If you believe all the marketing spiel about the glycaemic index (GI), then candy bars are a great food.

After all, they have a lower GI than pound cakes, sodas and even - watermelons. But consider that a watermelon is lower in calories and has more essential nutrients than a candy bar, and the whole GI thing sounds a little dodgy.

So what is GI? Well, it is an index that ranks foods based on the impact they have on our blood sugar.

High glycaemic foods cause blood sugar to rocket, then come crashing down, which triggers the appetite and causes one to crave more food (and carbs). There would therefore be a tendency to overeat.

Also, high blood sugar triggers the release of excessively high levels of insulin, which may signal your body to store the sugar as fat. This is especially so if your muscle cells are not starved of energy.

It is usually better to ensure balanced blood sugar levels - and in turn, balanced insulin levels. This helps to maintain a steady stream of sugar to our muscle cells and reduces the likelihood that our body will store fat.

The problem with GI though, is that it does not take into account real-world portion sizes. The GI of a food is derived from an amount of the food that gives 50g of digestible carbs (this includes starch and sugar but not fibre).

For candy, this works out to just three-quarters of a king-size bar. But one would have to eat five cups of diced watermelon to get the same amount of carbs.

It is thus flawed to compare these two foods as such.

Also, GI is not a fixed number. It can vary by as much as 54 per cent from person to person, according to a study by University of Toronto (Canada) scientists. It can also vary within the same person - the effect of a high-sugar drink on blood glucose drops by 12 per cent for 12 hours after an intense workout.

And consider that we seldom eat foods in isolation - adding fat or fibre to a meal slows the absorbtion of glucose in the bloodstream. This further affects the usefulness of the GI.

For people watching their weight, the total calories consumed is the most important factor in managing one's weight. Going back to the candy - watermelon example, it is clear that low GI does not necessarily mean low-carb.

Is the GI useless then?

Not necessarily. To address the flaws of the GI, Harvard University scientists came up with the glycaemic load, which takes into account both a food's GI and its portion size.

To calculate a food's glycaemic load, divide its GI by 100 and then multiply that by its grams of digestible carbs (total carbs minus fibre).

A glycaemic load of 20 or higher is considered high; 11 to 19 is medium, while 10 or below is low.

For more on GI and your health, grab your copy of this month's Men's Health.