A study by University of Massachusetts Amherst researcher Barry Braun compared a group of volunteers who sat all day (they even used wheelchairs to visit the bathroom) to a group that didn't sit down at all. The difference in energy expenditure, he told The New York Times, was hundreds of calories – but the level of appetite hormones and hunger in the two groups remained identical. This is significant, because after a very hard workout, you feel hungry. And hundreds of calories a day - let's say 200 - means half a pound weight loss each week.

So how can we use this insight. Sit less. Stand rather than sit at work if you can. Fidget, get up and down if you are watching the telly. Even unvigorous movement uses up those calories.

Most of us put weight on very slowly, just a few calories too much every day. So if you eat a bag of crisps (130cal) and a Flake (170) or their equivalent every day you can expect to put on a pound in under a fortnight. To use up 300 cals, you need half an hour of high impact aerobics.

So it means that if you want to lose weight, exercise is the second thing to do. The first is to eat less, and the indulgent foods are easier to give up than meals.

Ravussin has found that a sustained 10% excess caloric intake results in a 7% weight gain. His calculations clearly imply (and are consistent with the observational evidence from population studies) that the obesity epidemic is primarily driven by increased food intake and not by decreased physical activity.

Exercise is important for general health reasons. It doesn't work as the prime weight reducer for most of us, not least, because after jogging for half an hour, we might reward ourselves with a bar of chocolate. Instant weight gain.

I looked at the label on a cheesecake in the supermarket today. Over 1400 calories per 100g. That is all right then, I will only eat a slice. But then it read Normal 70g slice (1/6 of the cheesecake) over 1,000 calories.

Half a day's calories in one slice. And if it were me, one slice would not be enough. Truly shocking.

It is these indulgent treats, on the whole, that make us fat.

And there is a whole market segment which the manufacturers call Indulgent Snacks. The other segment is Healthy Snacks. The manufacturers want to sell this stuff to us, and they make these things very delicious. 

Treating ourselves once in a while is nice. Having a food treat every day heaps on the fat. Check the label and prepare to be shocked.

What is a portion? Well, portions have certainly got bigger over the last decade or so. A Beefeater restaurant steak plate from the 1960s (seen in an antiques shop recently) looked really small to me. We have got used to eating off really huge plates. We eat out much more than a generation ago, and we load our plates at the buffet to get our money's worth. All this makes it harder for us to assess a portion.

And when we buy a pre-pack meal from the supermarket, what looks like a meal for one, on close reading of the package, is a meal for two. On the plate it looks like a portion for one, (or maybe 1 and a tiny visitor). I am starting to think that it is calories for two but quantity for one. Look at an interesting portion control discussion here.

The King Size 84g Mars Bar was phased out following a House of Commons report, which showed that a Snickers bar had more calories than a full steak meal. It was replaced by the Mars Duo - two bars now at 91g (about 400 calories - equivalent to a plate of chicken, potatoes and veg). Research shows that once a pack is opened we eat it, so the unintended consequence of the House of Commons report is that we eat even more! For those with a sweet tooth and a desire to lose weight, buy fun packs. One little bar is a portion.

If you hold onto the idea that one formal meal is round about 500 calories, then when you read the calorie content of a sweetie bar or other snack, you can get an idea of the relative benefit of eating it. I was once given a delicious slice of chocolate cake - and was horrified to see that it contained 1,500 calories. A belt buster to be sure!

Well, what I really mean is Look at your food as you eat it. Keep as much of the evidence in front of you.

If it is chocolate bars you are eating, keep the wrappers to remind yourself that you are on the 3rd. Just a little nudge to tell you that you have had enough, for our stomachs don't measure calories in.

We eat with our eyes. If we use a small plate, and it is filled to the brim, then we eat happily. The same portion on a giant trencher makes us feel disappointed and neglected. And same again, in the eat all you can stuff your face with buffets, the vision is so exciting we fill our plates time and time again. And until 20 minutes is up, our stomachs don't signal that we are full. And if we are eating fast, by that time we are tightly bagged up. (I like this expression very much, I heard it first from my rather posh sister-in-law.)

Brian Wansink (what a hero) did some research on students. inviting them to a chicken wings supper. Those whose plates were cleared away ate more than those where the bone heap grew before them as they munched they ate 28% less). As Brian says, "If you could look back and see all of the handfuls of potato chips you’ve already gobbled before shoveling in another, you’d likely hestitate before reaching back into the bag".

Estimating portion size is difficult for all of us, but developing rules of thumb will be handy. Because we have got used to larger portions in restaurants, our estimate of calorie content of a meal gets confused.

Research by Brian Wansink and colleagues shows that everyone (normal weight, underweight and overweight, men and women) has difficulty in estimating the calorie content of a large plate of food, whereas they accurately assess the calories in a more normal portion.

If we are overweight, we tend to have larger portions. That means that those of us who are overweight are making these wrong estimates frequently. As a result, we are underestimating the calorie content of most of our meals and so we put on weight. What to do?

These researchers suggest that you should estimate the calories and then double them. A normal meal might be 500 calories. If you are overweight and you know that you are eating a large portion, then advise yourself that this meal probably contains 1,000 calories - half your day's allowance.

Counting calories is dull, but having a fair idea of the calorie content of your meals is handy. It allows you to develop useful rules of thumb when you are seriously trying to control your eating.