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We took a couple of days between Anzac Day and the weekend to make a short holiday on the Wanganui River. At the place we stayed we were presented with meals that were impossibly huge, a diet that would super-size us in no time. In 2002-2003 one in three adult New Zealanders was overweight (excludes obese) and one in five adults was obese (MOH). Some people are quick to ascribe their weight problem (if they identify it as such) to extrinsic factors, such as genetics, time or financial constraints, marketing, or poor social support. However you rationalise it, being overweight or obese comes down to simple mathematics. The only certainty among all the theories is that energy and mass are interchangeable.

Einstein had the answer

Einstein's famous equation, E=mc2, relates the concept that all mass has an energy equivalence, and all energy has a mass equivalence. On this basis:

• You cannot create mass from nothing;
• The source of mass is energy;
• Less energy in leads to less mass out;
• Mass can be reduced by conversion to energy.

Now energy isn't bad: we need it to keep moving, breathing, or even thinking. Think of yourself as like a car: you need a certain amount of fuel to get from A to B. If, when you come to the end of your journey, you still have fuel in the tank, you can store it there for later use (in the case of the body, as fat). If you finish the journey with an empty tank, you're going to have to refuel to meet your ongoing energy requirements.

Let's apply this to food, or "body fuel". Food has mass; that mass stores energy (and some types of food mass hold more energy than others). Eating food converts this mass to energy. Energy is either used by the body or, if there is excess, converted back to storage mass (fat). We can thus envisage three scenarios:

1. Food mass > Energy requirement = Weight gain;
2. Food mass = Energy requirement = Weight stable;
3. Food mass < Energy requirement = Weight loss.

If weight loss is our aim, how do we ensure that food mass is less than our energy requirement? We can either take on less food mass (and therefore less usable energy) or we can increase our energy requirement by exercising, thus "burning away" energy stored in fat mass. In short:

1. Eat less;
2. Exercise more.

If you are gaining weight you are eating too much and/or not exercising enough. Period.

How much is too much?

Exercise is a simple concept; any physical activity helps burn Calories (or kJ, the unit of energy). Some types of exercise are more effective at burning up Calories than other types of exercise, and people will vary in the types that they are willing (or able) to do. Eating less seems to be more effective than exercise alone, although the combination of diet and exercise is more effective still. So how does one "eat less"?

Have you heard people say "But I eat lots of salad!"? Me too. It's the "lots" that's the problem. Eating the right kinds of food is not a licence to eat more of them. Eating "less" is about:

1. Choosing low Calorie foods over high Calorie foods and;
2. Choosing smaller portions over larger portions.

You should not try to compensate by eating large portions of a "healthier" food, or you will be unsuccessful in attempting to loose weight. Let's say that a high-Calorie food "HighCal" has 200 Calories per 100g, whereas a lower-Calorie food "LowCal" has 100 Calories per 100g.

Standard portion 300g of HighCal = 600 Calories
Lower portion 150g of HighCal = 300 Calories
Standard portion 300g of LowCal = 300 Calories
Larger portion 600g of LowCal = 600 Calories

In this example, a 300 Calorie meal equates to a smaller portion (150g) of HighCal or a standard portion (300g) of LowCal. But if we eat twice as much of the LowCal (600g) in order to "fill up", we get twice the Calories of the smaller serving of HighCal!

Most of us probably have little idea how many Calories are in the food we eat. Have a look at these 200 Calorie meals, and these 300 Calorie meals. Do you stop after a handful of M&Ms, or do you finish the packet? Do you load up your plate to the extent that you can't see the porcelain? If so, do the math. If you just ate a 300 Calorie snack that you didn't need, unless you go for a 20 minute run at the gym (or similar) to compensate, those M&Ms will end up 'round your middle.

You cannot fault the arithmetic.

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