By vertically ill he means that while I'm upright with no obvious complaints, I'm not necessarily feeling great and have low energy. He's right but at least I've scored better than Celebrity Big Brother contestant Danielle Lloyd (37%) or the average Irish person (44%) and Holford says people who follow his diet can hope to achieve a score of 80% plus after three months. Translated, this means that if you choose your foods wisely, you can look forward to more energy, a sharper mind, better mood, clearer skin, lower weight and freedom from pain, or feeling as close to 100% as you possibly can.

It's a wonderfully seductive promise made more so by Holford's emphasis on prevention rather than cure. Obesity, as he points out, is now the second-largest preventable cause of premature death, after smoking, and has lead to an increase in diabetes, high blood pressure and high cholesterol levels.

"The point is that from 'vertically ill' comes what I call 'horizontally ill', which is where you develop one of these diseases and you're shipped off to your doctor. Anything that we can do to encourage people to take action earlier, to get the health score about 80% and then be in the green is going to minimise your chance of ever preventing one of these common and preventable diseases, " Holford says.

But most diet and health programmes also make similar promises, so what is the 100% Health Programme's unique selling point?

Perhaps, more pertinently, while the online questionnaire is free, what exactly do you get for the �24.95 it costs to get a personal programme tailored to your responses?

Holford's food plan is based around the concept of the Glycaemic Load, which might be considered the evolutionary dietary step on from Atkins and low GI (Glycaemic Index).

Atkins works by limiting the amount of carbohydrate you can eat and while it certainly gets results, it's not considered to be a healthy diet in the long run.

Low GI is based around the idea of eating slow-release carbohydrates as opposed to fast-release ones (eg sugar, white bread) but doesn't concern itself with quantities, so technically you could be eating a low GI diet and still consume a lot of sugar.

GL however takes the quality and quantity of carbohydrates into account giving you greater flexibility. Take watermelon, for example, which has a GI of 72, meaning it's forbidden on low GI diets, although Atkins says it's fine because a 120g slice of it has only contains 6g of carbohydrate. To find out the GL, you divide the GI rating by 100 and multiply it by the carbohydrate content, giving the watermelon a GL rating of 4.

Once you know how it works, you can play around with what you eat. Two bowls of porridge or half a bowl of cornflakes? They'll both have the same effect on your blood sugar and your weight. An entire 300g punnet of berries or 10 raisins? Again, they'll have exactly the same effect.

The three essential components of the GL diet are to eat no more than 40 GL a day; to always eat protein with your carbohydrates (some almonds with your apple for example) and to graze rather than gorge. It worked for Danielle Lloyd.

After her poor initial 37% score (she also reported low energy, stomach pains and irregular periods), she got up to 76% in just six weeks by following the diet and recommended supplements, with the added bonus of losing half a stone.

And in terms of it being a way of eating, as opposed to a fad diet? People generally find it feasible to maintain it, Holford says. "Certainly, they have the knowledge as to what they need to do if they slip off the wagon, " he says.

"I'm a firm believer in the 80/20 rule where if you do the right thing 80% of the time, you can get away with the 20%. When it's the other way around, that's when you're in trouble."

The GL Diet

Breakfast

"Oatflakes, hot or cold, with a tablespoon of ground seeds . . . say pumpkin seeds because they're very high in Omega 3 and 6 . . . with some fruit, like berries, apples or pears, and some milk or soya milk."

OR "Scrambled eggs on a wonderful wholegrain Irish soda bread and even better if I had a bit of salmon with the eggs. And I want eggs from chickens that are fed flax seeds which makes the eggs very high in Omega 3."

Snacks

"I might have an apple mid-morning or a pear midafternoon, with some seeds or nuts like a small handful of almonds or pumpkin seeds. Another very good snack is something like rough oat cakes with some hummus, cottage cheese or peanut butter.

Lunch and dinner

"Half of the plate to be non-starchy vegetables . . . it could be a salad, or peas, broccoli, carrots. A quarter of the plate could be starchy vegetables so this is where we're talking about potatoes, pasta and rice.

I'd recommend three boiled potatoes if you're trying to achieve weight loss or the equivalent of a cup of brown rice. If you choose something like quinoa, then you can have two cups because it's higher in protein.

The other quarter needs to be protein, a handful of chicken or fish or egg, lentils, beans or tofu."

Treat

"Chocolate is in fact a very potent antioxidant. Yes, it does contain stimulants but if you choose a dark chocolate . . . 70% . . . it's not so bad. You don't have to deprive yourself."

The GL diet