Kidney stones were once regarded largely as a problem of middle age. Today, however, doctors are increasingly seeing young adults, including people in their 20s and 30s, arriving with the excruciating pain of a stone moving through the urinary tract. The question is not simply why a young person develops a kidney stone. The more interesting question is what has changed in the way young people eat, work, exercise, sleep and hydrate.

Dehydration hits harder than people assume. It does not always arrive dramatically, with a parched mouth, dizziness or collapse. Sometimes it works quietly, hour after hour and day after day, leaving the kidneys with too little water to dilute the substances they are trying to eliminate. When urine becomes concentrated, calcium, oxalate and uric acid can reach levels at which crystals begin to form and grow. A young office worker may spend eight or ten hours at a desk, drink several cups of tea or coffee, remain in air-conditioning for much of the day, postpone drinking water because of work, exercise in the evening, sweat heavily and then fail to replace the lost fluid. None of these habits alone necessarily causes a stone. Together, however, they can create an environment in which stones become more likely.

The oxalate problem is hiding in everyday food. Oxalate occurs naturally in foods such as spinach, amaranth leaves, beetroot, nuts, peanuts, sesame, chocolate, tea and some legumes. It is not a poison, and these foods are not inherently unhealthy. The problem arises when oxalate is absorbed from the intestine and eventually reaches the urine, where it can combine with calcium to form calcium oxalate crystals—the most common type of kidney stone.

There is an important twist. Dietary calcium can actually be protective because it binds oxalate in the intestine and reduces its absorption. This means that someone trying to prevent stones should not automatically eliminate calcium from the diet. The more sensible approach is to maintain normal dietary calcium, preferably with meals, while avoiding excessive quantities of high-oxalate foods if one is particularly susceptible. The kidney-stone story is therefore not about one villainous food. It is about the interaction between food, water and individual body chemistry.

Then there is citrate, one of the body’s natural defences against kidney stones. Found abundantly in lemon and other citrus fruits, citrate can bind some of the calcium in urine, leaving less free calcium available to combine with oxalate. It can also interfere with the growth and clumping of crystals. This is why lemon water has a reasonable scientific logic behind it: it combines fluid, which dilutes the urine, with citrate, which can inhibit crystal formation. Lemon is not a magic cure and cannot be relied upon to dissolve an obstructing stone, but adequate hydration and sufficient urinary citrate can be valuable components of stone prevention.

Modern medicine and modern consumer habits add another complication. Painkillers are readily available, and many people take them casually. Non-steroidal anti-inflammatory drugs such as ibuprofen, diclofenac and naproxen do not ordinarily cause kidney stones, but excessive or prolonged use can stress the kidneys, particularly when a person is dehydrated. Supplements deserve similar caution. High doses of vitamin C can increase urinary oxalate in some people, while excessive protein intake and certain fitness regimens can alter urinary chemistry. Supplements are not automatically dangerous, but the assumption that something sold over the counter must be harmless—or that more must be better—is a poor principle when the kidneys are concerned.

Perhaps the biggest change is environmental and behavioural. Human beings evolved to be physically active, to obtain food through considerable effort and to live without constant access to processed food and sugary beverages. Modern life has reversed much of this. We sit for hours, work in air-conditioned rooms, order food at the touch of a screen and can go through an entire day without realizing how little water we have consumed. The body, however, has not been redesigned or re-evolved for the convenience of the smartphone age.

A kidney stone does not necessarily appear overnight. The crystal may begin forming long before the first attack. There may be no pain, no obvious warning and no dramatic symptom to tell a 30-year-old that the chemistry of his urine has been quietly changing. The eventual attack can therefore appear sudden even though the process may have been developing for years.

The stone that forms at 30 may be the product of habits that were never designed for a desk-bound, air-conditioned, chronically under-hydrated life. The kidney quietly concentrates urine and maintains the body’s delicate chemical balance, but it has no alarm bell that tells us a crystal is slowly forming. Often, there is no warning until that accumulation becomes a stone, begins to move and the pain finally arrives. By then, the kidney has been keeping the score for years.