As we grow older, many things change — we get tired more easily, our joints are stiffer and occasionally ache, sleep comes with more difficulty, and things we used to do with ease can become chores. But by taking care of ourselves when we’re younger and changing a few things as we get older, we can avoid many of these problems.

Of course, we all pay with our health for the indiscretions of our youth. Studies have shown that the risk of dementia and brain diseases increases with the more infections (and vaccinations) a person has had in his or her life, and lack of exercise increases the likelihood of many types of disease, not just brain diseases.

Scientists are also learning that as we age, our cells change and age with us, becoming what are known as senescent cells.

Normally, cells divide a certain number of times (a quantity called the Hayflick limit) and then they stop renewing. After that, they just get older. And those aged cells in organs and tissues don’t work as well as young, renewed cells.

The fact that most cells in the body are renewed many times over a lifetime means, in part, that we have a different body at different times in our lives. And when we reach old age, that body may contain many senescent cells.

The problem is that senescent cells leak in two directions — too much calcium is allowed to enter the cells, and excitotoxins leak out of the cells.

Excitotoxins can be very noxious, causing widespread inflammation and destruction of surrounding tissues. This happens especially in the brain.

Immune cells also age, functioning less well and releasing destructive enzymes and excitotoxins into tissues and organs including the brain. Microglia (the primary immune cells in the brain) and astrocytes (which contain most of the excitotoxins in the body) leak these destructive substances outside brain cells, where they do the most damage.

In fact, when they are inside of cells, excitotoxins do no harm at all.

Many things worsen this process, including excessive alcohol consumption, lack of exercise, insufficient sun exposure, and lack of sleep.