FOR as long as I can remember, certain numbers have inhabited my head: Five fruit and veg a day, 8 hours of sleep, 2000 calories, 2 minutes of tooth brushing, eight glasses of water, 10,000 steps.
For want of a better term, these are the “recommended dailies”: numbers that are etched onto our brains as the ones we should live by. But for all their influence, it is unclear whether sticking dutifully to them actually makes a difference to our health. So I decided to get to the bottom of things, attempting to live my own life by the numbers (see “Living the dream”, page 41) while also digging into some critical questions about them: who came up with these figures in the first place? How solid is the scientific basis for these magic numbers? More to the point, is sticking to them really worth the effort?
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Brush your teeth for 2 minutes, twice a day
Keeping your teeth clean isn’t just a way to avoid the dentist’s drill (and bill) – the bacteria that cause gum disease have also been linked to an increased risk of developing Alzheimer’s. People with gum disease also have a of having a stroke or heart attack, and while a causal link hasn’t been shown, there are suggestions that gum disease increases markers of inflammation in the blood.
The benefit of brushing is that as it removes thin films of plaque bacteria from the teeth, it breaks up bacterial communities, disrupting the opportunity for social evolution. This essentially stops them evolving into super bacteria that are experts at causing decay.
Where the “rule” about brushing for 2 minutes, twice a day comes from isn’t clear (Public Health England is unsure of its origins, despite the recommendation appearing on National Health Service sites), and there is conflicting evidence on how much, and how often, is actually needed.
A at the University of Göttingen in Germany concluded that brushing once a day could be sufficient for good oral health, but argued that since most people aren’t particularly thorough, twice a day is probably better.
As for how long to brush, in 2012 the Academic Centre for Dentistry Amsterdam, in the Netherlands, looked at this and that when people brushed for 2 minutes, they removed around 41 per cent of the plaque on their teeth, while those who brushed for 1 minute removed just 27 per cent.
Unfortunately, 41 per cent is still quite low, which perhaps suggests that we should brush for even longer. A 2009 article in the Journal of Dental Hygiene found a , noting that “even after 3 minutes of brushing, some plaque removal still appeared to be occurring”.
Yet, as we all know from TV advertising, only nine out of every 10 dentists agree. Santosh Kumar Tadakamadla, a research fellow at Griffith University’s School of Dentistry and Oral Health in Australia, says that some researchers argue that brushing with hard bristles can damage the teeth by causing erosion. But with the British Dental Health Foundation reporting in 2010 that , more is probably better for most people.
Although people who have prior experience of tooth decay or who consume a lot of sugar between meals might need to brush more than others, Tadakamadla says that 2 minutes twice a day should be fine, particularly when paired with fluoridated toothpaste to protect the enamel.
Take 10,000 steps
In the mid-1960s, a small, plastic pocket watch-like device went on sale in Japan. Called the “manpo-kei”, it was the . Roughly translated, manpo-kei means “10,000 steps meter”.
Why 10,000? “It likely originated as a marketing tool,” says I-Min Lee, an epidemiologist at Harvard University. Not only is 10,000 an easy number to remember, the character for 10,000 in Kanji, a script used in Japanese, .
This health target, then, didn’t originate with science. Can we scrap it and have a nice sit down? Herman Pontzer, an evolutionary anthropologist at Duke University in North Carolina, studies the Hadza, hunter-gatherers in Tanzania, as a window into how humans lived thousands of years ago, and the levels of activity our bodies were built for. In a recent study, he measured more than 2000 days of Hadza activity, and found that Hadza men aged 18 to 75 walked on average 18,434 steps a day, while women in the same age range walked 10,921. The Hadza tend to avoid many of the chronic illnesses of Western societies that are related to inactivity.
So should women dedicate time and energy to getting those extra 921 steps? Perhaps not. In a , Lee and her colleagues found that women who averaged 4400 steps a day had lower mortality rates than those who took fewer than 3000 steps. Mortality rates decreased as step rates increased up until around 7500 steps, beyond which no additional mortality benefit was observed.
Lee’s study only examined mortality – not quality of life – and she doesn’t want to dissuade anyone who does get their 10,000 steps. “Stepping more is better, but we don’t necessarily need to reach that 10,000 number,” she says. She studied older women, but says her findings are applicable to “a broader group of people who aren’t very active”.
Another thing worth remembering is that the step counter on your wrist or in your pocket doesn’t really measure steps at all, but the motion of your hands or hips. According toTessa Strain at the University of Cambridge, who studies the epidemiology of physical activity, step counts are a rough proxy for energy expenditure, and . Not all steps, then, are created equal.
The rate of energy expenditure in a given task is given as a “metabolic equivalent” or MET. One MET is the energy expended at rest. According to , developed by William Haskell, a cardiovascular health researcher at Stanford University in California, walking slowly on level ground requires 2.8 METS; walking briskly, say at 3.5 miles per hour (5.6 km/h), is 4.3 METs; and walking briskly uphill accounts for 5.3 to 8 METs, depending on how steep the ground is.
