-- The Human Body as a Vast Chemical Factory
A hormone is, broadly speaking, any chemical substance produced by the body's endocrine glands. The word comes from a Greek verb meaning "to set in motion," and that's a good description. Hormones are released into the bloodstream and travel throughout the body, regulating metabolism and development, growth and maintenance, reproduction and our responses to change. More than eighty types have been identified, including familiar names like estrogen, testosterone, dopamine, endorphins, adrenaline, and insulin. The emotions that seem to come out of nowhere? Those are driven by hormones. The physical differences between men and women? Also hormones. Each hormone is produced in a different part of the body, serves a different purpose, and works through a different mechanism. In this sense, the human body really is a vast chemical factory.
Hormones circulate in the blood and influence virtually everything — emotions, appetite, body weight, energy metabolism, and diseases ranging from diabetes to cardiovascular disease to cancer. There is hardly anything in the body they don't affect. This is why pregnant women experience such dramatic mood swings. Two hormones are responsible: estrogen and progesterone. Estrogen promotes feelings of happiness and well-being, and it even improves cognitive function. Progesterone, on the other hand, can cause irritability and anxiety. During pregnancy, these two hormones fluctuate dramatically, leading to intense emotional ups and downs. The same hormonal cycle, playing out across the menstrual cycle, is why mood swings tend to be more noticeable in women than in men.
Do only women experience hormone-driven emotional changes? Not at all. When men go through andropause — the male equivalent of menopause — testosterone drops sharply while estrogen actually increases. This is why a man who was the picture of toughness in his younger years might, in middle age, start volunteering to do the dishes, tearing up at TV dramas, or getting sentimental watching autumn leaves fall. If you notice an older man in your life who has become chattier than usual or started listening to old love songs, the best advice is to just leave him be. Think to yourself, "Ah, the estrogen levels are rising," and let him have his moment.
All the hormones I've mentioned so far are sex hormones. What about the others? Insulin, cortisol, and growth hormone are among the most well-known. These hormones are released into the bloodstream and circulate throughout the entire body — brain, heart, organs, and muscles. They're essential for brain function, cardiovascular health, digestion, immune defense, and movement. Without them, the body simply can't function.
Hormones are divided into two main categories based on what they're made of. Peptide hormones are made from proteins and work through receptors on the cell surface. No matter how much of a peptide hormone the body produces, if the receptors are blocked or not working properly, the signal doesn't get through and problems follow immediately. Oxytocin, the main subject of this book, is a peptide hormone. Steroid hormones are made from cholesterol and other fats. They don't need a surface receptor — they pass directly through the cell membrane and act on the nucleus. The most well-known steroid hormone is cortisol.
Cortisol is released from the adrenal cortex — small glands sitting on top of the kidneys — when the hypothalamic-pituitary system sends out adrenocorticotropic hormone (ACTH) in response to stress or low blood sugar. Cortisol raises blood sugar, suppresses immune function, and ramps up the metabolism of fat and protein. When chronic stress keeps cortisol elevated all the time, it becomes a driver of diabetes, belly fat, muscle loss, and weakened immunity. But cortisol isn't all bad. In the short term, it triggers acute inflammatory responses that protect the body — some wound-healing ointments are actually designed to mimic its effects. The problem comes when cortisol stays high day after day, which erodes immunity and opens the door to metabolic diseases, depression, and other mental health conditions.
These hormones influence everything — the brain, the organs, and all the systems in between — which is why they're connected to our emotions, our immunity, our muscle and fat levels, and the diseases we're vulnerable to. But here's the key point: our daily habits also shape how hormones are produced and how they function. We aren't just passive passengers on a hormonal ride. Through regular exercise, healthy eating, positive thinking, and stress-relieving activities, we can take an active role in managing our hormones — making them work for us rather than controlling us.