You know that your body is made of cells - but just how many? It’s actually not all that easy to answer that simple question. But recently, scientists have made a pretty good effort. And their final count is…37.2 trillion.
Calculating the number of cells in the human body is tricky. Part of the problem is that using different metrics gets you very different outcomes. Guessing based on volume gets you an estimate of 15 trillion cells; estimate by weight and you end up with 70 trillion.
The answer researchers came up with is 37.2 trillion. They broke down the number of cells by organs and cell types, counting everything from eyeballs to intestines to knees. Incidentally, of those 37.2 trillion cells, 50 billion are fat cells and 2 billion are heart muscle cells.
What Rate do We Replace Our Cells?
It's a neat idea, and one that has caught the popular imagination. Here's how the story goes: Every seven years (or 10, depending on which story you hear) we become essentially new people, because in that time, every cell in your body has been replaced by a new cell. Don't you feel younger than you were seven years ago?
It is true that individual cells have a finite life span, and when they die off they are replaced with new cells. Red blood cells live for about four months, while white blood cells live on average more than a year. Skin cells live about two or three weeks. Colon cells have it rough: They die off after about four days. Sperm cells have a life span of only about three days, while brain cells typically last an entire lifetime (neurons in the cerebral cortex, for example, are not replaced when they die).
Recent research has confirmed that different tissues in the body replace cells at different rates, and some tissues never replace cells. So the statement that we replace every cell in the body every seven years or every ten years is too general to be accurate. Researchers now know that:
Neurons in the cerebral cortex are never replaced. There are no neurons added to your cerebral cortex after birth. Any cerebral cortex neurons that die are not replaced.
Fat cells are replaced at the rate of about 10% per year in adults. So you could say that on average, human beings replace all their fat cells about every ten years.
Heart cells are replaced at a reducing rate as we age. At age 25, about 1% of cells are replaced every year. Replacement slows gradually to about 0.5% at age 70. Even in people who have lived a very long life, less than half of the heart cells have been replaced. Those that aren’t replaced have been there since birth.
So why does all of this matter? It matters because each time a cell is replaced, it is actually created from recycled cell parts: mitochondria being the most important.
Mitochondria is a cell part that is responsible for cell energy, and at the macro level, your total mitochondria is response for the natural energy you feel every single day. Think of them like batteries that hold power. Your batteries can be old and weak, or they can be new and super charged.
As a cell ages, its mitochrondria become weak, damaged and destroyed. As a cell is replaced, its mitochondria are rebuilt and assembled anew. You can actually speed up this renewal process, through intermittent fasting because it promotes autophagy. More on autophagy later, but really, if your energy levels are low, consider a nutritionally supported intermittent fasting program as an experiment to see if it helps.