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Aging refers to the physiological changes we experience during our lifespan. It’s also an inevitable part of life.

After all, our cells aren’t made to last forever. The structures and functions in our cells decline over time.

But why does this happen? For decades, scientists have been studying the subject. There are currently more than 300 theories on why we age, and experts are learning more every day.

Let’s explore why humans age, and how you can slow down the effects.

Aging can be categorized into two types and due to two types of factors, intrinsic and extrinsic.

Intrinsic aging vs. extrinsic aging

Intrinsic aging is a genetically predetermined process that occurs naturally. Extrinsic aging is a result of outside factors chose by you, such as where you live, your stress levels, and your lifestyle habits (like smoking).

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Cellular aging

Cellular aging is due to intrinsic factors. It’s related to the biological aging of cells.

Cells are the basic building blocks of the body. Your cells are programmed to divide, multiply, and perform basic biological functions.

But the more cells divide, the older they get. In turn, cells eventually lose their ability to function properly.

Cellular damage also increases as cells get older. This makes the cell less healthy, causing biological processes to fail. Cellular damage accumulates over time, too.

Damage-related and environmental aging

Damage-related and environmental aging is related to extrinsic factors. It refers to how our surroundings and lifestyle affect how we age.

This includes factors like:

Over time, these factors can damage our cells and contribute to aging.

Everyone experiences both types of aging. However, each form of aging varies from person to person, which explains why we age in different ways.

It’s generally accepted that aging is caused by multiple processes, rather than one reason. It’s also likely that these processes interact and overlap with each other.

Here are some of the most prominent theories:

Programmed theories of aging

Programmed aging theories say that people are designed to age and that our cells have a predetermined lifespan that’s encoded into our bodies.

Also called active, or adaptive, aging theories, they include:

  • Gene theory. This theory suggests that specific genes turn “on” and “off” over time, causing aging.
  • Endocrine theory. According to this theory, aging is caused by changes in hormones, which are produced by the endocrine system.
  • Immunological theory. Also called the autoimmune theory, this is the idea that the immune response is designed to decline. The result is disease and aging.

Programmed theories have many supporters. However, they suggest that habits linked to longevity, like quitting smoking and exercise, are useless. This is likely inaccurate, as research has continuously proven that these habits affect life expectancy.

Error theories of aging

Error theories, or damage theories, are the opposite of programmed theories. They hypothesize that aging is caused by cellular changes that are random and unplanned.

Error theories of aging include:

  • Wear and tear theory. This is the idea that cells break down and become damaged over time. But critics argue that it doesn’t account for the body’s ability to repair.
  • Genome instability theory. According to this theory, aging happens because the body loses its ability to repair DNA damage.
  • Cross-linkage theory. This theory claims that aging is due to the buildup of cross-linked proteins, which damages cells and slows biological functions.
  • Rate-of-living theory. Proponents of this theory say that an organism’s rate of metabolism determines its lifespan. However, the theory lacks solid and consistent scientific evidence.
  • Free radical theory. This theory suggests that aging is due to the buildup of oxidative stress, which is caused by free radicals. But some say this theory fails to explain other types of cellular damage seen in aging.
  • Mitochondrial theory. As a variation of the free radical theory, this theory says that mitochondrial damage releases free radicals and causes aging. The theory lacks hard scientific evidence.

Genetic theory of aging

The genetic theory proposes that aging primarily depends on genetics. In other words, our life expectancy is regulated by the genes we got from our parents.

Since genes have predetermined traits, it’s thought this theory overlaps with programmed theories of aging.

Genetic theories include:

  • Telomere theory. Telomeres protect the ends of your chromosomes as they multiply. Over time, telomeres shorten, which is associated with disease and aging.
  • Programmed senescence theory. Cellular senescence occurs when cells stop dividing and growing, but don’t die. This theory suggests that this causes aging.
  • Stem cell theory. Stem cells can turn into other cells, which helps repair tissue and organs. But the function of stem cells declines over time, potentially contributing to aging.
  • Longevity gene theory. This is the idea that certain genes extend lifespan. More research is necessary.

The limitation of genetic theories is that they disregard the importance of external factors. In fact, it’s estimated that just 25 percent of lifespan is influenced by genetics. This suggests that environmental and lifestyle factors play a major role.

Evolutionary theory of aging

Natural selection refers to the adaptive traits of an organism. These traits can help the organism adjust to their environment, so they’re more likely to survive.

According to evolutionary theories, aging is based on natural selection. It says that an organism begins aging after they have reached their peak of reproduction and have passed down adaptive traits.

Evolutionary theories include:

  • Mutation accumulation. This theory presumes that random mutations accumulate later in life.
  • Antagonistic pleiotropy. According to this theory, genes that promote fertility early in life have negative effects later on.
  • Disposable soma theory. The theory claims that when more metabolic resources are directed toward reproduction, the less is put toward DNA repair. The result is cell damage and aging.

These theories are still being researched and require more evidence.

Biochemical theory of aging

Another theory is that biochemical reactions cause aging. These reactions occur naturally and continuously throughout life.

This theory is rooted in various concepts, including:

  • Advanced glycation end products (AGEs). AGEs develop when fats or protein are exposed to sugar. High levels may lead to oxidative stress, which speeds up aging.
  • Heat shock response. Heat shock proteins protect cells from stress, but their response decreases as we age.
  • Damage accumulation. Normal chemical reactions damage DNA, proteins, and metabolites over time.

In recent decades, life expectancy around the world has increased. This is due many factors, including:

  • better nutrition
  • improved hygiene
  • access to health care
  • improvements in modern medicine

These factors can protect our cells and reduce cellular damage, increasing life expectancy.

In most parts of the world, women live longer than men. This is due to several biological, social, and environmental factors.

Women, on average, have more estrogen than men. Estrogen is the female sex hormone. It’s been found to have anti-inflammatory and immune-boosting effects, which may protect women from certain diseases.

In contrast, the male sex hormone testosterone may suppress the immune system. Men typically have more of this hormone.

There are also behavioral differences between men and women. Generally, compared to men, women:

Though aging is inevitable, it’s possible to slow down some of the effects. You can do this by following healthy lifestyle habits.

Here’s how to slow aging:

  • Eat a nutritious diet. Focus on fruits, vegetables, whole grains, and lean protein. Limit processed foods.
  • Stay active. Exercise reduces the physical and mental effects of aging. Aim for 30 minutes a day, 5 days a week.
  • Avoid tobacco. Tobacco speeds up aging. Quitting is often difficult, but a doctor can help create a cessation plan that works for you.
  • Drink alcohol in moderation. Alcohol increases your risk of chronic disease.
  • Attend regular checkups. Routine checkups with a doctor are the best way to prevent or treat disease early.
  • Know your family history. Discuss your family’s health history with your doctor to have appropriate screening or get early treatment for potential chronic diseases.
  • Engage your brain. Protect your cognitive function by doing brain exercises.
  • Wear sunscreen. Sunscreen protects your skin from UV radiation, which can lead to premature aging.

Aging is likely caused by a combination of reasons. Some theories suggest cells have a predetermined lifespan, while others claim it’s caused by error and damage. Other theories say that aging is due to genetic, evolution, or biochemical reactions.

Aging is normal, but following a healthy lifestyle may help you live longer. Habits like eating well, exercising regularly, and wearing sunscreen can reduce your risk of disease and improve your quality of life.