In recent years, water bottles have grown almost absurdly large, while health influencers claim that drinking more water can improve skin, sharpen the mind, boost exercise performance and solve many other problems.

Yet hydration expert Tamara Hew-Butler argues that much of this advice ignores how carefully the human body already controls its water balance.

She has studied hydration for more than two decades and serves as medical research director of California's Western States Endurance Run, a demanding ultramarathon in which poor drinking decisions can have serious consequences.

Her experience offers a useful reminder that both dehydration and excessive drinking deserve equal scientific attention.

The first myth is that everyone should drink six to eight glasses, or about two litres, every day.

In reality, water needs vary according to body size, activity level, climate and diet.

A small office worker sitting indoors will not lose as much water as a large athlete training under the sun, so giving both people the same target makes little sense.

According to Hew-Butler, people should generally replace the water they actually lose. Although we cannot measure that loss easily, the brain constantly monitors the concentration of the blood and creates thirst when more fluid is needed.

Contrary to popular belief, feeling thirsty does not mean dangerous dehydration has already begun; for most healthy people, it is simply a timely signal to drink.

A second myth is that only plain water counts. Coffee, tea, soup, fruit, vegetables and smoothies all contribute to daily fluid intake.

Coffee is often described as dehydrating because caffeine is thought to act as a diuretic, but the water in an ordinary cup usually outweighs that effect.

Alcohol is different, since it can increase urine production and contribute to fluid loss.

The third myth is that drinking extra water, even without thirst, must provide additional benefits. The body regulates water levels tightly, so surplus fluid is normally removed through urine.

Evidence that excessive water improves skin, digestion or immunity remains limited, although drinking before meals may sometimes reduce appetite.

More importantly, overhydration can be dangerous. If someone drinks several litres within a short period, the sodium in the blood may become severely diluted, causing hyponatraemia.

This can produce headache, nausea and bloating; in extreme cases, the brain may swell, creating a life-threatening emergency.

Estimates suggest that consuming three to four litres within an hour may be enough to trigger symptoms in some people.

The final myth is that sports drinks are essential whenever people exercise. These products contain electrolytes, such as sodium and potassium, as well as carbohydrates that can supply energy during prolonged activity.

However, most ordinary workouts do not cause enough electrolyte loss to require special replacement drinks.

Even after several hours of exercise, many people can restore lost minerals through normal meals.

Sports drinks become more useful during exceptionally long, intense events, particularly in unfamiliar heat, when athletes are sweating heavily and running low on energy.

The broader lesson is simple: hydration is personal, not a competition.

For most healthy adults, drinking when thirsty, obtaining fluids from both food and beverages, and paying attention to unusual symptoms are more sensible than chasing a fixed daily number.

Bigger bottles may be fashionable, but more water is not automatically better, and the body's own signals are often the most reliable guide.