I was arguing last night that one cause of obesity might be reduced metabolic rates in warmer housing, workplace, and transport internal environments.
Today I thought I’d take a stab at estimating what the difference might be, using Humphreys and Nicol’s 1970 BRS study of Theoretical and Practical Aspects of thermal comfort. For this I needed to estimate what mean room temperatures prevail today, and prevailed in the 1950s. And also what clothing levels prevailed now and then.
From my personal memories of life in a detached English house in about 1955, I guessed at a mean temperature of about 12º C in winter, largely because only two rooms had any heating in them (in the form of coal fires). And even these rooms were seldom very warm. I’d estimate that they were never much warmer than about 18º C, and the bedrooms and kitchen and bathroom and hall were about 10º C. So a mean internal temperature of 12º C seemed about right.
I also remember how my grandfather dressed, in flannel trousers and tweed jacket and shirt and waistcoat with woollen long johns underneath. Given an ordinary suit has a thermal resistance of 1 clo, I guessed that his outfit was about 1.3 clo.
Then I compared it with now, when many people keep their houses very warm, maybe 25º C, and wear jeans and T-shirts that are probably about 0.5 clo.
Given these numbers, I plugged them into the equation for metabolic rate in the Humphreys and Nicol study (right), and found that in the 1950s the required metabolic rate to maintain thermal comfort was about 88 Watts per squ metre of body surface, while the required metabolic rate today was about 60 Watts per square metre. 88 W/m² corresponds to standing doing light work, while 60 W/m² corresponds to sitting.
Which seemed about right. In most homes today, people seem to spend most of their time sitting. But back in the 1950s there was quite a lot that needed doing. Back then we had 3 meals a day, and they all required cooking to be done. But on top of that there was hand-washing clothes and sheets, squeezing most of the water out of them in a mangle, and hanging them up to dry. There was also ironing, and house-cleaning. And the two coal fires needed to have coal added periodically, and the coal hods beside them had to be filled once or twice a day from the coal shed outside. And on top of that there were clothes needed to be darned. The result was that the household was pretty much a hive of activity all day. And I used to help out, laying fires, fetching coal, blending cake and pastry mixes. We had to make more or less everything ourselves.
Nowadays the clothes are put in a washing machine which may also include a spin and tumble drier. Cooking just means sticking something in a microwave for a few minutes. And if your socks get holes in them, you just buy new ones. And you have a vacuum cleaner that you can run over the floor every few days. So there’s not very much to do, and you can spend most of your time sitting and watching TV or listening to the radio or updating your Facebook page.
88 W/m² over 1.6 square metres of body surface area for an entire day works out at a bit over 12000 kJ / day, or 2900 Calories per day. And 60 W/m² works out at a bit over 8000 kJ/day, or 2000 Calories per day. According to Chris Snowdon, 2000 Calories/day is our current energy consumption per day. And according to George Orwell, 2900 Calories/day is what people were consuming in 1946. So we’re now consuming a third less food energy than we did 70 years ago.
How might that have resulted in an obesity “epidemic”? One plausible explanation is that people tend to carry on eating the same amounts of food, and also wearing the same clothes, as they are accustomed to. And this will mean that they will be generally eating more than they need to, and the excess energy gradually accumulates as body fat. Even if people do reduce their food intake (as clearly thy have been), their response is likely to always be lagging behind their actual circumstances, like a car cornering on the outside of a bend.
The same would be true if they needed to consume more food and wear more clothes. The lagging response to this new circumstance would be that they would eat too little and wear too few clothes, and get very thin. The obesity epidemic would be followed by an anorexia epidemic..
The lagging response would have been particularly notable when people moved from a coal-fire-heated 1950s house into a centrally-heated, double-glazed 1980s house, but carried on wearing the same clothes and eating the same food as they had before. And this probably happened very frequently when 1950s houses were demolished and rebuilt, as was happening continually throughout the 1960s, 70s, and 80s.. Since it was generally the working classes who made this sort of rapid transition as they were moved out of their back-to-back houses into tower blocks, the problem would have been the greatest there – the middle classes instead gradually adding central heating and double glazing and loft insulation, and gradually adapting over a much longer period,
Other factors may not have helped. Recent decades have seen a slightly warmer climate than 70 years ago (“global warming”), and this would have meant slightly higher room temperatures, and a need for an even more greatly reduced food energy intake.
And just when smoking would have helped reduce appetite, smoking is being very strongly discouraged, making it even harder for people to reduce food consumption.
One may speculate that clothing fashions and diet fads may have been as much driven by the need to wear less less and eat less as any purely cultural trend. For example, mini-skirted girls would have lost heat energy faster than their more staid peers, and been slimmer as a consequence. And if they were always dieting, it was because they always needed to be reducing their food energy consumption. Culinary fashions might also be explained in the same way. Nouvelle Cuisine, for example, consisted of minute portions of exquisite food. And something like this might be the culinary future, with food producers hiring armies of chefs to make their products more tasty, while simultaneously reducing their size and energy content.
What about regular exercise? A half hour of brisk walking every day would burn off about 650 kJ or 155 Calories. That’s the energy content of about half of a small bar of Cadbury’s milk chocolate – a drop in the ocean.. If exercise is to be used to offset the effect of living in warm houses equipped with the latest labour-saving devices, while eating the same amounts of food as in prior eras, the exercise must be both intense and prolonged. And this may explain the rise of gymnasiums offering just such intense and prolonged varieties of exercise. Just like mini-skirts, they are a way of preventing obesity.
In summary, the argument being made is that the gradual introduction of warmer houses with ever more numerous labour-saving devices over the past 70 years has required a continual compensatory reduction in food energy consumption and clothing. Such adjustments were gradually made, but more slowly than was necessary. The result was a long term obesity “epidemic” that was the result of steadily falling energy consumption.This is not the fault of food manufacturers. Nor is it the result of gluttony. The problem is instead that people tend to eat and dress in the way to which they are accustomed, but what was once customary and adequate has now become excessive..