A food with a high GI will break down very quickly in the digestive system, as a result releasing glucose into the bloodstream and providing what is commonly known as a “sugar rush” – a short-term boost that will give a person a sense of energy and well-being. A low GI means that a food will break down slowly, giving slow-release sustained energy throughout the day, and will therefore not have the common “crash” effect present when a sugar rush expires and leaves the person with an energy debt, feeling shaken, tired and often depressed.

The GI diet was developed by research scientist Dr. David Jenkins nearly 30 years ago at the University of Toronto, and came about as a result of a study into which foods would help diabetics control their blood sugar level. The Glycaemic Index is concerned with how quickly the energy is released into the body, and the higher GI a food has, the quicker it is broken down in the digestive system. Thus a food with a low GI is considered a good thing.

Foods with high GI include corn flakes, croissants and white bread – traditionally breakfast foods which are considered to kick-start a person’s metabolism. Adherents to the GI diet point out that these foods may get the day off to an energetic start, but are partially responsible for a mid-morning “crash” which can leave you hanging on waiting up to two hours for lunch to top up your energy levels. It is generally held that a better option is to eat fruit or grainy breads for breakfast, as their lower GI will keep a steady level of energy going for most of the day.

The diet was developed by and named for the US physician Dr Robert Atkins, based on his theory that too many diets concentrated on saturated fat as the major problem causing weight gain and heart disease and that a more beneficial diet could be built around the principle of cutting out refined carbohydrates (such as sugar and flour) and “trans fats” (unsaturated fats with no real nutritional value, used for their greater durability and their usefulness for baking). All these technical points tended to get brushed aside, though, as the Atkins diet had a few more headline-grabbing properties.

Chief among these properties was the fact that Atkins allowed for a liberal amount of all meat products which, as a consequence, led to a great number of people deciding that they could happily go on a diet – and quite happily give up on bread – if in so doing they could eat sausage, bacon and egg for breakfast every morning and eat steak every night. Though this wasn’t a particularly wise approach, and indeed was not advised by Atkins, the more scientific side of the Atkins Nutritional Approach (to give it its full name) didn’t fit into the headline-and-short-article approach favoured by mass media outlets.

A heavily technical approach to the Atkins Diet certainly bore results and saw weight loss, with losses of up to ten pounds a week not uncommon. But the process of ketosis, which drove the weight loss in this approach, led to bad breath, body odour and in some cases the sparse growth of thick black body hair, and the pick and mix approach taken by some adherents proved incompatible with weight loss. Atkins Nutritionals, the company founded by the doctor, filed for bankruptcy in January 2005.

The diet was developed by the Californian biochemist Barry Sears, after he became aware that the men in his family were disproportionately prone to heart attacks. Fearful that he would die from an attack like his male relatives, Sears drew on his knowledge to create a diet that would significantly reduce all risk factors and allow him to break the tragic cycle. The diet that he created is based on a ratio or quota system which recommends that the adherent takes their calories from carbs, protein and fat, with a percentage value assigned to each. Specifically, 40% of calories should come from carbohydrates, with 30% each coming from the other “zones” – thus no one source has a dominance and the dieter can benefit from a balanced diet.

Independent studies comparing different diets have reported that as far as set diets go, the Zone has a high success rate, with weight loss being significant but measured, and fat loss being accompanied by a simultaneous gain of muscle mass. Additionally the adverse effects reported most often by dieters, such as fatigue and hunger, were rarer than with other diet plans and, where they were reported, greatly reduced. Conversely there are those who point out that little study has been done into the long term effects of the diet, and that it over-emphasizes protein in a person’s diet – which critics say could be more likely to cause heart attacks than prevent them.