Muscle mass comprises between 30 and 50 percent of your body’s total weight – the more the better. The human body is composed of more than 650 muscles, they are the primary user of calories in the active human body, and it’s the storage facility for most of the body’s protein. Muscles are the motors which move the skeleton – the system of levers we use to interact with our environment – and are therefore responsible for our physical relationship with our surroundings. Fat, on the other hand, is where calories are stored, not used. Mostly, fat just lays there, using very few calories itself but hoping you’ll use the calories it stores as fuel for muscle contraction. But body fat itself is not the problem. The processes that allow for the accumulation of body fat are the problem. Accumulating body fat means that there is an imbalance which must be addressed, usually by correcting the quality and quantity of both the diet and the physical activity schedule. For you, gaining muscle is more important than losing fat. Muscle is important metabolically, in a much more significant way. Muscle tissue does much more than just move you around – it keeps you alive, and the more you have, the more alive you get to stay. Muscle tissue is the body’s most important end user of blood sugar, and of insulin too. Muscle burns most of the calories you use during the day. Type II diabetes is very strongly negatively associated with the health and size – of your muscles, because the activity that makes muscles big and healthy is also the activity that uses and regulates blood sugar and insulin. It is not an overstatement to say that the activity that keeps muscles big is also the activity that prevents type II diabetes. This is very important, in a country with diabetes in almost 10% of the population. Diabetes is a very bad deal, because it shortens life expectancy by an average of 10 years, and makes your shorter life more expensive and less fun. Muscle tissue also performs several other important jobs besides moving you around. It modulates immune function by providing an active repository for immune system cell component proteins – very important for a long illness, and it serves as a receptor site for sex hormones, thereby regulating some of their functions as well. But mainly, muscle tissue burns calories, by using energy when it generates the force of contraction, and the processes by which energy is used are processes the human body is designed to perform. When your muscles fail to perform these processes – in other words, when you sit squarely on your buttocks, failing to use your muscles – the machinery gets disassembled because there is no market for it, and maintenance is expensive. And the less muscle mass you have, the harder it becomes to keep fat from being deposited. Muscles burn both fat and carbohydrate for fuel, and the bigger the fireplace, the more fuel the house can burn. So the presence of this machinery (muscle) is very important, but its loss is a normal part of aging, for several reasons. The only practical way to slow or reverse these processes is to subject the body to the type of stress that makes muscles need to be bigger, and the thing that most people become less likely to do as they get older. If loss of muscle mass is a function of aging, maintaining that muscle mass requires that you do the things that would actually make it grow in a younger person – strength training and eating enough quality protein and calories to enable its growth. So the question we started with – Which is more important, the loss of body fat or the increase in muscle mass? – is easy to answer. But if this is so, why do the vast majority of people who start an exercise program do so with the expressed intention of “losing weight,” by which they really mean losing bodyfat? Because they don’t know any better, unfortunately. The popular culture’s perspective on the subject is pretty uniform: Healthy = Skinny. And as usual, I’m telling you that this is wrong. Not only is it factually incorrect, it can get you prematurely dead, depending on your age. Skinny is a bigger problem for some people than it is for others. Clinical Illness Myopathy is an acute sarcopenia – the short-term loss of muscle mass experienced by very sick or injured people who cannot preserve muscle mass, due to the illness or injury itself and the inability to eat or move enough to preserve muscle function. The loss of 40% of lean body mass is usually fatal, and in older people the loss of muscle mass is a very accurate predictor of mortality. This cannot be overemphasized: the older you are, the more important your muscle mass becomes to you, whether you know it or not. If you get sick or hurt, your muscle mass is your bank account for healing up and getting better – the more you have in the bank, the longer you can hold out when things get weird. This means that being old and skinny is not a useful thing if you want to enjoy your last decades. The young can get away with being skinny, because the young don’t usually get chronically ill or injured.
In fact, the healthiest older people are a little overweight. The very popular (because it is easy to calculate) Body Mass Index, or BMI, assigns the terms Very Severely Underweight, Severely Underweight, Underweight, Normal, Overweight, Obese, Severely Obese, and Very Severely Obese to everybody in the human race, based on their height-to-weight ratio. This seems a little broad to me, since it fails to take muscle mass into account. A person can be 5’8” and either be a fat slob or a muscular freak at the same BMI. But since most people are not muscular freaks, the BMI works – in a broad sense. The medical community has recently begun to admit that “overweight” is a better place to be if you're not a kid anymore. With a BMI calculated at 29, I am “Obese.” I am not worried about my Body Mass Index. BMI offers some useful predictive capacity for health problems in younger sedentary populations, but its usefulness for older people is very limited.