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Lecture Notes

There are three main ways that we expend energy, or you could also classify this as our human metabolism, how we burn energy in our body, or how we use calories.

And the first of these, which makes up about 60 to 75 percent of the energy that we expend, is resting energy expenditure, or your resting metabolic rate. And here this is the caloric requirement to run all of your essential bodily functions while you're at rest. And so this is pretty stable within individuals but differs based on age. So as we get older our metabolism (or our resting expenditure) decreases (gets slower). So we're burning less calories as we get older at rest. It also differs by biological sex. Men typically have higher metabolism than women or burn more calories than women. Thyroid hormones can also-are also strong regulators of this resting energy expenditure. The amount of lean muscle mass we have. So if you have higher muscle mass, you have a higher metabolic rate, you're burning more calories even when you're at rest and not using those lean muscles.

And finally, another factor that can affect (not the only other factor, but one other factor that can affect your resting energy expenditure is if you're in severe caloric restriction. So if you have some reason that you're in a severe caloric restriction situation, your resting metabolic rate is going to go down, so your body's going to try and conserve the energy that you already have in your body when you're essentially starving yourself. So that's the resting energy expenditure, and there are some other differences between individuals, but you can read about those in your book.

The second way that we expend energy is through the thermic effect of food, or the energy needed to eat and process food. And as you can see in the pie chart here, that's only about ten percent, so this is not a way that we really use to increase our energy expenditure. We use that amount to both eat and process the food. It differs based on the type of food you eat. Eating more calories will increase the thermic effect of food, but this is not a great way to increase your overall energy expenditure because you're also increasing caloric intake. And so this is not a category that we talk a whole lot about because it's not very effective for increasing your overall energy expenditure.

And the final category, which we have the most influence on, would be your physical activity. And here it can range anywhere from 15 to 30 percent of your total energy expenditure. And this really captures the energy required to contract and relax your skeletal muscles, and this is the area that differs the most between individuals, and it's the best way to increase your energy expenditure or metabolism. Your metabolic rate increases while you're being active, but the awesome thing is that is also remains elevated after you're done being active. So it's not just during your bouts of physical activity where this-your body's using extra fuel, it also continues after you finish that activity.