Home > Diet and nutrition > Weight loss Updated 25 September 2013 The Fast Diet The Fast Diet, also called the 5:2 Diet, has taken the world by storm. Here's what you need to know. 3 Pin It Shutterstock Related Diets that protect the heart Check Glycaemic index tool » Count Calorie counter » Quiz Ready for weight loss? » Ask DietDoc » What 100 calories of fruit looks like Should you try the Paleo diet? The latest diet craze to hit the world is the Fast Diet, also known as the 5:2 Diet or the Intermittent Fasting (IF) Diet. The basic principle of this diet is to eat normally for five days and semi-fast for two days of a week. Does it work or is it dangerous? DietDoc comments. One of the main reasons The Fast Diet: The Secret of Intermittent Fasting - Lose Weight, Stay Healthy, Live Longer, written by Dr Michael Mosley and Mimi Spencer, is so popular, is that it appears to be relatively easy. Instead of having to count calories or kilojoules, follow intricate rigmaroles, cut out whole food groups as required by the Atkins and all other high-protein, high-fat, zero-carb diets, or learn how to apply a low-glycaemic index (GI) diet (which takes some insight, practice and dedication), the authors of the above-mentioned book assure their readers that the Fast Diet is very easy. All it requires is that users restrict their energy intake on two days per week (preferably on Mondays and Thursdays) to 500 kcals (2 100kJ) a day for women and 600 kcals (approximately 2 500kJ) per day for men. This two-day semi-fast is supposed to produce a loss of about 0.46 kg a week for female users and a slightly greater weight loss for males. The authorsDr Michael Mosley describes himself as someone who studied medicine and qualified as a doctor in the UK, but immediately after qualifying joined the BBC where he has spent 25 years producing science and medical programmes for the UK broadcaster. I am not sure if a person who has not practised in his field or specialised in nutrition, is fully qualified to influence the dieting habits of thousands of individuals worldwide, but that is something each person who elects to follow his dietary instructions will have to decide for him- or herself.I was also concerned to read that Mimi Spencer, the other author of this new diet book, is a writer who appears to be taking the 5:2 diet a bit too far. She admits that she regards herself as an "evangelist" of the Fast Diet and proudly confides that she has reduced her BMI from 21.4 (which is generally regarded as below ideal) to 19.4 which sets off alarm bells that she may be on her way to becoming anorexic.AnomaliesIt is important to note, that Mosley and Spencer do state that followers of the Fast Diet should "not overeat on the normal days". They suggest that on "feeding days" women should consume about 2 000 kcal (8 400kJ) and men about 2 600 kcal (10 920kJ) per day. How feasible this will prove to be, remains to be seen. There is a body of research to indicates that we humans tend to overeat after fasts or periods of severe energy restriction. Even skipping a single meal often results in overeating at the following meal or for the rest of the day, which is why nutritionists emphasise that it is not a good idea to skip meals because what you lose on the swings will certainly be made up on the roundabouts.Then there is the potential that reducing your energy intake drastically for two days a week may well cause the body to switch off its weight loss systems to conserve energy instead of losing fat. This could well backfire and lead to weight gain or failure to lose any weight. Keep in mind that the human body is programmed to always attempt to achieve a state of homeostasis or balance. If you deprive your body of energy it does not automatically follow that the body will only burn available energy in the fat depots. The body can just as easily reduce its basic metabolic rate to conserve energy in the face of perceived starvation (the lean times) to maintain this balance, a strategy which will not result in weight loss.Money spinnersThe fact that Mimi Spencer is cashing in on the success of the initial book and has just published the Fast Diet Recipe Book in collaboration with Dr Sarah Shenker, a nutritionist, which provides "150 delicious, calorie-controlled meals to make your fast days easier", indicates that what started out as a simple concept may soon become a lot more complicated. Based on prior experience with other diet fads, I predict that we will soon hear about "Fast Diet Ready-Made Meals", being sold at popular pharmacies and supermarkets around the world. It is perfectly feasible seeing that the majority of diet gurus such as Patrick Holford, Tony Ferguson, Dr Atkins, etc, expanded their scope for profit by producing handbooks, recipe books, and diet products once they had caught the attention of a public desperate to lose weight.Potential dangersAt first glance the Fast Diet appears most innocuous, but there are certain considerations that must be mentioned for the safety of persons who may be tempted to try such diets. Patients with reactive hypoglycaemia, diabetes, particularly type 1 diabetes, or those using diabetic medications such as insulin, glucophage, etc, pregnant women, very active individuals and anyone with a tendency or past history of eating disorders, should not use fasting as a means of losing weight. Unfortunately many individuals who suffer from diabetes are overweight and will be tempted to try the Fast Diet. This can have dangerous consequences because diabetic patients always need to adjust their use of medications (both oral or injected diabetic medicines), to their food intake. So if you are diabetic and using oral anti-diabetic agents or injecting yourself with insulin, please discuss the concept of the Fast Diet with the doctor who is treating you before you try out fasting and develop a hypoglycaemic coma induced by lack of food and the effect of your blood glucose-lowering medications.Individuals who fast, may also automatically tend to ingest too little liquid (food contains relatively large amounts of liquid) and develop dehydration which can be potentially harmful and should be avoided. So while fasting is an accepted practice in many of the world’s great religions (Catholics fast during Lent, devout Greek Orthodox Christians are advised to fast for 180 days a year, Jewish people fast during Yom Kippur, and Muslims fast during Ramadan every year) (Mahan et al, 2011), the new Fast Diet craze may well induce individuals who should not be fasting to this practice with negative consequences. Always take your unique circumstances into consideration before trying out a new diet fad and if in doubt don't do it! (References: Mahan LK et al, 2012. Krause’s Food and the Nutrition Care Process. 13th Edition. Elsevier) (Photo of diet apple from Shutterstock) Dr Ingrid van Heerden (DietDoc) Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Health24.com. 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