You have probably heard of BMR before; it is an abbreviation of Basal Metabolic Rate. ‘Metabolism’ is the aggregation of all of the complex chemical reactions which occur within every cell of your body in order to maintain your existence, and your Basal Metabolic Rate is the minimum amount of energy which is required to fuel all of those reactions, measured while you are asleep. Those reactions and processes include things like protein turnover, which is where proteins are broken down and re-synthesised in every cell of your body, bone metabolism which is a similar process within your skeleton, action potential activity in your nervous system and filtration in your kidneys amongst literally thousands more. As you can see, you don’t actually have to do anything for this energy to be used.
RMR is very similar, but it is measured while you are awake, and seeing as most research is done using RMR we will talk about that instead.
Note: Your RMR is usually a little higher than your BMR, by 10% or so. It may or may not surprise you to know that your RMR will make up somewhere around 60-70% of your total energy expenditure if you are moderately active. This will, of course, go up or down if you are very inactive or an athlete respectively, because other factors start to make a bigger difference. But the key thing to note here is that you cannot directly affect to any substantial degree the largest component of your daily calorie expenditure.
This is not to say that this number is static as it can be affected by certain environmental factors including calorie balance (broadly speaking, it will go up and down slightly when you are eating more and fewer calories than you require, respectively), hormonal interactions, medications/drugs, your age and health status. It is, for most practical purposes, however, more or less static and determined by your bodyweight and body composition (which describes your balance of fat mass and everything else). In some extreme or unusual cases your RMR could be wildly different to what is expected, but that’s beyond the scope of this course.
Each tissue within your body requires a different amount of energy to maintain itself day-to-day. Fat tissue, far from being inert or dead, requires about 4.5kcal per kg per day (so 90kcal if you have 20kg of fat mass, roughly) whereas muscle requires about 13kcal which is of course more, but not so much more that we should buy in to the idea that you radically speed up your metabolism by building muscle, because you don’t. On top of that, your liver requires 200kcal per kg whilst your heart needs a humongous 400kcal per kg. Of course, you have significantly more skeletal muscle than liver or kidney tissue and as such your muscle takes up a lot more total energy per day than these smaller organs. Bones require energy too, as does every single tissue in your body at different rates. When all of this is added together, you have an approximate figure for your RMR.
As you can probably work out, your BMR/RMR will vary wildly due to differences in tissue mass. If you’re tall or short, carrying a lot of bodyfat or very lean, carrying a lot of muscle mass or very slight, your BMR will be affected and you will probably sit outside of the standard estimated ranges because your tissue distribution is unusual. On top of this, around 15% of RMR is not predictable from tissue mass, although you would expect that it should be. This fast or slow metabolism phenomenon seems to be natural genetic variance and there is very little you can do about it, but when put into real numbers, the difference is not going to be huge. If two people should have an RMR of 1750kcal, that means that the variance is around 260kcals at a maximum, or 130 calories up or down on their calculated RMR. While some people may indeed have slower metabolic rates than others, the difference equates to a large banana or less, and therefore general estimates can be used and considered to be ‘pretty good’. We’ll show you how to make these estimates in a later section of this module and then we’ll show you how to verify them later in the course.