#155 - Sleep & Nature's Call

Matthew P. Walker (0:02)

Hi there, it's Matt here, and we're back for another episode of the podcast.

Today, let's discuss a measuring jug, the sort you'd get in any kitchen, the one that holds one liter made of clear plastic, and you probably don't trust it past the halfway mark. It has one function and one function alone to tell you how much of something there is. If you fill it to somewhere between three to 500 milliliters, you've got yourself a typical adult bladder capacity. The whole point of today's episode is how quickly does that capacity fill up and when is it supposed to normally fill up throughout the day?

The problem commonly associated with this reservoir is the nocturnal visit to the toilet. It's often joked about or dismissed as the cost of getting older or perhaps a personal failing best kept quiet. It is in fact a medical condition which has a medical name, nocturia.

It's clinical definition was determined by the world's continents experts in 2002 and has been discussed in research papers for many years. And I'd like to ask a slightly silly question. Is it really your bladder waking you at night? Quite a lot of the time, it very often isn't. Something else is to blame. And the bladder is taking the fall. Sometimes it's the kidneys that haven't noticed it's nighttime. Sometimes it's the throat that closes up. Sometimes you're already awake, and the bladder just provides somewhere to go.

So keep hold of that measuring jug, because by the end, you'll be looking at it very differently. Before 2002, no one knew how often was too often. Each study used a different method, and the results from these studies weren't comparable. The International Continence Society resolved this issue in 2002, as science tends to do, by using a committee.

From that moment on, it was formally established that nocturia is defined as having to get up at night to urinate, and the research community now had a standardized tool to compare data. In 2019, the definitions and the way the condition is measured were refined on the back of 16 years of research by the same group. In 2010, a different panel entirely made up of urologists, urogynecologists, nurses and behavioral therapists defined a normal adult bladder as one that empties about eight times a day and no more than once in the night. Give a couple of hundred women of normal lives two-day bladder diaries and barely one in eight ticks every box for normal bladder function. Then, the number people actually report. Send a questionnaire to 6,000 randomly selected adults aged 18 to 79 from a national population registry and about 6 in 10 reply.

At two or more trips per night, the age standardized prevalence works out at roughly one adult in eight and about the same for men and women. Scrutinize 43 community studies over nearly 20 years and at two or more void trips, anywhere from 2 to 17% of men aged 20 to 40 are affected and 4 to 18% of women of the same age. For both men and women over 70, it ranges roughly from around 30 to 60%. Reduce the requirement to one bathroom trip per night, and these older men go to about 70 to 90% plus. And younger men go at a rate of just over 10 to 35%.

The rates for older women for one trip were roughly 75%, whilst younger women went 20 to 44%.

Overall, younger women suffer the nighttime void more than younger men, and the curve generally reverses with age. Then we get to the flip side of normal, since the trips must originate from somewhere. The adage of eight glasses of water a day is the only dietary recommendation nearly universally repeated by everyone, but entirely unsubstantiated by anyone.

Search for the literature backing up this figure in healthy adults, in temperate climates, and you will find none. The official numbers substituted for it, about 3.7 liters per day of total water intake for men and 2.7 for women are survey medians. These numbers reflect what a random national sample of people happen to drink, include the water in food about a fifth, and do not decline with age. These amounts were never prescriptions. And the amounts of fluid that our bodies process is not constant across the lifespan. We can follow water through 5,604 people from 8 days old to 96 in 23 countries, using a labeled isotope. Water turnover peaks around 30 in men, around 4.3 liters per day, and holds a plateau from 30 to the mid 50s in women, around 3.4, then drops to around 3 liters in men, and just under 3 liters in women after 70 That includes water through the body from all sources, including food and metabolism. It is not a drinking target. The amount actually available for drinking is much lower than that. So the figure on the bladder side of normal is more of a committee room convention. And the figure on the drinking side of normal is based on a set of survey averages.