Perhaps rather surprisingly, around 70% of our daily expenditure of energy is used simply to maintain life - what scientists call basal life processes.

So that will include the energy needed by your heart, lungs, brain, kidneys, muscles and the like solely to keep you alive. And believe it or not almost a third of that energy is used by your liver.

The basal metabolic rate (BMR) is the scientific term for the amount of energy you use while at rest in a neutrally temperate environment when your digestive system is inactive.

You can forget the scientific mumbo jumbo, but you must remember one key thing.

Your BMR will account for about 70% of the energy you use on a daily basis. And that means 70% of the calories consumed too.

I had no idea about this, but it is explosive information in the hands of someone who wants to lose weight. Because I had no idea that I needed that amount of calories just to function properly.

Take the man from the back of the crisp packet as an example. His recommended daily intake of calories is 2,500kcal. Of that around 70% will be used by his body to keep him alive. Something we take for granted, but something that requires an enormous amount of energy. About 1,750kcal in this instance.

Every minute of every day, your body is burning calories. You don’t have to be pounding a treadmill or pumping iron. The wonderful machine that is the human body is burning calories at a ferocious rate even while we sleep.

Compare that 1,750kcal expended by bodily functions to the calories burned when our man does 60 minutes of weight training - 250kcal. Just about enough to burn off the calories consumed after eating a cheeseburger or a chocolate bar.

So whether we like it or not, we all have to consume enough calories to maintain life. And the number of calories needed is probably greater than most people think. Certainly more than I thought anyway.

And that’s why crash diets don’t work.

If you cut out too many calories, your body doesn’t get the calories needed to function properly and it goes into survival mode, slowing down your metabolism - the rate at which your body burns all those calories you shovel into your mouth.

Yes, you may lose weight initially, but at the risk of damaging your health. And when those hunger pangs get the better of you, back on goes any weight loss and then some because your body’s metabolism is still burning calories at a slower rate just in case the famine returns.

On top of the calories we need to maintain our basal metabolic rate (BMR), we need energy to go about our daily lives. The more active your life, the more calories you will need.

There’s a simple formula for working out how many calories you actually need a day.

First of all, work out your BMR. To do that do one of the following sums:

Women: weight in kg x 2 x 11 = your BMR
Women: weight in lb x 11 = your BMR

Men: weight in kg x 2 x 12 = your BMR
Men: weight in lb x 12 = your BMR

I’m 81kg so my BMR is 81 x 2 x 12 and that equals (he says reaching for a calculator) 1,944kcal or 1,900kcal for cash.

Now we have to factor in your lifestyle - the more active you are the more calories you will need to maintain your current weight.

If you are inactive multiple your BMR by 20%.

If you are fairly inactive multiply your BMR by 30%.

If you are moderately active (exercising two or three times a week) multiply your BMR by 40%.

If you are active (exercising three to four times a week) multiply your BMR by 50%.

And if you are very active (exercising daily) multiply your BMR by 70%.

I suppose I lead a moderately active life. I walk about a mile or so a day and try to get to the gym two nights a week, but for much of my waking hours I am sitting in front of a computer screen or a TV. So that would be 30% of 1,900 or 570kcal.

So aged 42, weighing in at 81kg and with a height of 5′7″ (170cm) and given my lifestyle, I need to consume about 2,470kcal a day to maintain my current weight.

Maybe I am the man on the back of the crisp packet after all.

If I consume any more I will put on weight and if I consume any less I will lose weight.

We have already ruled out crash diets. The secret of successful weight loss is to shed those pounds gradually, thus preventing your body going into survival mode. No more than 1-2lb or one half to 1kg a week or it starts to worry.

Now we know one other thing. To burn off a pound of fat requires 3,500kcal of energy - which equates to 500 calories a day in order to lose a pound of fat in a week.

If I need to consume 2,450kcal a day to maintain my current weight, I will need to get that down to 1,950kcal a day through a combination of eating less calories and doing more exercise to lose weight safely and without troubling my body’s basal efforts.

To try for any more would probably backfire because of the calories I need simply to maintain my body functions. If my body thinks it’s famine time, it will simply enter survival mode and any dreams of long term weight loss will go out of the window.

So that will be my goal. Knocking 500kcal off my current daily intake in a bid to lose a pound or thereabouts a week. I need to lose 22lbs (10kg) to go from “overweight” to a “healthy” weight so at a pound a week that gives me 22 weeks. Which by my reckoning takes me to the middle of May or thereabouts.