This is a massively complex area, but here are my thoughts (taken from an essay I'm writing on theories of obesity) -
Excess calorie consumption affects different people in different ways and part of this is down to genetics. A study with identical twins showed that enforced over eating and inactivity led to weight gain (no surprise there) but dramatic variations between sets of twins (but not the twins themselves). The simple thermodynamics of calories in less calories out just doesn't work with a system as complex as the human body.
The Set Point theory is an interesting illustration of the difference 'metabolism' can make, but within rather than between individuals. Basically the theory states that if your body is at a certain weight, it will try and remain there. So if you weigh 65kg your body will try and remain 65kg. Overeat and you might put on a bit of weight, but the result will be that you feel less hungry (probably due to the hormone leptin) and you'll quickly lose that weight.
It works the other way too - it can be hard to lose weight as the body resists the weight loss. If you lose weight, the body goes through hormonal and metabolic changes that persist even once weight loss has plateued. In other words, lose weight and your metabolism slows, making it far easier to put the weight back on. This has been shown to last for at least 6 years and is possibly permanent. So if you put on a significant amount of weight and then lose it, you will have to eat fewer calories to maintain your old 'thin' weight than before the weight gain.
In our evolutionary past, losing a significant amount of weight would have been a likely precursor of death. The body responds to this by flipping into starvation mode, becoming more efficient to make do with the restricted food intake. Muscles become more efficient, converting to type II endurance fibres. After losing weight, muscles burn 20 to 25 percent fewer calories during everyday activity and moderate aerobic exercise. The body is trying to get back up to "normal" weight - its set point. It is a matter of some debate whether the amount of time the body spends at a given weight makes a difference. If you've been overweight for 10 months or 10 years does this affect the above process? Nobody is sure yet.
So if the body doesn't want to change weight, why does it? Well, the change in metabolism and balance of hormones outlined above gives us a clue. Rather than thinking of obesity as simple overeating, perhaps it is better thought of as a malfunctioning metabolism. If all your hormones are working as they should be then the body will resist weight gain. Screw them up, and your natural defences break down. The double whammy is that you gain weight because your metabolism is screwed, and then the process of losing weight screws up your metabolism even more.
Of the various hormones that help to regulate our appetite and body weight, two of the most important are leptin and insulin. Hormones are chemical messengers in the body - a generalised signal that is picked up by relevant cells in the body. However, these cells can become unreceptive to this signal and fail to act as they should. This is generally termed resistance. Leptin resistance interferes with the body's ability to regulate its own weight; insulin resistance interferes with the body's ability to regulate its energy supplies, and can lead to diabetes.
Excess calorie consumption affects different people in different ways and part of this is down to genetics. A study with identical twins showed that enforced over eating and inactivity led to weight gain (no surprise there) but dramatic variations between sets of twins (but not the twins themselves). The simple thermodynamics of calories in less calories out just doesn't work with a system as complex as the human body.
The Set Point theory is an interesting illustration of the difference 'metabolism' can make, but within rather than between individuals. Basically the theory states that if your body is at a certain weight, it will try and remain there. So if you weigh 65kg your body will try and remain 65kg. Overeat and you might put on a bit of weight, but the result will be that you feel less hungry (probably due to the hormone leptin) and you'll quickly lose that weight.
It works the other way too - it can be hard to lose weight as the body resists the weight loss. If you lose weight, the body goes through hormonal and metabolic changes that persist even once weight loss has plateued. In other words, lose weight and your metabolism slows, making it far easier to put the weight back on. This has been shown to last for at least 6 years and is possibly permanent. So if you put on a significant amount of weight and then lose it, you will have to eat fewer calories to maintain your old 'thin' weight than before the weight gain.
In our evolutionary past, losing a significant amount of weight would have been a likely precursor of death. The body responds to this by flipping into starvation mode, becoming more efficient to make do with the restricted food intake. Muscles become more efficient, converting to type II endurance fibres. After losing weight, muscles burn 20 to 25 percent fewer calories during everyday activity and moderate aerobic exercise. The body is trying to get back up to "normal" weight - its set point. It is a matter of some debate whether the amount of time the body spends at a given weight makes a difference. If you've been overweight for 10 months or 10 years does this affect the above process? Nobody is sure yet.
So if the body doesn't want to change weight, why does it? Well, the change in metabolism and balance of hormones outlined above gives us a clue. Rather than thinking of obesity as simple overeating, perhaps it is better thought of as a malfunctioning metabolism. If all your hormones are working as they should be then the body will resist weight gain. Screw them up, and your natural defences break down. The double whammy is that you gain weight because your metabolism is screwed, and then the process of losing weight screws up your metabolism even more.
Of the various hormones that help to regulate our appetite and body weight, two of the most important are leptin and insulin. Hormones are chemical messengers in the body - a generalised signal that is picked up by relevant cells in the body. However, these cells can become unreceptive to this signal and fail to act as they should. This is generally termed resistance. Leptin resistance interferes with the body's ability to regulate its own weight; insulin resistance interferes with the body's ability to regulate its energy supplies, and can lead to diabetes.
12 years
