YOU might think you control your own behaviour, but actually much of the time your hormones do.
Even the tiniest swing in hormones, down to a billionth of a gram, can have a dramatic impact on the body and affect your behaviour, metabolism, sleep, mood, immune system, puberty, sexual experience, and how you eat, grow, hate, love, and think – to name but a few.
But what are these powerful chemicals that control our lives? Randi Hutter Epstein, author of Aroused: The History Of Hormones And How They Control Just About Everything (£19.99, Norton), explains that hormones are "loopy chains of amino acids" produced in the nine key endocrine glands.
These are the hypothalamus, pineal and pituitary in the brain, the thyroid and parathyroids in the throat, adrenals in the kidneys, pancreas in the abdomen, and the ovaries and testes in the pelvis.
Every cell has markers that direct hormone signals to precisely where they need to go, she says – but hormones rarely work alone, and a dip in the amount of one hormone interferes with other hormones, in a domino effect that can throw a host of bodily functions off-kilter.
Here, Epstein, a respected medical writer, Yale University lecturer and adjunct professor at Columbia University's journalism school in the US, outlines nine ways hormones affect our health and wellbeing...
1) 'I-feel-full' hormone affects fertility
"The world was wowed with the discovery of leptin, the 'I-feel-full' hormone," says Epstein.
"New evidence suggests it not only controls appetite, but may be linked to infertility. Leptin increases after meals, and chronic low levels of the hormone due to severe starvation alters other brain hormones that, in turn, dampen hormones necessary for conception."
So, people who eat very little and have lower leptin levels may also be less likely to conceive. This may help explain why women who have anorexia or other eating disorders often have trouble getting pregnant.
2) Obesity reduces testosterone
If you lose weight, you'll help boost your testosterone levels, the male sex hormone that plays a key role in the development of male reproductive tissues such as the testes and prostate, as well as promoting increased muscle and bone mass, and the growth of body hair.
"Contrary to popular notion, taking testosterone doesn't burn fat," stresses Epstein.
"Taking testosterone when you're in the normal level to burn fat is a myth. You need to cut calories, not take testosterone shots."
3) Hormones produced by being overweight make you eat more
You think the fat cell is just a blob of fat? Think again; it secretes hormones too, altering your drive to eat.
"The fatter you get, the more your hormones will lure you to the kitchen," says Epstein.
4) Growth hormone isn't just for growing
As well as stimulateing growth, cell reproduction and cell regeneration, human growth hormone (HGH) helps balance sugar, metabolise proteins and fats, maintain heart and kidney health and stimulate the immune system.
5) Childbirth hormone promotes love and trust
Oxytocin, the hormone that 'squeezes' the womb to help in childbirth and gets the breast milk flowing, also influences feelings of love, trust and empathy, says Epstein.
"Brain research is revealing the ways oxytocin is tied to social behaviour," she says, "but buyer beware of the oxytocin-filled supplements touted to help you lure a lover.
"These supplements haven't been shown to get into the brain and have an impact, nor are there studies to show that oxytocin boosts bonds between two adults. Also, many of these supplements haven't gone through any kind of quality control, so you have no idea what you're getting."
6) Man boobs from too much testosterone
Testosterone converts into oestrogen in the body, so men who are taking too much testosterone are likely to get unwanted man boobs, says Epstein, who points out that men are often lured to take testosterone because it's been advertised as helping boost libido.
"For men with low testosterone – identified through two blood tests and assessed by an accredited laboratory – going from low to normal range will boost libido and combat fatigue," she adds.
7) Lower menopausal oestrogen levels affect much more than periods
As well a woman's periods stopping at the menopause, lower oestrogen levels can lead to changes in the brain and nervous system, leading to things like mood swings, memory loss, problems focusing, irritability, fatigue, hot flushes, night sweats, stress and anxiety and vaginal dryness, triggering painful sex.
"The link between oestrogen and hot flushes has something to do with a brain cell receptor that controls temperature," explains Epstein.
"In other words, low oestrogen cobbles your internal temperature control mechanism. It's also tied to fluctuations in adrenaline, the fight or flight hormone, making some women feel anxious for the first time."
8) Embryos all look alike until hormones kick in
Around six to eight weeks after conception, the human foetus has two sets of ducts, one of which can develop into the male reproductive organs (scrotum and testes) and the other into the female reproductive organs (ovaries and uterus).
If the foetus is genetically male (XY chromosomes), the embryonic testes will produce anti-Mullerian hormone causing the Mullerian (female) ducts to disappear.
"Before that, we all look alike," says Epstein.
9) Progesterone can make you sad
Changing progesterone levels can contribute to abnormal menstrual periods and menopausal symptoms, and the hormone is also necessary for implantation of the fertilised egg in the uterus and for maintaining pregnancy.
But, in addition, Epstein says progesterone can trigger moodiness and sadness among some susceptible women.
"That's why some women, but not all, may feel glum on the birth control pill, which is a mix of oestrogen and progesterone," she explains.
"For some women, progesterone can trigger depression, others get moody, and some don't seem bothered. The point is that if you go on birth control or hormone replacement therapy and start to feel emotionally different (depressed, sad), you should let your doctor know. There's a link between hormones and moods."
