Look around the world today, and you will still find many areas where people live subsistence lives, scratching a living from the soil with little if any machinery to assist them. Heavy manual work of this sort requires from 3,000 calories a day (women) to 4,000 calories (men). If that work is being done at low temperatures, add another 1,000 calories a day to maintain body heat. Factor in strenuous exercise, as with polar explorers who insist on walking, skiing or roller-skating across the ice caps, and energy requirements reach 6,000 calories a day and more. At these very high levels of energy expenditure the diet changes dramatically, as only fats and oils contain the calorific density needed, and even then it seems impossible not to lose weight. Even on a diet containing around a pound of lard and 6,000 calories a day, a recent exploration team recorded body weight losses of 10% and more during their arduous trek.

For most of us, life has become much less physical. Where clothes were once beaten on rocks at the water’s edge or on washboards, and then wrung out by hand, there came a series of progressively more efficient labour-saving washing machines. My mothers first top-loader washed the clothes in one tub, after which the sodden and lead-heavy clothing had to be hauled out, dragged through a (hand) mangle, and thence into a centrifuge before it was carried out to the clothes line. How laborious, compared to the do-it-all washing machines on offer today! And who beats carpets any more? Or clears out and then makes up the coal fires every morning? This explains why energy requirements today are, on average, a mere 2,000 calories a day.

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