The article by Pontzer raises important and interesting points. It is very relevant in India, where activity levels in urban populations are worryingly low. In one report in the journal nature, this was measured at about 4,000 steps per day. So the bar of 10,000 steps is a reasonable ask, for an approximate doubling of activity. But is it enough? Will it specifically work for weight loss? Here is an important message don’t worry about body weight just being active is good for the heart, muscles and brain.
It is a reasonable hypothesis that Pontzer argues, that for good health, we might eventually be setting the bar too low for the target number of steps per day (it might not be 10,000 steps). The origin of the magic number of 10,000 steps per day for good health is mysterious, but it is a convenient way to translate walking activity. Let’s take walking, for say, an hour per day. There is the apocryphal saying that ‘a walking man can cover a distance of one league in an hour’.
A league is very roughly about 5 km, and assuming a step distance of half a metre for a normal height Indian, will require about 10,000 steps per hour. For those with even shorter steps, the number is higher of course. But does the shorter person with more steps per hour spend more energy? And if one runs this distance, what happens? Many complexities here, but eventually, the number of steps means little in terms of the energy spent.
Walking on difficult terrain, or up a slope is more tiring, so more energy is spent. The step count tells us nothing about this exertion. That said, in a simple framework, it is advantageous to emulate more active lifestyles (that require 15,000 steps per dayor more) relating to better health and longevity. Set the bar higher. But questions still remain.
The big one: Will stepping more help me lose weight? That answer is never simple. Let us start with the question of effort, translating into energy spent. Does it take more effort to transport a heavier body for the same number of steps? Logic says yes. This means that a heavier person will benefit more from the walking, and should spend more energy; and lose more weight. But more of that later the weight loss story is complicated. Before that, let us look at how we rate effort and activity in physiology and nutrition.
The energy spent (heat) in physical effort is difficult to measure, but can be calculated more easily from the air breathed (oxygen consumption)during an activity. A metric for ‘effort’ can be derived by dividing the ‘energy spent during the activity ‘by’ the energy spent at rest (also called basal)’. Thus, being at rest would calculate a value of 1 for the activity. This metric (also called the physical activity ratio or PAR; the term ‘met’ is also used) would therefore increase as the effort of the activity increased: for example, walking at normal speed would give a value of about 3-4, and running would give a value of 6.
Dietitians use this value to calculate how much energy you need based on your reported activity. They take the PAR of all reported activities in a day and multiply it into the energy spent at rest (called resting energy expenditure, measured in calories), to obtain the required daily energy intake. So, if the average PAR for the day was 1.5, then multiplying this into the resting energy expenditure (say 1,000 calories) would give an energy requirement of 1,500 calories/day. But note the point of interest here the PAR is a ratio, where the body size is a factor in both the numerator and the denominator.
Body weight effectively cancels out in the ratio, meaning that the PAR should be independent of body weight. Dietitians love this point a single PAR value for any given activity can be used for people of all sizes, reducing their need to look up PAR tables. But things are never that simple, and the theoretical independence of body weight for the PAR is now proving wrong.
Careful measurements have shown the PAR actually increases as the body size increases, for a range of activities. This means a larger person would spend relatively more energy in the same activity as a smaller person. There is also an efficiency of movement: for those habitually doing some form of labour or activity, over time, there is an economy of movement, or efficiency, that results in a lower PAR. This is adaptation.
How often have we heard the refrain “I lost a little weight initially when I started exercising and/or dieting, but now my weight loss is zero?” As body size decreases, the energy spent (PAR) in the effort also decreases, asking for the effort bar to be raised. In crude translation, increase the steps. So, does the step count target help lose weight? Yes and no: it depends on the exertion effort for the steps, and initially could help in a small way towards weight loss. Complex enough? Wait, there is the elephant in the room our behaviour.
Behaviour is mysterious and not always predictable. As an example, let’s take the discovery of contraceptives: this protective method actually increased risky sexual behaviour. That was an unintended consequence.
