Better Weight Management

Better weight management through science

The problem of weight management

Most affluent people in the world have a problem with weight management. Obesity is associated with higher odds of encountering a wide variety of deteriorative illnesses. As an example, if you have diabetes in your family, two factors which can change your odds of transitioning to full blown diabetes are obesity and muscle mass. Genetics may load the gun, but what fires the trigger is partly in your hands.

Generally, people `slow down’ as they grow older. In a quiet and insiduous way, this leads to a weight gain if old diet habits are held intact.

Perhaps more than half of affluent people are on a time-trend of increasing body weight. Most people who try to lose weight fail to make a significant dent on the problem.

In the West, a body mass index of 25 is considered the threshold of obesity. If you’re at 24.9, you should not be satisfied: you’re at the threshold of obesity. Other ethnic groups need different standards. In Singapore and in India, the authorities are pushing 23 as the threshold. There are significant health benefits of pushing further down below a target of 23.

Conservation of energy

The energy you take in, minus the energy you use, is your energy imbalance, also termed `energy gap’. This gets converted to fat. The exchange rate is 7.8 kcal (also written asCal or food calories) per gram of fat. In other words, when your body has piled up 7800 kcal of extra energy, this shows up as 1 kg of weight gain. Conversely, to lose 1 kgrequires an energy deficit over a set of days that adds up to 7800 kcal.

Measurement of the energy you take in

This is done by adding up the calorific value of everything you eat. E.g. the US FDA has a good database which can guide you on the calories per gram of the various things that you might eat.

As a thumb rule, the energy-intensive foods are fat, cereal and meat. You can go far with eating fresh vegetables (which average 0.2 kcal/g), fresh fruit (which averages 0.5 kcal/g), egg whites (which average 0.5 kcal/g) and milk with 2% fat (which is 0.5 kcal/g).

Everything else is troublesome. Surprisingly, lean meat (e.g. trout at 1.2 kcal/g) is less dense than cereals (which are 1.3 kcal/g for boiled rice and twice that for the typical commercial bread). Commercial food which does not report calories per gram is dangerous because their path to making it tasty involves laying on the butter or cream. Butter is 7.2 kcal/g. A tablespoon of butter is 102 kcal, which is the same as 566 grams of tomato.

It’s a bit painful to log everything that you eat. I tried to do measurement and it was hard to track the weight and energy content of a lot of things that we eat every day. Matters are made worse by the fact that cooking (or the lack thereof) modifies the calorific value of food. The energy gained from eating cooked food is not the linear combination of the energy content of the raw materials. In short, it’s hard to have more than a hazy idea of how much energy we’re taking in.

Measurement of the energy that you use

The key term here is `metabolic equivalent’ or METs. The energy cost of sitting quietly works out to 1 kcal per kg per hour. (I always found it surprising, how there was a round number here). In other words, if a person weighing 100 kg sits tight for one hour, he burns 100 kcal. Sitting tight is termed a 1 MET activity.

Scientists have compiled tables which show the energy intensity of various activities [pdf]. From this table, we see that walking at 5 kph is 3.3 METs. Running at 8 kph is 8 METs.

As an example, a person weighing 70 kg who runs at 8 kph is burning 560 kcal per hour or 9.33 kcal per minute. Since the human body stores fat at 7.8 kcal/g, this corresponds to losing 1.2 grams per minute of a 8 kph run.

Kcal per km for running, across a wide range of velocities

Can one roughly guess what the METs associated with running at a certain speed? Suppose you do 1 km at x1 kph vs. x2 kph where x2>x1. The time taken to do this will be 60/x1 vs. 60/x2 minutes. To hold the energy expended to run 1 km constant, the energy outgo per minute has to be x2/x1 times. In other words, if you switch from 5 kph to 10 kph which halves the minutes per km, and the METs double, then your energy per kilometre is unchanged.

Remarkably enough, the human machine is rather efficient, and so the cost of running at (say) 10 kph turns out to be much like 2x the cost of running at 5 kph. (This came as a bit of a surprise to me; I had expected efficiency to degrade as you go faster. I guess at these velocities, air resistance doesn’t matter :-)).

So for all people, running works out to roughly 1 kcal per km per kg. That is, a 70 kg person who runs 1 km burns roughly 70 kcal at a wide range of velocities.

For a person who is 60 kg, running this same 1 km burns roughly 60 kcal at a wide range of velocities. The exchange rate of 7.8 kcal per gram then implies that a person with weight 60 kg uses 7.7 grams of fuel (i.e. fat) to run a kilometre. So for a person with weight 60 kg, eating one snickers bar (of 57 grams or 271 kcal) is counteracted by running4.5 km. I await the day when a television advertisement for a snickers bar features an attractive 60 kg woman who holds the familiar brown widget up to eye level and says: “It powers me for a full 4.5 km run”.

Mpg of the human machine

The human machine is rather efficient at converting fat into locomotion. A litre of human fat is 918 grams. This yields 7160 kcal. A 70 kg person running at 8 kph is burning 560 kcal an hour, so a litre of human fat can power him for 12.8 hours or 102 km. In other words, the human machine obtains an efficiency of 102 kpl or 242 mpg, when a person weighing 70 kg runs at 8 kph.

Losing weight by eating ice

Here is another amusing calculation. Suppose you eat a kilo of ice. Your body spends 116 kcal in heating this up to body temperature. This corresponds to a weight loss of 15 grams.

Good data about energy outgo is essentially infeasible

It’s a bit painful to log your energy outgo. I tried to do measurement and it was hard to track activities and their METs for a lot of things that we do every day. So most of us only have a hazy idea of how much energy we’re using every day.

Input and output are hard to measure but the energy gap is not

So in short, your body is a system where energy goes in every day, where energy is used every day, and it’s hard to get a numerical fix on what goes in and what is used. However, while the input and the utilisation are hard to measure, it is actually feasible to measure the energy gap.

The logic is simple: the body stashes away the energy gap at an `exchange rate’ of 7.8 kcal per gram. So if you watch your weight, the rate of change of weight can be converted into a measure of your energy gap.

The input and the usage of energy are both hard to measure, but the change in weight is the summary statistic through which you can infer the energy gap.

Measurement of energy gap

So what we’re after is a measure of your weight gain or loss, measured in the units of grams per day. E.g. if you know that you were losing 10 grams a day, then you know that your energy gap is 78 kcal a day. This is not bad, for if you’re losing 10 grams a day, then this is weight loss of a kilo every 100 days and 3.65 kilos a year, which is fine for most people.


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