It's 8.05 on Wednesday morning. I've got a heavy dumbbell on my shoulders, doing endless repetitions of yet another excruciating exercise.

Squeeze your quadriceps as you come up.' What?' I shout above the din of the music.

What I told you. Squeeze your quadriceps'.

But my muscles don't do foreign languages. In English please.' Front thighs.' The instructor seems a nice man, though I've long been convinced that in his previous life he was a torturer in the Spanish Inquisition.

I am in the bowels of what used to be the Cannon Row police station, close to the old cell block. There are bars on the windows, to stop us getting out.

It's now the House of Commons gym, where I'm taking part in the regular early morning "Body blast" aerobics and weights lesson, along with seven other sad souls.

I've not properly warmed up, so I'm all to well aware of various aches and pains.

And, as ever, my mind drifts to all the things I'd rather be doing - sleeping, having a gentle cup of tea in bed with my wife, even watching Burnley Reserves on a wet Wednesday would be better than this.

But we all keep going. Half way through the lesson I start to feel better.

By the end of the lesson I have a good sense of achievement, and it's off to the Ministry of Justice building, and then to the Commons' chamber for oral questions.

I'm lucky, in two respects. First, my metabolism is such that I use up a lot of energy.

Second, like everyone else of my generation, food rationing was around for the first seven years of my life.

Although I was born after the war (in 1946) it's often forgotten that rationing was more stringent in the immediate post war years than it had been during the war itself.

And my family, like millions of others, were hard up.

We had good healthy food, but there was never enough of it.

I don't remember anyone at my primary school who was overweight, and just two people were at the secondary school I attended.

Roll forward to today, go into any school, for any age group, anywhere in the country, and you'll see far too many pupils who have a significant weight problem. So do more and more of their parents.

In 2005 England had the highest rates of obesity in the European Union, according to new figures.

It was up by 50 per cent among children, while another study predicted this week that by the year 2050 most Britons will be obese.

The health risks, from diabetes to an increased risk of heart disease are obvious.

When I was Foreign Secretary I used to think that I was eating, as well as speaking for Britain. I'd get off a plane, go straight to a meeting, there'd be food.

Then there would be a working lunch or dinner - headphones on for the simultaneous translation, food (often very good food) put in front of you. I often succumbed to the temptation, and the pounds went on.

But in the end I got a grip. I started to insist that I went to a gym or swimming pool wherever I was in the world.

For some people with a weight problem I have only sympathy.

People's metabolisms do vary; and it's just a fact that if people are depressed they are likely to eat for comfort.

But if we want to avoid obesity becoming our number one public health problem, there is plenty we can all do to try to tackle it, by eating healthily (and I don't mean by trying some cranky diet) and taking a bit of exercise on a regular basis.