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The science behind the fasting diet

The science behind the fasting diet

We’ve seen the cabbage soup diet, the milkshake-only diet, the diets which restrict your calorie intake all week, and also the carbohydrate diet. Now it’s all about fasting.

The latest craze to sweep a nation is to eat as you would normally for five days of the week, and then on two days of your choice, cut your calories down – to 600 if you’re a man and to 500 if you’re a woman.

The diet has been named the 5:2 diet and has been championed by Dr Michael Mosley, a scientist and presenter for BBC. He researched the practice of intermittent fasting and discovered that it reduces the risk of a range of diseases from diabetes, asthma and arthritis to cardiovascular disease and cancer. He believes it can also slow the ageing proves and improve brain function.

With such bold claims it’s no wonder everyone has jumped on the fasting bandwagon, but what is the science behind it, and how can semi-fasting for two days improve our health?

If you starve yourself, your body can’t feed on the calories you’ve consumed from food and instead it turns to the glucose that’s stored in your blood stream. When it’s finished there it goes for the glucose stored in carbohydrate molecules called glycogen. From there it turns to your fat stores for energy and glucose. If you think about marathon runners, after a long period of running they need to top up their glucose supply because they get fatigued as their bodies begin to feed on their fat stores and their energy is sapped.

If you just continue to starve yourself then your metabolism slows down and the whole process is simply not sustainable. But the idea of the 5:2 diet is that you don’t completely starve yourself, but rather you intermittently fast.

The theory behind the Fast Diet – from Dr Michael Mosley and food writer Mimi Spencer - is that you eat fewer calories but only some of the time.

A study run by neuroscientist Mark Mattson involved mice and found that intermittent fasting increased levels of a protein called brain-derived neurotrophic factor – or BDNF.

This process stimulated new brain cells in the area of the brain responsible for memory.

The study involved Mattson feed one group of mice a diet of junk food, while a different group of mice ate a low fat diet and fasted for part of the week. The mice that had fasted had more BDNF, whereas the junk food mice simply became fat and disorientated.

If you go right back to hunter-gatherer days, people would go for long periods of time without food. In order to find food the theory is that our brains would have needed to be sharp in order to find food during this period of fasting.

So the brain’s response to fasting was a survival skill, according to Mattson.

Our blood sugar levels rise when we eat foods high in carbohydrate, sugar and starch. In response our bodies produce insulin to regulate our glucose levels.

Studies have shown intermittent fasting boosts the effectiveness of insulin to store glucose and break down fats, something called insulin sensitivity. "Increased insulin sensitivity will reduce your risk of obesity, diabetes, heart disease, and cognitive decline," Dr Mosely writes.  

The fasting diet has other claims, including that it can help to reduce the risk of cancer and slow down the ageing process, but many of the studies that have been undertaken have involved mice, and not humans so it is still not conclusive.

Theory and successful weight loss from many who undertake the fasting practice have meant that this diet has led to more research, but before embarking on any diet you must always make sure that you speak to your GP and ensure you are fit and well enough to do so. Alongside diet we mustn’t forget that exercise is vital for a healthy lifestyle, and you may find that by restricting your calories on days you are doing heavy workouts, means that you don’t have the energy you need. If that’s the case, consider dropping your exercise levels on fasting days, and pushing it up on the other five. On the other hand, don’t believe that just doing this diet on its own will ensure a long and healthy life. You must never use diet as a substitute for staying active. If you are taking the philosophy from hunter gatherer humans that they used to fast, then you also need to remember that hunter gatherers were on their feet walking, and often running for most of the day, something that few of us do now.

Also, it works best if for five days of the week you are eating sensibly, rather than stocking up on cakes and pastries on your non-fast days!

Dr Michael Mosley upholds the benefits of the diet and he has done it himself. He is doing a talk on October 19th at St Thomas’s Church in Wells, Somerset entitled “The Fast Diet How can you enjoy a longer, slimmer life?” as part of the Wells Festival of Literature.