I have recently introduced you to the book Body by Science by Doug McDuff, MD and John Little. This book offers a clear explanation of the actual science of exercise, how activity relates to hormones, and how this determines what happens in your body. In short, it answers the question I am frequently asked. “Why am I gaining weight when I am working out every day?”

In the introductory article I summarized his key points. In this article I will address one of those key points. The effectiveness of exercise is all about hormones, fat metabolism and blood glucose levels. Hormones signal the body to burn fat and to store fat. High intensity training works the major muscle groups to exhaustion, uses up glucose, and encourages the body to burn fat and build muscle.

First, here’s a little background on fat. Fat is not the evil it is made out to be – either the fat we eat or the fat on our body. Fat (and the ability to store fat) is why humans have survived the many droughts, famines, and lean times in our history. Fat is how we store energy for future use. As I like to say, there were no refrigerators in the Garden of Eden. Having food available 24/7 is somewhat new in human history. Our ancestors ate when food was available and stored the excess as fat, allowing us to survive.

There’s also a popular myth that our hunter-gatherer ancestors were way more active than we are today, but there is little evidence of that. In fact it is not very likely. Since they didn’t have food all the time they would need to conserve their energy to find food and survive! Modern obesity is not from lack of exercise, but from too much food (and poor quality food at that – although that’s another series of articles!). For most of human history fat storage was essential to survive. Unfortunately today it now leads to obesity and chronic disease.

As mentioned earlier the key to exercise is how it impacts hormones. To understand this we need to look at how our body accesses and stores energy. The following explanation is simplified for the purposes of this discussion. It starts with our body breaking down food into useable components. One of these is glucose which goes into the blood stream. From there, glucose enters cells and is used to produce energy. Excess glucose, beyond what the cells are calling for, needs to be stored. The first storage sites are the muscles and liver where it is stored as glycogen. After that it is stored in fat cells.

The main benefit of high intensity training is the depletion of glycogen stores from the muscles. When glycogen is moving out of storage the hormone called hormone sensitive lipase is released. This allows body fat to be mobilized and burned for energy. Eventually the glycogen levels will be restored in the muscles coming from this mobilized fat.

In traditional cardio the glycogen is not released to the same extent. The muscles retain some glycogen, glucose remains in the blood stream and insulin is required to move the glucose out of the blood stream. Since the body does not sense a need to build glycogen stores the glucose is stored as fat. This may also have the effect of keeping insulin elevated which ultimately can lead to insulin resistance. Also when insulin is high, hormone sensitive lipase is inhibited.

Another hormone that gets into the act is called leptin. Leptin is our satiety hormone – it tells us we are full. The more fat we have, the more leptin is produced so that our appetitive will decrease and our body fat level will stabilize. A modern day problem has become leptin resistance. Similar to insulin resistance our body is not able to respond to all the excess leptin being produced so we are not properly getting the signal that we are full and should stop eating.

The bottom line is that HIT will lower insulin, leptin, and cortisol levels while increasing growth hormone while traditional cardio or “aerobics” will have the opposite hormonal effect.

A few other points of interest:

One of the big truth stretchers of the fitness industry is the measuring of “calories burned” on machines. We have what is called our basal metabolic rate. Just by living we burn calories. The BMR is how many calories you burn just by being alive at that moment. The calories burned being recorded by the machines are inclusive of those calories. The “calories burned” is highly misleading as if you stood on the machine and did nothing you’d still be burning calories!

Another basic truth is that no exercise per se burns a lot of body fat. As explained earlier, we survived by being very efficient with our fat. It is what kept us alive. So, if we were burning lots of fat by hunting and gathering or any other activity, it would jeopardize our ultimate survival.

In conclusion, our body developed ways to survive food scarcity so we could endure as a species. It was not designed for food abundance. Our modern diet which is high in carbohydrates keeps our blood glucose levels elevated, which keeps our insulin levels high, which keeps our glycogen stores high, so we are not able to burn fat.

And one last key factor is hydration. Most people do not drink enough water. Being properly hydrated improves our liver function which will support fat metabolism and improves cell structure so hormone receptors work properly. And, drought precedes famine. If we are not properly hydrated we are sending an evolutionary signal to our body to hold on to fat as it prepares for the upcoming famine.

Bernard Rosen, PhD is a Nutrition Consultant and Educator. He works with individuals, groups, and at corporations to create individualized nutrition and wellness programs. His office is in Mequon, WI. To learn more or to schedule an appointment, e-mail at bernie@brwellness.com, call (262) 389-9907 or go to www.brwellness.com.