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14 May 2024

How hormones rule our lives

From sex to eating, birth to body temperature, our physical selves do what our chemical masters tell us.

By Pippa Bailey

In 1849, the German scientist Arnold Berthold noticed that, compared to non-castrated chickens, capons did not display certain behaviours and physical characteristics such as crowing or the development of the comb and wattle. When the birds’ removed testes were transplanted back into their abdomens and they developed new vasculature, these secondary sexual characteristics were restored. Something, Berthold observed, was being carried from the testes into the bloodstream that directed these changes.

Of course, that something was testosterone. It was another 50 years before these signalling molecules would be given their name, “hormones”, from the ancient Greek for “impetus”. In 1902 the British physiologist Ernest Starling and his brother-in-law William Bayliss discovered the hormone secretin while studying digestion, then thought to be solely controlled by the nervous system, and hypothesised that secretin-like substances coordinated other processes too. In the following decades, several Nobel prizes were won for the identification and isolation of some of today’s best-known hormones: oestrogen, testosterone and progesterone.

Today, the simplistic labels we give them – melatonin is “the sleep hormone”, cortisol “the stress hormone”, oxytocin “the cuddle hormone” – suggest some popular knowledge of hormones’ wide-ranging role in human function. But the endocrinologist Max Nieuwdorp’s The Power of Hormones, published in Dutch in 2022 and now in English, reveals how little the Everyman understands about the extent of their powers.

Hormones, created by endocrine glands, travel around the body in blood and other fluids, and most have “a central regulatory function; they can either set processes in motion or inhibit them”. The hormonal “headquarters” are the hypothalamus and the pituitary gland, found in the centre of the brain behind the eye sockets. Together, they control “both our nervous and endocrine systems, like army generals keeping a close eye on all the troops”.

From the moment a foetus begins producing its own at 12 weeks until old age, hormones regulate sex, conception, birth, growth, body temperature, sleep, circadian rhythm, hunger, digestion and metabolism. In chronological chapters, The Power of Hormones follows how they rise, fall and interact with each other in pregnancy, infanthood, childhood, adolescence, adulthood, and two distinct stages of later life – after 60, and after 80.

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The description of testosterone and oestrogen as the “male” and “female” hormones is, in a sense, misleading. Men and women have the same hormones, though their roles and levels vary between the sexes. For instance, follicle-stimulating hormone (FSH) and luteinising hormone (LH) are vital to the menstrual cycle, conception and pregnancy. Men also produce both FSH and LH, which play key roles in the life cycle of sperm cells. LH and FSH also, as scientists discovered when considering why their production – seemingly illogically – increases after the age of 60, aid bone density, fat storage, body temperature and memory.

Teenagerhood is not the only time both men and women experience huge hormonal upheaval. Babies go through a “mini puberty” before the age of two, during which a boy’s testicles (24 times smaller than those of an adult man) can produce adult levels of hormones, and the growth of follicles in girls’ ovaries causes a surge in oestradiol in the blood comparable to levels at the time of ovulation in a woman. It is not yet understood why this “hormonal circus” takes place, but Nieuwdorp considers that it could be a “test drive” for the real thing.

Adults also experience a “second adolescence” in their sixties, when men and women become more similar in hormonal terms. In women, a “change of the hormonal guard” means testosterone takes over control from oestrogen. Androgens are still dominant in older men, but they also have three times as much oestrogen as in their younger years. Sex hormones decrease even further at 80, which causes a decline in muscle quality and impairs function of the optic nerve and ears. At this age, sleep quality also worsens, disrupting the circadian rhythm that determines the production of hormones over a 24-hour period. This “loss of hormonal flexibility”, Nieuwdorp writes, “is a reliable predictor of dementia and premature death”.

Women can add the menopause and, potentially, pregnancy to this list of life-changing hormonal surges. During pregnancy, the demand for hormones is so great that the pituitary gland doubles in size. Everything from your metabolism (human placental lactogen ensures the mother burns fat for their own energy, leaving the simple sugars for the child’s growth) to your immune system (in the third trimester progesterone lowers the body’s defences so it doesn’t attack the foetus) are temporarily altered. Following childbirth, it takes two years for mothers’ hormone levels to return to their usual balance.

