And not only have we been eating significantly more calories, we're also burning significantly fewer.
Even the efficiency of our homes may be contributing to our increased metabolic efficiency as well (and metabolic efficiency is a great thing if calories are scarce and a bad thing if they aren't).
Because we maintain our homes at such a constant temperature (and don't have to do any physical work to do so), our bodies don't have to work very hard to maintain our body temperature. Before the days of central heating, just maintaining body temperature (keeping cool in summer and warm in winter) was a big job that required a lot of calories.
Even two hours in the gym may not be enough to compensate for all of the "work" our bodies no longer have to do. In the past, people had to burn a lot more calories just to live their daily lives, maintaining body temperature, fighting illness and injury, obtaining and preparing food, cleaning and maintaining their homes and clothing, ... there are so many ways in which we are able to "save" energy (and therefore calories) in the modern world.
We also sleep less, and there's some interesting studies that show that sleep deprivation can suppress metabolism. So as we're eating more, we're also burning less - a lot less. So much less that it can't be compensated for by even daily aerobic activity (which most people are NOT getting - if everyone who had a gym membership went even once a month, many if not most gyms couldn't accomdate them - there wouldn't be room in the building to hold everyone).
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