- [Voiceover] In this video, we’re going to talk about body mass or what is it in your body that determines how much you weigh? Well, you can kind of think of it like a scale, where the things that determine whether you’re heavy or you’re light are based on opposing forces, where some things cause you to put energy into your system, and there are other things that cause you to get rid of energy, or that means you lose energy or energy out. And as you might remember, food is a type of energy. So we’ll put diet in this column as a way that you get energy into your bloodstream. If you eat a lot of sugar or carbohydrates or fats, that’s energy for use. But then remember, once we’ve eaten things there are hormones that regulate whether we store things or we get rid of them and just discard what we ate. One hormone is insulin, and that regulates blood glucose levels. There’s also leptin, which is related to blood lipid levels, and then there’s also another hormone that’s called ghrelin that’s related to whether the stomach is full or not. These three hormones help regulate whether the energy or the food that we’ve just consumed is stored or if we just let it pass through and become waste that we get rid of. Now the things that we do that get rid of energy or consume energy include things like our basal metabolic rate or the amount of energy we burn just at rest. That comes out to be, in the average person, about 2000 calories in a day. What about from just getting up, moving around, and being active? Well, in the normal person, activity or the use of energy to perform tasks comes out to be about 500 calories in the average person. And finally, there’s this phenomenon that’s called diet-induced thermogenesis, and that’s just the energy that we use or we burn to store the food that we eat, and that comes out to be about 10% of the amount of calories we bring in from our diet. So this means on average, whether we are eating more or if we are doing things. So this means on average, whether we are a person that tends to eat more and be sedentary, or if we’re a person that’s more active and eats less and burns more calories, that helps determine what our body mass will be. But one of the things I want to highlight is the effect that hormones can have on this process, on determining whether we have a high body mass or a low body mass. And the best example of this is a mouse experiment. In this experiment, three mice were studied over time. The first mouse, which for simplicity sake let’s say it was a normal mouse. So there’s nothing genetically unusual or different about this guy. The other two mice, however, had some specific gene that was knocked out. Knocked out meaning missing. So there was a gene that was not there that the normal mouse had. And so, what they did next is that they took these two mice and they fed them, and they gave them equal amounts of food and let the mice eat whenever they wanted to and saw what happened. Well, over time the normal mouse just stayed the same. No difference. But when they let the genetically altered mouse eat for as long as it wanted to, this guy got huge. The researchers found that when this gene was missing, these mice tended to eat more, because they were hungrier. But then the unusual thing was that when they took this same strain of mouse that had this exact same gene knocked out or missing and then fed them, but also injected the serum from the normal mouse or injected blood from the normal mouse, they found that over time the mouse looked pretty much the same as the normal mouse, and it was this experiment that led scientists along the way to determine that this gene was actually encoding the hormone leptin. Leptin is a hormone that lives in our fat tissue, our adipose tissue, and goes to our hypothalamus, this part of the brain that tells us we’re not hungry. In people or in mice that don’t have any leptin, these guys are always hungry. And so, they just keep on eating. So this just goes to highlight how important it is to have the correct hormones flowing through the bloodstream to talk to your brain based on the nutrients or the diet that you have to regulate your body mass.