Tuesday, 27 January 2009

Why are thin people not fat? (And why am I fat?)

"Why are thin people not fat?" was a Horizon documentary shown last night. It's available on iplayer (possibly only in the UK). The programme followed a small group of young, naturally thin adults who were monitored for weight, measurements, body fat percentages and so on while eating huge amounts of food and avoiding exercise over a few weeks.

From memory, the following points came up:

*Naturally thin people found it difficult to put on extra weight. Two people could not manage to eat all the extra food.
*One person put on extra muscle instead of extra fat and increased his metabolic rate.
*All the participants easily lost weight at the end of the study

Meanwhile, some other obesity research was looked at:
*A virus which causes chickens to gain weight is more common in fat people
*Once laid down, fat cells don't go
*Evolution has selected for a tendency to fatness
*Small children fall into two groups - those who continue to eat when full, and those who lose interest in food when full
*People seem to have a set weight point and adjust their diet and exercise naturally to maintain that weight.
*Fat people who lose weight, have an appetite for extra food.

There was more, but that's a quick sum up.

I was particularly interested in this story, because for most of my adult life I wasn't fat. I was naturally slim - not thin, but within a "healthy" BMI range. I seemed to be very much like the naturally thin people on the programme. I didn't have any desire to overeat. I lost interest in food after I was full. I was never overweight as a child.

Now my weight has changed, but I still behave much like a naturally slim person. I have a normal appetite. I don't overeat. My house is full of chocolate and other treats left over from Christmas but I only have the occasional piece now and then - not because I'm trying to resist but because I don't want it. So the puzzle is - why am I eating as I have done for all my adult life (apart from a few misguided attempts to diet), but steadily gaining weight?

The programme didn't give any explanations for somebody like me. (Unless you count the virus idea people seem to be pooh-poohing at the moment).


  1. It could be simply that people naturally put on weight as they get older; or it could be that you have a medical problem such as low thyroid or PCOS. Don't panic, but do mention it to your doctor, who can test to see if something has changed that should not.

  2. Thank you - I'm not panicking - it has been going on for a while. My doctors know about it. I'm high risk for thyroid problems, so I do like to have that checked every few years. I don't have symptoms of PCOS (apart from weight gain) so they're not interested in that. I suspect a natural tendency to gain weight later in life, combined with something else - I have a couple of theories. But I was hoping that the programme would look at naturally thin people who DO become fat, rather than just assuming that naturally thin people stay thin. The participants all looked like they were in their 20s.

  3. My doctor told me that when we become older our metabolism slows slightly and we need less calories, it's that simple. However, many people continue to eat the same amount as when they were young. I guess to avoid weight gain one has to eat a little less, exercise more, or simply accept that even slim people put on a few pounds as they age.

  4. the older you are the less you do [physically]more sitting,less doing...so if you still eat the same over a year but do less over a year you will slowly grow heavier.Eat less as you age or increase activity,such as walking to make up for increased sedentary behavior

  5. Anonymous and anonymous, above. I agree that calorie requirements tend to change as we get older. But if the conclusions of the programme were right, how does that fit in? What controls that natural set point? When I was younger I stayed much the same weight however active I was, so presumably my appetite adjusted accordingly. Why doesn't that happen now? If it was simply a case of appetite staying the same, then you would expect naturally thin people to lose weight if they exercised and put on weight if they became more sedentary. But that isn't what happens. You'd also expect that everybody would put on a substantial amount of weight as they got older (unless they increased their activity to compensate). Again, that doesn't always happen. Plenty of naturally thin young people grow into naturally thin older people. I think there's a lot more still to learn about the "set point" idea.


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