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As an adult male, your life used to be fairly active where your body used up about 2000 Calories per day to do all its necessary processes and another 200 Calories per day to power some exercises, for a total of 2,200 Calories of food energy used each day.
During that day, you ate an assortment of food which contained a total of about 2,200 Calories of food energy.
But then you SLIGHTLY INCREASED the amount of food energy you ate, up to 2,300 Calories per day. You didn't notice that small difference.
But now your body was taking in 2,300 Calories and only using up 2,200 Calories. The result is that you were adding 100 Calories of food energy each day. It turns out that about 3,600 Calories of food energy becomes one pound of human bodyfat. So, after you had added that 100 Calories per day on each of 36 days in a row, you added one pound of bodyfat to your body. In a year of this minor change, you will have added enough food energy to become TEN POUNDS of bodyfat! In five years, you would have gained 50 pounds, and never even realized why.
There is a second way this can happen. Say that you no longer exercised as much as you used to. Maybe more TV or computer games. But instead of burning up 200 Calories each day for exercise, that now drops to 100 Calories per day. So now you are still eating the original 2,200 Calories of food energy each day, but now you are only using up 2,000 for Metabolism plus 100 for exercise, or 2,100 Calories each day. The result is like before, where you would be gaining about 100 Calories of food energy every day. Again, after 36 days of that, your body will have gained one pound of bodyfat. After a year, the gain would be about 10 pounds.
(There ARE exceptions to the common result of a good workout burning
off 100 or 200 Calories!
Researchers in Antarctica generally need to eat around 6,000 Calories
each day to maintain their body weight because their bodies radiate
immense amounts of heat due to the constant sub-zero temperatures.
Olympic-level athletes need to eat impressive amounts of Calories
each day. I understand that Michael Phelps, the swimmer with many
Olympic Gold Medals, needs to eat around 12,000 Calories every
day, which is partly due to constant physical exertion but is also
partly due to his body having to radiate and conduct large
amounts of heat to the cool water which he is nearly always in.
When professional football players retire, many continue to
eat the large quantities of food that they had to eat during their
career, and they often rapidly gain huge amounts of weight, which they
then have great difficulty getting rid of.)
There are cheap and silly devices which are sold which claim to be able to monitor bodyfat content. The two most common are Bodyfat Calipers and devices which measure the electrical resistance of your body. Both can give you numbers that can make you feel happy, but the fact that both are commonly WRONG by up to 15% of bodyfat, they become useless. If you get a reading from such a device that says you are at 20% bodyfat (a healthy number) the reality could be either that you are actually at 35% (morbidly obese) or 5% (which few professional athletes can accomplish).
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago