Body Temperature: a Metabolic Thermostat

Ilana Katz MS, RD, CSSD

It seems the climate in most areas is a topic of conversation these days. It’s not that we are making small talk with weather conditions, but the unusual climates are setting in. Freezing way before winter starts, or a few hot days emerging in the midst of a cold spell, and vice versa for Summer. So how does a cyclist (or endurance athlete) know how to dress, or even eat accordingly, when they could experience four seasons in a matter of hours. In this article I intend to address how temperature changes ones metabolism, and if you as an athlete can use varying nutrition to take the most advantage of what Mother Nature has to offer.

As body temperature increases, so does metabolism. Thus temperature and metabolism become directly related and metabolism itself is directly affected  by all daily living components, that is eating, sleeping, exercise and physical exertion.

Body Temperature and Metabolism

Working muscles cause the internal body temperature to increase. The physiology of blood transporting oxygen as needed  and removing the byproduct requires the metabolism of calories, or as we know it, food energy. This further translates into the more activity, the higher the body temperature, and the ultimate requirement an increase in the metabolism of more calories. So the question becomes what happens in cold weather. A cold body temperature decreases the chance of burning more calories. So one would think that  if you have not dressed warmly enough for a long ride in winter, you may jeopardize your chances of burning calories. Actually, the opposite is true, and here is a good time to introduce the difference between internal and external body temperature. Internal body temperature will rise when a body is active, this is a given. When externally the temperature is extremely cold, physiology takes over to ensure the internal temperature is safe and warm, hence the effect of shivering. Simply put, movement of bones and muscles become involuntary to generate heat. So how does food choices on a long ride effect this, actually very little. The important thing is the cyclist is giving the body enough food to provide the fuel the metabolism is demanding.

Types of Metabolism and Body Temperature

Most of the calories burned on a daily basis actually takes place during resting periods, also known as basal metabolism. Basal metabolism includes all of the body’s natural functions required to sustain life. The basal metabolism varies from individual to individual, typically affected by health conditions, body composition and body temperatures.

Through your cycling efforts, more calories become metabolized due to the increase in the body’s temperature through working muscle tissue. Calories required for this and other physical activity account for some of the calories needed for basal metabolism, but not as many as most people think. In other words, if you catch yourself allowing a feast fit for a king because you rode a couple of hours, you may be missing the point, especially if you are trying to get leaner. For an optimal burn in fat, not every calorie needs to be replenished post work out, and furthermore, the body has a lot more potential to store calories (stored calories = fat) as it becomes more of an efficient machine. Aerobic endurance (which cycling certainly is) has the potential to teach your body to become efficient, which also means, you would require less fuel to get through the same ride today, as you did a year ago.

The amount of calories metabolized through exercise and muscle exertion does however increases the body’s basal metabolism over time, and certainly temporarily after a long ride. But remember basal metabolism, is different to the fuel needed for during a long ride.

There are many calorie calculators (apps and online) that can give you an average caloric burn per hour. But be warned, these calculators are all basing the calculations on the middle of a bell shaped curve (average) of what you enter in to them.  For example the amount of caloric burn for an average size male (170 lbs)  riding at about 16 – 18 mph is about 800 calories. My point is, there is no way the calculator can know where this individuals basal metabolic rate lies, what his muscle mass versus fat mass is, if he has a history of long consistent endurance training or is a newbie – all these factors, and more, affect this calculation.

There is a silver lining even as we become metabolically efficient, and that is that merely just eating and digesting food,  the body metabolizes some extra. This natural process, often referred to as the thermic effect, increases through the process of eating healthy foods and remaining active. Dehydration and excessively decreasing body temperature reduces the body’s ability to digest food properly.

Muscle Demands

Both muscle strengthening (resistance exercises) and aerobic or cardiovascular activities such as cycling, increase body temperature and break down muscle tissue. In order for muscles to grow and repair broken down fibers, the muscle tissue requires calorie metabolism in order to carry out the functions. This is a continual process which has further demands on the metabolism to work hard consistently, even at rest. In other words, during sleep, muscle groups continue to repair. This is a great physiological explanation of why breaking down and repairing muscle with the correct recovery nutrition increases ones metabolism. In other words, that same 170 lb male in the example above, may actually burn 1000 calories versus 800 per hour cycling, if he has more lean muscle and less fat.

In Summary

The human body regulates internal temperature in order to maintain a steady rate in which normal functions occur. As mentioned above, during the winter or in a very cold room, a human will normally shiver. This natural process represents the body fighting in order to keep muscles warm and the body temperature stable. The direct opposite of this situation, when external temperatures become extremely hot or even warm, the body sweats in order to keep the body cool. This process does not increase metabolism and neither does shivering, simply because neither of the two raise or lower body temperature, but rather fight to maintain a stable rate. Therefore it is irrelevant as to what the temperature of the food you are eating on the bike, in hot or cold extremes, as long as the food is fuel to compensate the metabolic burn rate.