Just like a thermostat in a house, your body has a sort of ‘set point’ for body fat that it prefers.
The human body likes to stay pretty much the same given the same environment and external stimulus. That means that if you eat the same things, and exercise (or not) the same amount, your body likes to maintain itself where it is for the most part. Since your body is a stimulus response device, it simply responds to the life you lead. So if you start to go for a walk every day, your body will adapt for a short time, and then stop adapting once it gets proficient at walking that far, that often because it has now adapted to that change, and needs not adapt any further.
With weight gain and lack of exercise, the body also adapts, and this time, it turns the thermostat in a direction you may not like; as you stay sedentary, the body gets rid of muscle, making your body less efficient at burning fuel and resultantly more overweight. It does this, because sitting around does not require muscle, so the body gets rid of what it doesn’t need. You may wonder why it doesn’t get rid of fat, but that’s different. Fat is inert, it doesn’t actually ‘do’ anything. Fat is a result of eating too much, and moving too little.
Getting back to the thermostat, what I wanted to point out, is the yo-yo effect of losing weight and putting it back on again because your body (and your lifestyle) was comfortable where it was. More often than I would like to see, people work out really hard, reduce some bodyfat, get a great result, and then relax and bounce right back up to where they were before. What happens is that the body returns itself to its previous comfort zone, or ‘set point’; your default thermostat setting.
Let’s say your body was ‘set’ for 35% body fat (which isn’t permanent by the way, it’s just what your body is used to), and you work out really hard and eat well so that you get down to 20%. Great! Now, if you slack off on the diet and exercise right after you get there, your body will yank you right back to 35% because it is still used to that level.
What you want to do is this: Get yourself to the percentage you want or can achieve, using exercise, good nutrition, rest and plenty of water. Let’s say 20% body fat. THEN, you need to continue to work out consistently and eat well so that you maintain that percentage for an extended period of time and this will ‘reset’ your thermostat! If you have been heavy a long time, that might mean you have to stay at 20% for 2 or 3 years to reset your new percentage. If you have been fit for many years and just recently let yourself go, it might only take 6 months to reset your body fat thermostat. The point is, if you change in a short period of time and then relax, you will yo-yo back to your set point. If you change and hold for a period of time, then your body will adjust to your new set point. Once you have the new set point locked in, your body will comfortably stay there with only maintenance efforts required.
I like to think of it as a plane taking off: Full throttle, full flaps and total effort for take off, and then once cruising altitude is reached, the pilot can throttle back and maintain altitude with less effort. By comparison, if the pilot were to go full throttle, take off, reach altitude, and then shut the engines off, the result would not be good! If you think about it, that’s what most people do: work hard for January, February and may March, then figure ‘hey, I made it! Time to relax and stop working out for a while’, and before you know it, they are right back where they started or worse. To make matters worse, now they are disheartened and upset with themselves. My bodyfat thermostat used to be set at over 25%. I reduced it to 7% (which was a pretty nice number, but temporary) and then held it at 14% for a few years. Now I can coast for periods of time and re-intensify again and all the while, my body sits in that 12% to 14% range without too much trouble; my new setting. I do notice though, that as I age, I do need to be more aware of my bodyfat and exercise at age 42, than I did at 32.
It’s key to understand that if it took you 15 years to get overweight, it will take at least a few years to stay fit. Don’t give up, hang in there and change your thermostat setting!