“The food we eat accounts for around 30-35 per cent of moisture intake”
The upshot is that if you have only done a few thousand steps but they were brisk and uphill, there is no need to walk around in circles to get to 10,000. And if you average nowhere near 10,000, the important thing to remember is that any steps are good, but at the lower end of the scale, more is better. In , Strain found that the biggest differences in health risk were between those doing minimal amounts and those doing slightly more. “I think recent efforts to promote the message ‘every move counts’ as strongly as an actual number is a really positive thing because I think it better reflects what we know about the association, and it is a universally understood message,” she says.
In short, don’t burn your trainers, try to move more and don’t forget that 10,000 steps was originally a marketing ploy.
Drink eight glasses of water
Whether you are walking 10,921 or 4400 steps a day, you will need to keep hydrated: but is it necessary to gulp down eight glasses of water every 24 hours? Water is clearly important stuff: it delivers nutrients and flushes out waste, helps cells maintain their shape, lubricates our joints and helps to regulate body temperature. It also keeps our brains working at full capacity. According to a 2018 study from the Georgia Institute of Technology, 2 hours of vigorous gardening in the sun without drinking can be enough to impair our cognitive functions. But do we really need eight glasses a day, no matter what?
That figure dates back to 1945, when the US Food and Nutrition Board of the National Research Council published dietary guidelines that recommended consuming . That adds up to 2 litres of water a day for those on a 2000-calorie diet.
Yet Tamara Hew-Butler, an exercise scientist at Oakland University, Michigan, says the guidelines were misunderstood. “That recommendation included all of the fluids in fruits and beverages, it wasn’t just about drinking water,” she says. In fact, much of our water intake comes from the food we eat. Population surveys from France and the UK published in 2016 found that food.
The 2-litre recommendation wasn’t just misunderstood: over time, it was simplified into eight glasses of around 250ml each, or 8 × 8 ounces glasses as per widely quoted in the US version. Hew-Butler believes bottled water companies capitalised on the misconception.
How much topping up we actually need varies considerably. For people with a history of or bladder infections,– but for everyone else it varies. “How can one particular recommendation be good for everybody?” says Hew-Butler. “If you’re bigger, if you’re more active, and if it’s hotter outside, you’re going to need more.”
The answer to how much more comes from an oft-ignored line in the original 1945 report, which stated that “sensations of thirst usually serve as adequate guides to intake except for infants and sick persons”.
Our bodies are set up to take care of this without the need to count glasses, says Hew-Butler. “The gene that governs water balance, it’s been conserved in nature for 700 million years – it’s in worms, it’s in insects, it’s in every mammal out there,” she says.
The main hormone that governs water balance is the antidiuretic hormone vasopressin, or AVP. In mammals, AVP participates in a feedback loop between the brain and kidneys, so that when the body loses water, AVP is secreted by the brain, which signals the kidneys to produce less urine. Thirst is activated once the kidneys have conserved all the water possible. If we drink more water than we need, then the brain stops the production of AVP and more urine is produced and we spend a lot more time in the bathroom.
If you really want to put a number on your water intake, says Hew-Butler, you could estimate the energy used during exercise and aim to take in 1 millilitre of water for every calorie burned. Drinking when you are thirsty, she points out, is far simpler.
Get 8 hours of sleep
Eight is clearly an appealing number – as well as glasses of water per day, it is often touted as the magic number of hours of sleep per night for optimum health.
This, at least, does seem to be based on research. While the number of hours varies from person to person, says Ciro della Monica, research fellow at the University of Surrey’s Sleep Research Centre, “Eight hours is an average ideal for a healthy adult”.
“There is tentative evidence that too much sleep can be harmful”
Sleep isn’t just satisfying – it’s a matter of life and death. Rats deprived of sleep die within a month and, in humans, insufficient sleep has been shown to impair physical and cognitive performance, impact mood, reduce alertness and worsen reaction time. In the longer term, insufficient sleep has been . Neuroscientist Maiken Nedergaard at the University of Rochester, New York, suggests that sleep activates a waste-disposal network in the brain that flushes out harmful metabolic debris, including amyloid protein, the build-up of which is associated with Alzheimer’s disease.
Multiple studies have also linked how much sleep you get and lifespan. While it varies a bit from the 8-hour recommendation, a of more than 1 million adults conducted by the American Cancer Society found that men who got 7 hours of sleep had lower death rates over the following six years than those who slept less or more, with those who slept just 5 hours experiencing “very high death rates”. This link has .
There is clearly some leeway in the 8-hour rule. Jerome Siegel, a sleep researcher at the University of California, Los Angeles, has studied pre-industrial societies in Tanzania, Namibia and Bolivia, and found they sleep on average between 6 and 7 hours – finding a “striking uniformity” across the groups.
According to a poll by YouGov, . A 2018 study from Western University in Ontario, Canada, suggests that these people are at an advantage, cognitively speaking. In a , 7 to 8 hours of sleep proved to be optimum for cognitive function, while participants who got just 4 hours nightly performed as if they were almost 8 years older than they were.
