run. (It's worth noting that we're talking here about a person who is well-trained-- they can store adequate glycogen, they have adequate enzymes, a well-developed cardiovascular system to deliver oxygen and glucose, etc...we're talking about a person who has developed each of these capabilities to near their fullest extent. We're saying that, with all systems optimized, heat buildup is the limiting factor in performance.) So this brings me to my suggestion: a human's ability to shed the heat produced within their muscles constrains their overall athletic performance. Getting better at delivering oxygen to muscles is great, but by design, we are constrained by our ability to shed heat. While humans have several ways of cooling, including heat loss through the mouth and airway and convection from blood flow at the skin surface, sweating is our primary means. And because my contention (not my idea, but one that I advocate) is that human evolution was strongly influenced by the need to cover distance, it follows that our capacity to sweat places a clear constraint on how humans and human ancestors moved around their environment, one that is far more profound than most researchers have considered. This idea is supported by theory and some observations, such as recent tests showing that a cooling device that sucks heat out through the fingers improves performance. If this weird device was shown to increase the metabolic ceiling-- if it allowed those Tour de France athletes to achieve not 5-7x BMR but even greater values-- that would seem to confirm the hypothesis that heat limits maximal metabolism.
Soon to come--Sweating Part 2: Observations on Human Sweating. What do we know about the biology of sweating? When did it likely evolve? What questions remain?
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