Why is it so hard to lose weight?

After several years of looking, I think I have finally found the answer.

So if people follow a calorie-controlled diet based on government guidelines, almost everyone will put on weight.

Let me explain…

Mifflin St Jeor

The minimal basal metabolic requirements (BMR) of a human being have been the subject of scientific study for more than a century.

The best estimate of our requirements are the Mifflin St Jeor (MSJ) Equations which state that the calorific requirements for men and women are given by:

Men:     BMR = 10 × [Weight in kg] + 6.25 × [Height in cm] – 5 × [Age in years] – 5

Women:     BMR = 10 × [Weight in kg] + 6.25 × [Height in cm] – 5 × [Age in years] – 161

This is the amount of food (expressed as kiloCalories (kCal) per day) required to maintain a given weight and do nothing else: no exercise at all.

A sedentary male lifestyle

The MSJ equations are generally multiplied by a factor to reflect the amount of physical activity one undertakes during the day. And there is considerable uncertainty about which factor applies to any particular individual!

The factor 1.2 is commonly chosen to represent a “sedentary lifestyle”. In a moment I’ll come back to whether this factor is justified or not.

But based on this factor, the blue line on the graph below shows how the actual calorific requirements of a man of my weight and height vary with age. The equivalent graph for women is shown in the next section.

Calories versus Age

The most striking thing about this graph is that the actual amount of calories I need to maintain my weight (1860 kCal/day) is 25% less than the government recommend (2500 kCal/day).

The difference is not a rounding error – it amounts to 640 kCal/day which is a reasonably-sized meal!

A man of my age living a sedentary lifestyle and following government guidelines would put on weight at a rate of several kilograms per year.

The second striking feature of the graph is reduction in calorific requirements with age. The slope of the graphs is 50 kCal/day per decade.

This means that if I was maintaining my weight in my forties, then unless I changed either my eating habits or my exercise habits, I would slowly begin to put on weight.

Eating 50 kCal/day too much amounts to putting on weight at around 2 kg per year.

Is the sedentary lifestyle factor 1.2 appropriate?

One way to assess whether the factor 1.2 applied to the MSJ equations is appropriate is to consider the calorific equivalent of some exercise.

For a man of my weight and height, running 1 kilometre uses up about 74 kCal.

So if I were to run 25 km per week, then this would allow me to eat about another 260 kCal/day and still maintain my weight. This is shown as the red line on the graph above.

Most people would consider running 25 km per week to be quite serious exercise. Comparing this amount of exercise to the work done in a sedentary day makes me think that the factor 1.2 is probably about right.


The equivalent graph for women is shown below

Calories versus Age Women

It shows a similar disparity between government recommendations and actual metabolic requirements, but not quite as dramatically wrong as for men.

Government Guidelines

The reason I searched out the MSJ equations was because I know from experience that if I eat anything close to 2500 kCal per day I put on weight.

Calorific intake is notoriously difficult to estimate with an uncertainty better than about 10%,  but the MSJ figure of about 1860 kCal/day for a man of my age weight and height seems about right.

The UK Government guidelines are – frankly – nonsense, and given that the UK has something of a problem with obesity – not least with people of my age – it would seem a sensible first step to just get this simple factual message about right.

One important step would be to emphasise the reducing calorie requirements with age.

Government guides in the US such as this one are closer to reality, but if you want real information I recommend this helpful calculator.

4 Responses to “Why is it so hard to lose weight?”

  1. edhui Says:

    My concerns are very similar to yours and I have often thought about this. I wonder why exercise is seen as the one big variable? To me, the obvious other variable is heat- statistics seem to treat environmental temperature as of negligible effect, yet we know polar explorers lose weight on 8000 kcal a day. Also, does it matter if we ingest hot or cold food and drinks?
    The other thing is that people talk about individual metabolisms, as if the actual efficiencies of our internal chemistry differ. But nobody seems to talk about digestive differences- that is the simple question of calories in minus calories out. To put it bluntly, what percentage of calories in comes out as poop, and does that differ between individuals or if you eat small meals or binge? And do these efficiencies change if you eat less- does the body extract a greater percentage of calories from food if you are nearer the basal input level?
    The body seems to be capable of homeostasis- if only weight varied in direct relation to calories eaten! It feels like you stay at a certain weight for years and then suddenly jump to another stable number.
    I think this is a fruitful area for official NPL research!

  2. protonsforbreakfast Says:

    Ed. First of all, regarding the temperature of food eaten (which you didn’t mention), it is generally tiny in comparison with the the chemical calorific value of the food. For example, 500 g of ‘mash’ might have 460 kCal of ‘food value’ but heating it to 60 °C will only add 20 more kCal of energy.

    Regarding environmental temperature, the calorific requirement to keep a body warm is massive but there are very large downsides to exposing one’s body to cold temperatures. Sitting still in even 15 °C will quickly become unpleasant. When it comes to polar explorers I think it is the ‘explorer’ bit that it is more important than the ‘polar’ bit.

    Regarding metabolic efficiency, I think how much unprocessed food comes out as ‘poop’ is exactly what metabolic efficiency measures.

    A recent article in Scientific American looked at the shortcomings of the ‘weight gain = calories in – calories out’ model of the metabolism and actually – within about 5% to 10% – it sort of makes sense.

    Homoeostasis does work roughly. But my calculations are showing that with regard to weight, the balance is exquisitely delicate. I suspect that in this modern world that we need to supply the feedback for homoeostasis manually by weighing.

    More on this soon

    • edhui Says:

      20kcal is nearly 5% of 460 kcal! And drinking tea all day instead of iced water…
      Is that metabolic efficiency? I think they measure metabolic efficiency by calometry rather than burning poop.
      The amount of energy used by the body as measured by calorific output must surely be independent to the food/poop calory ratio. To put it crudely, if you have a dodgy meal at night and the world falls out of your bottom the next day, you won’t have taken in the same number of calories as you would have if your digestion stayed normal. I imagine there’s got to be variation between people as to how efficient their guts are at extracting calories from food. And differences in diet- I remember eating corn on the cob as a kid, presumably not chewing properly, and being amused by its reappearance apparently unaltered a day later. Can’t have got any calories out of that, but there is a textbook calorie count for corn on the cob, isn’t there?
      Anyway- more on this when you can please- it’s a delight to hear about weight loss from someone who can look at it rationally.

  3. Is weight homeostasis possible? | Protons for Breakfast Blog Says:

    […] recent discovery of the Mifflin St Joer equations (link) has allowed me to  simulate my weight over time, and my calculations are allowing me to […]

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