On the other hand, there is tentative evidence that too much sleep can also be harmful. Long sleep duration has been associated with , diabetes, stroke, coronary heart disease and obesity. Correlation isn’t causation, though, says della Monica. It hasn’t been shown if long sleep causes these health outcomes or if they are consequences of underlying diseases.
Intriguingly, some lucky people can get away with very little sleep without the biological consequences. In 2019, biologist Ying-Hui Fu at the University of California, San Francisco, found two DNA mutations, in genes called ADRB1 and NPSR1, that allow people to thrive on just 4 hours of sleep. Fu found the genes by studying families who reported feeling fully rested after much less sleep than normal. When she engineered the NPSR1 mutation into mice, they slept less, but their memory and health didn’t seem to be impaired. Fu speculates that these mutations could be very recent and therefore may continue to spread. So, while in the distant future people may be able to sleep less than you and stay healthy, for now we should aim for at least 7 hours.
Eat five a day
Everyone knows they should be eating five portions of fruit and veg a day, yet according to government surveys, and . Which makes the , that 10 a day is even better, seem even less achievable.
The five-a-day figure is based on the World Health Organization’s . Five a day comes from taking 80 grams as an approximate serving of fruit or veg – it is the weight of a small orange, a handful of blueberries or a carrot, for example. According to the WHO, this figure was based on “epidemiological evidence of an increased risk of cancer with low intakes of certain fruits and vegetables and their contributions to micronutrients as well as to dietary fibre”.
In the 2017 analysis by the Imperial College team, led by Dagfinn Aune, however, eating 800 grams (or 10 portions) of fruit and veg a day was preferable to the well-established five, with the team estimating that this change could prevent 7.8 million premature deaths globally every year.
The group found that compared with someone who ate half a serving of fruit or vegetables a day, people who ate five a day saw a 14 per cent drop in risk of coronary heart disease, while people who ate 10 had a 24 per cent reduction. Similar patterns emerged for stroke and heart disease – when it came to all-cause mortality, five-a-day-ers had a 24 per cent reduction in risk compared with half-a-portion-ers, while 10-a-day-ers had a 31 per cent drop.
Aune stresses that if 10 a day feels overwhelming, do what you can. “It’s important to note that we did see benefits even with modest intakes versus very low or zero intakes, so it is not about all or nothing,” he says.
Aune’s research also found that particular types of fruit and veg seemed more beneficial, including apples and pears, cruciferous vegetables like cabbage and kale, berries, and citrus fruits. Focusing on quality over quantity may be a better strategy than encouraging people to aim for a set number of portions, he suggests.
2000 calories
When it comes to what we eat in general, many of us are aware that we should be eating somewhere around the 2000-calorie mark. Pontzer, however, says that this is also a misconception. In his book, Burn: The misunderstood science of metabolism, Pontzer says that the figure originated in the 1990s when the US Food and Drug Administration (FDA) asked the nation to self-report their intake. In the study, women claimed to consume between 1600 and 2200 calories a day, while men confessed to eating anywhere between 2000 and 3000. The FDA pinned 2350 as the average and then rounded the figure down, further simplifying the message and to avoid overconsumption in women. Even in the original research, then, 2000 calories per day was an estimate. On top of this, Pontzer says that people tend to– “if you ask somebody how many calories they ate, it’s basically a random number generator”. When you actually measure how many calories people eat, says Pontzer, women actually take in closer to 2500 calories a day on average while men consume more like 3000. Is that a problem? Not necessarily.
When it comes to what we should eat, Pontzer argues that if your weight is stable, then “what you’re consuming is also what you’re burning”. Needs vary from person to person based on our body mass and the demands we put on our bodies: for example, Pontzer estimates that a 45-kilogram gymnast with 10 per cent body fat would need 2200 calories a day. “The one-size-fits-all policy is have a look at the bathroom scale,” he says.
Pontzer concedes that if you do manage to figure out your individual needs, then keeping track of your calories can be a helpful guide for maintaining a healthy weight and avoiding the slow creep of weight gain. The number 2000, however, isn’t necessarily where the answer lies.
Living the dream
For one week only, I ticked all the boxes. I stuck to 2000 calories, ate my greens, guzzled water and went to bed on time.
At the end of day one I had a headache, was fed up of counting glasses of water and frustrated because needing to pee kept interrupting my 10,000 steps.
On day two, though, I started to feel the benefits: I felt less bloated than usual, and was noticeably happier and more energised. By the end of the week, my leg muscles felt tighter from all the walking, but I was contemplating setting fire to the world’s supply of kale.
Overall, it was a success, but it required constant effort, and I’m not sure I could keep it up for the years that research-based health outcomes are based on. Still, a few weeks after my experiment, I have managed to stick to a more manageable 7500 steps a day, ensuring at least some of them are vigorous. Two minutes of toothbrushing, feels like half an hour and who has the time? I leave my water and my sleep up to my body. I leave kale alone, full stop.
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