Nieuwdorp writes that fathers also undergo smaller hormonal changes: testosterone levels fall in the last trimester of pregnancy and dip again after birth, presumably because the male hormone “can exacerbate physical aggression and increase libido; traits we would rather not see in new fathers”. He also posits that men experience a menopause of sorts: androgen deficiency in the ageing male, or “Adam”. Testosterone levels don’t stop abruptly, but they do decrease by around 1 per cent a year from the age of 30. But NHS guidance says that the symptoms some men in their forties and fifties report – including irritability, depression, loss of libido and poor concentration – are likely unrelated to hormones. Nieuwdorp’s uncritical inclusion of Adam may irritate the third of women who experience menopause symptoms so severe their work and social lives suffer.

Most surprising to me was the role of hormones in appetite and digestion, understanding of which could prove instrumental in solving the obesity epidemic. Somatostatin and ghrelin control the gradual release of food from the stomach into the small intestine. Glucagon-like peptide-1 and cholecystokinin trigger the production of digestive juices. Peptide YY (3-36) is released to signal when the tract is full. Cortisol raises blood sugar in response to perceived stress. Thyroid hormones dial up or down our metabolism. Insulin helps cells use glucose from the bloodstream as energy, and leptin, produced by fat cells, suppresses appetite. You might think this delicate interplay would work to prevent obesity and its ensuing health risks.

It doesn’t. If you lose weight, for example, fat cells shrink, meaning they produce less leptin, so you feel less full. And ghrelin, which signals when the stomach is empty, is present in higher concentrations in obese people’s blood (though a brief google reveals some disagreement in the literature over this). “Our body seems to do all it can to maintain an increased weight,” Nieuwdorp writes. On an evolutionary level this makes sense: the body prepares defences against possible scarcity. Humans “went out hunting for our food and, in search of calories, populated the whole world”. Most of us get heavier throughout our lives, and those who lose weight often struggle to keep it off. “It’s as if we have a type of built-in thermostat that keeps our weight precisely at the right level: the set-point weight.” Your body works to maintain this “right” weight, which can increase if we overeat consistently. Some animal studies support this, but though Nieuwdorp presents it as a fait accompli, set-point weight theory remains a theory: there have been few clinical trials into its existence in humans.

The book’s weakest chapter is on “Homosexuality and transgender people” – not least because this grouping suggests a link that does not exist, and that there is, as Nieuwdorp admits, “no conclusive evidence that sex hormones influence the formation of an LGBTQI+ identity”. Nieuwdorp’s support for transgender people and medical transition is clear. This position is not in itself a problem, but his refusal to engage critically with concerns (even if to dispute them) about the routine prescription of puberty blockers to children is. He makes no mention of the five European countries, including the UK, that have restricted their use. The Cass report released in April found that there is “no good evidence on the long-term outcomes of interventions to manage gender-related distress”. Instead, Nieuwdorp writes that the practice “marked a significant milestone for the acceptance and emancipation of transgender people”, leads to better cosmetic results and reduces psychological suffering. It was Nieuwdorp’s native Netherlands that in the Nineties pioneered the prescription of puberty blockers for people as young as 12 to treat gender dysphoria, part of the so-called Dutch Protocol. Perhaps, on this, he is simply too close to his subject.

It is valid that Nieuwdorp’s clinical experience and research has led him to support the prescription of puberty blockers, or set-point weight theory, or the existence of a male menopause, but their unquestioning presentation made me wonder what else he presents as fact that is contested. In an online age in which well-meaning but often unsubstantiated advice proliferates on social media about how to “regulate your cortisol levels” or “hack your hungry hormones”, there is a need for a book on hormones that is truly myth-busting. I am not confident that The Power of Hormones is it.  

The Power of Hormones: The New Science of How Hormones Shape Every Aspect of Our Lives
Max Nieuwdorp, translated by Alice Tetley-Paul
Simon & Schuster, 352pp, £25

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[See also: The rise of the worried well]

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This article appears in the 15 May 2024 issue of the New Statesman, The Great Stink