My question is regarding the body fat composition test. I want to know why age is considered when comparing the objective number of mm of fat. Why is bodyfat percentage different for a 25 year old then for a 75 year old with the same number of mm of fat?
Your question on why is there a different measuring equation for a 25 year old versus a 75 year old is legitimate. It seems that body fat should be body fat, no matter who it is "hanging on." However, we must consider several factors before choosing one of the many body fat equations that are out there for calculating skin folds. The explanation will be rather "long winded," but it may give us a better view on the difference.
The age, sex and fitness level of the person seems to affect the end result. The results are based on the above mentioned and age related changes in the body. Adipose tissue changes, like everything else, when we age. There is white and brown adipose tissue. White adipose helps us keep warm, acts as a cushion from impact and jarring and acts as an energy reserve. Brown adipose is somewhat vascular, which gives it the color, has a moderate amount of mitochondria in it and keeps us warm under non-shivering cold conditions. Adipose surrounds internal organs and lays just beneath the skin surface and through out muscle tissues on some people.
Adipose tissue is lipid filled cells called adipocytes. These adipocytes weave throughout the collagen fiber webbing that lies under our skin. Apparently, some obese people who get "dimply" look around the hips and thighs have their adipocytes pushing up through the network of collagen fibers against the skins interior surface. Adipose tissues contain WBCs and pre-adipocytes. The contents are about 90 percent triglycerides and the rest proteins and water. There is very little water in fat stores.
How does that affect our 25 and 75 year olds? Well, after about 50 years old, the body starts to shift around, especially if you do not workout. Bone density starts to decrease, as does cartilage repair. The only cartilage that seems to keep growing is the stuff found in your ears and nose. Tendon and ligament weight is decreased, and connective tissue restoration is slowed, affecting wound healing. Muscle cells are also decreased, affecting body mass. The 25 year old has a body mass made up of 30 percent muscle, 20 percent fats and 10 percent bone. The 75 year old has 15 percent muscle weight, 40 percent fats and eight percent bone.
With these differences in LBM, there must be equation differences. It does not mean it is super accurate, but it gives a difference of opinion. There are several things to consider. The first, most of the body composition measurements are made on very active or Olympic type people. Or they are made on total couch potatoes. There are a few developed on the “average” gym participant.
It is sometimes difficult to believe skin fold calculations. You think the person is real lean, and he or she comes out with an unbelievable number. Or they are rather corpulent and show 10 percent body fat. Getting good skin folds on people can be tough. They should not train before a pinch test and should not have any lotion on the skin's surface. Take three measurements at each site and take the average. And, when in doubt, throw them in the water or underwater weighing, also known as hydrostatic weighing. I hope this is helpful.