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Why exactly do humans lose subcutaneous fat tissue with age?
It appears that we either lose fat cell volume or we might actually be losing the number of fat cells with age.
One hypothesis is that as the vascular system weakens with age, the fat cells on the perimeter of the body shrink and die because they don't have enough blood flow to sustain them. The problem with this hypothesis is that subcutaneous fat loss is very gradual and starts during young adulthood and continues. It also happens in very fit individuals too.
Another hypothesis I read before was that subcutaneous fat is loss because of a drop in estrogen levels after menopause. This hypothesis doesn't explain why men lose subcutaneous fat with age. Also, women start to lose fat on their hands and upper cheeks long before menopause starts.
Short answer
We do not loose fat (cells) as we age; on the contrary - we tend to gain fat by about 1 or 2% per decade. The place where it gets stored does change; instead of going into subcutaneous regions, it gets stored around the abdomen. A sex hormone imbalance might be the cause of this gain in visceral fat.
Background
What a coincidence! I just finished an article in the Sci Am May issue of this year (2018) on exactly this topic, literally just minutes ago.
In young folks, fat tends to accumulate under the skin, while in older folks this tends to shift to accumulation of fat around the abdominal viscera (the internal organs). Bottom line: we don't loose fat overall, we just store it elsewhere. What's more, as we age we store more fat as our base metabolic rates drop.
Why this happens is no known, but there are some interesting findings:
Sex hormones are involved; women in menopause (estrogen levels drop) gain more visceral and belly fat and loose it around the hips (they go from pear to apple);
However, a drop in estrogen is not the cause, as certain ailments lead to belly fat in young women with plenty of estrogen;
Androgen-using females (transgenders) gain more visceral fat.
These findings together have lead to the idea that a sex hormone imbalance might be the cause. However, some findings counter this idea:
Afro-Americans have less visceral fat, while Asians have more;
Blocking FSH results in less visceral fat, which may have other mechanisms of actions than estrogen.
$\begingroup$The question I'd ask, though, is whether the accumulation of fat is actually due to age, or whether it's a side effect of sedentary, inactive lifestyles increasing with age. E.g. this guy doesn't seem to have a lot of subcutaneous or visceral fat: adn.com/sports/2018/07/23/…$\endgroup$
How sad, you didn't get a response. You got a person saying that people don't lose total body fat with age (which you never claimed, or ask about), and a person saying that visceral fat gain isn't inevitable with aging. Neither of these answers deals with your question. I think it's a common psychological flaw in humans that, when they can't answer a question, they produce a random unasked-for question from the same semantic field. Unfortunately, I also can't provide a thorough response, but I will try to at least narrow it down:
To my knowledge, the reason is not vascularization; decrepitude of peripheral vascularization was suspected for a time, but we have seen that the predilection sites of fat loss readily take fat grafts, and those fat grafts that survive the initial post-transfer phase are "permanent". Since both transferred fat and "original" fat would depend upon vascularization, the difference can't be blood supply, but must be something about the local fat itself. As has correctly been pointed out, loss of subcutaneous fat "begins" long before menopause in women. Indeed, it does not "begin" at all (to my knowledge), but rather, the loss is an ongoing process from birth, although interrupted by localized plateauing at times. Another notable feature in facial fat loss is that its predilection sites are also predilection sites for bone volume loss: maxillary and zygomatic regions. To our knowledge, the cellular processes of fat atrophy and bone remodelling are distinct; osteoclasts do not dissolve fat cells. To me, this indicates that it would be a sensible hypothesis that the process of the body removing facial volume is not a predictable side effect from cellular senescence, but rather that it is, like baldness, a programmed event which might be triggered by the epigenetic clock of aging - a programmed event that simply takes aging as its trigger, but isn't itself necessitated by senescence.
$\begingroup$This fits with my understanding of the subject nicely. Welcome to the site Dr. K. Please take our tour and refer to the help center for guidance, enjoy the site.$\endgroup$
$\begingroup$Welcome. There are a lot of unsubstantiated claims in your answer. Can you add more references, most notably in the 2nd paragraph of your answer.$\endgroup$
$\begingroup$Thank you. In fact, success of fat graft procedure is a common knowledge which does not not require sources, I think. It is easily found in any popular or scientific source on facial rejuvenation surgery. But I edit a source for predilection sites of bone and fat loss.$\endgroup$
