The framework proposed in the journal Cell in 2013 reflects current thinking among scientists who look into human aging. Here we list some other theories, many outdated but some that offer useful insights. One way to categorize the theories is into two types: (1)programmed theories and (2) damage theories.
When the hormone system breaks down, the body ages. It is true that many glands stop producing hormones (e.g. ovaries) or become slower to release hormones (adrenal gland) and this makes the body less able to reproduce and react to the environment (two things that drive natural selection). So the aged body is indeed less fit than the young one in an evolutionary perspective. But a decline in hormone activity does not explain most of the symptoms of aging.
Hayflick limit theory of aging. In the lab, somatic cells divide a maximum of about 50 times before entering senescence. This is called the Hayflick limit. The idea was that this limit played out in the body and contributed to aging. But it is known that cellular senescence is not directly related to the aging of the body as a whole.
The immune system is complex and not well understood. However, it is known that the effectiveness declines over time. As it declines, the body is subject to more diseases. Another malfunction can make the immune system turn on the body, leading to autoimmune diseases. Is aging itself an autoimmune disorder? No. The aging of the immune system increases risk of mortality so it does contribute to aging.
This is how many lay people with some understanding of biology or physiology imagine aging occurs, but scientists do not think so. The characteristics of old age are not due primarily to molecular damage. The body’s cells turn over so by the time you are 70s few of your cells remain from when you were 20. Damage occurs to biomolecules - cell membranes, DNA, etc, but the body has processes to repair the damage. (If there were no repair, we would die pretty quickly. Heavy molecular damage can be fatal - that’s what happens when people die from poisoning or injury.)
This theory gained purchase because it’s easy to understand if you know a little chemistry. Oxidation is sometimes the way food spoils. Food preservatives are anti-oxidants.
The idea is that superoxide and other free radicals cause damage to nucleic acids, lipids, sugars, and proteins. Free radicals form in the normal course of living, and the body has natural antioxidants to keep them from doing too much damage.
Some foods we eat contain chemicals that can be classified as antioxidants. In laboratory glassware - in vitro they call it - these chemicals stop or slow oxidation reactions. People got the idea that if you swallow antioxidants - either in food or dietary supplements - you reduce oxidation inside the body and hence illness, disease, or aging.
While it has never been resolved whether antioxidants reduce your odds of contracting illness in the short run, their effects on long-term aging are even less clear, and nobody seriously things aging is analogous to food spoilage.
This is an old idea - first articulated in the Nineteenth Century - and it appeals to us because it seems logical. Machines wear out so people think human bodies will, too. But no one who understands biology and physiology accepts this theory any more. Living bodies are not the same as inanimate machines. Bodies repair themselves.
DNA is the molecule that carries genetic information and cell division (reproduction) relies on duplication of DNA. There are errors during the duplication, and sometimes these errors make the cell cancerous. While DNA damage is a partial explanation for cancer, it is inadequate to explain the characteristics of aging.
One old theory - no longer widely believed - was that chronic inflammation caused the worst physical maladies as we age. Elderly people must therefore be plagued by low-grade, chronic, systemic inflammation in this view, even though empirical data shows that not to be the case. It is true that inflammation is generally a negative thing. It is often painful and inflammation is a symptom and sign of another disease. Even if the underlying cause cannot be identified, inflammation is considered to have negative effects. Inflammation of the cardiovascular system increases the chance of heart disease. Anti-inflammatory drugs such as aspirin exist and some plants are thought to have beneficial life-extending effects in the diet because they stop or slow inflammation of tissue.
The network theory of aging posits that aging is indirectly controlled by the network of cellular and molecular defense mechanisms. There is some merit to this idea, but it does not tell the whole story of why the effects of aging develop.
The remodeling theory, which was put forward to explain immunosenescence, is the gradually adaptive net result of the process of the body fighting malignant damage and is a dynamic process of optimization of the trade-off in immunity. Immunosenescence is inflammaging (aging related, low-grade, chronic inflammation) and plays into indirect outcomes of inflammation, including thromboembolic disease such as cardiovascular and cerebrovascular events.