The number of steps (and little gadgets that measure them) are similar in principle. If you spent an hour a day stepping up to the golden requirement of 10,000 steps, ask yourself: what did you do for the rest of the day? Did you feel ‘protected’ by getting to your goal,? Did you then become a couch potato for the rest of the day (risky stuff indeed nibbling and grazing all the while) you felt protected, remember? So think about the consequences of this behaviour. Sitting, instead of standing, means that considerable postural effort (that spends energy) against gravity is lost, with a lower total energy expenditure.
You are throwing away the good you achieved in walking. So, do not think that the more you walk, the more your total daily energy expenditure will automatically be. There was an excellent experiment published in CellPress that actually showed this (Pontzer again).
Measurements of daily physical activity (by activity monitors) were compared with measurements of total energy expenditure. Up to a point, these measurements matched meaning, as activity went up, so did daily energy expenditure. Beyond this point, as the daily activity increased (as measured by the activity monitor), the relation of activity with total energy expenditure went flat it plateaued out.
It is therefore not a ‘given’, that the total energy expenditure for the day faithfully increases as physical activity increases (Pontzer called this an additive model: activity energy expenditure adds to other expenditures). It does not work that way. In practice, the total energy expenditure was ‘constrained’ or kept down such that it remained in a fairly narrow range.
The biological reason for this is not fully known. It may be due to adaptations in the way the body allocates energy to other non-muscle based tasks, like those relevant to the activity of the organs for example. This is complex, but easier to surmise is a simple behavioural change in the way we might respond to increasing physical activity. We might have more rest periods in between, particularly sitting or lying down, which can reduce daily energy expenditure.
So, as we increase energy expenditure by stepping or running, we also decrease it dramatically by ‘vegging out’. OK we are equating steps to an activity monitor’s output in the Pontzer study, and that is reasonable, but the point coming out here is this: is a single metric of 10,000 or 15,000 steps going to be that relevant to body weight maintenance or even weight loss? No, not unless you are active in all dimensions of a day. So, should your lifestyle aspirations become unidimensional? The short answer is,no, it should not. If what you think is a helpful intervention becomes your sole goal, you might be disappointed in terms of your weight. Remember body weight should not be the target here think of your cardiovascular, muscular and mental health as well.
So, it looks like the number of steps is not a panacea for weight loss. What else is there, if weight loss is the goal? The answer probably lies in the energy intake, rather than the energy output. Our food environment is now so adverse for weight maintenance the availability of cheap ultra-processed foods (they can be remarkably energy dense) is increasing.
Eating out in restaurants or fast food outlets is also simply a way of packing away about 1,000 calories per meal (more than half the daily requirement of a sedentary person), as a study in India showed.
One hears (some with nostalgia) about the hunter-gatherer lifestyle in the distant past, where intense hunting activity was followed by equally intense eating and then by intense rest. Our lazy 'inner ape' calls the shots about our activity (Pontzer’s words), harking to our ape relatives “who are impressively sedentary, resting and sleeping for 18 hours a day”.
Well, they ate foods that were pretty much poorly digested, and certainly low in energy density. If they had access to our energy dense foods, they might well be extinct. Let us circle back to the arguments above: without restraint in eating, and a conscious avoidance of ultra-processed food (now well-defined on the internet), simply stepping up your daily steps is not an answer to maintaining or losing weight in a sustained way.
Finally, for whom should we set the bar at 10,000 or 15,000 steps? We take it for granted that the step target is independent of age. I think it is inconceivable to ask the elderly to be so active without risk (my active ‘inner squirrel’ says that’s nuts!). If one has always lived a good (diet-wise) and active life, perhaps these goals are possible at advancing ages. But starting out, at any age one needs to ease into what one can do. Running for two hours a day? That is very good if you can, but don’t be overambitious: start small and ramp up.
Make sure that the running path is good and even, free of dogs that chase your inner squirrel and of footpath bikers, with breathable air. If not, weigh the risks you take, and aim as high as you can without encountering other risks. Just be active all day. Sit less. Don’t chase magic bullets. Eat moderately. Eat right. Eat minimally processed foods. And yes, walk more.
The author is the Professor of Physiology and Nutrition at St John’s Medical College, Bengaluru; Head, IAEA Collaborating Centre in Nutrition; Margdarshi Fellow of Wellcome Trust/DBT India Alliance and past-president, Nutrition Society of India.
