Medically Reviewed by Dr Rob Hicks
Fasting doesn't seem like fun. It conjures up images of supermodels who exist on salad and diet drinks just to keep their size zero figures or maybe those drastic dieters who leave it to the last minute and need to lose half a stone before a wedding in a week's time!
It seems pretty extreme and there's always the suspicion you'll just put the weight back on when you stop, that's if you have the will-power to start in the first place!
The buzz at the moment though is over alternate day fasting or intermittent fasting. There's some evidence that short periods of fasting could not only be a way to lose weight but also be potentially good for your health.
The latest discussion about fasting has been provoked by a BBC Two Horizon documentary presented by Dr Michael Mosley.
He looked into the science, met the experts and tried it for himself. The upshot of the programme was he lost weight and apparently became healthier.
It seems pretty extreme and there's always the suspicion you'll just put the weight back on when you stop, that's if you have the will-power to start in the first place!
The buzz at the moment though is over alternate day fasting or intermittent fasting. There's some evidence that short periods of fasting could not only be a way to lose weight but also be potentially good for your health.
The latest discussion about fasting has been provoked by a BBC Two Horizon documentary presented by Dr Michael Mosley.
He looked into the science, met the experts and tried it for himself. The upshot of the programme was he lost weight and apparently became healthier.
Do you lose weight fasting?
The simple answer is yes. It may not be the most practical - or safest - diet, some people use fasting as a way to lose weight or to cleanse the body of toxins, although experts say our bodies are perfectly equipped with organs that already do the job.
When you fast, your body is forced to dip into energy stores to get the fuel it needs to keep going, so you will definitely lose weight.
The big question is how long you will keep that weight off. Because food was often scarce for our ancestors, our bodies have been genetically programmed to combat the effects of fasting.
When you eat less food, your metabolism slows down to conserve energy. Then, when you go back to your usual diet, your lowered metabolism may cause you to store more energy, meaning that you will probably gain back the weight you lost and possibly even put on more weight when eating the same calories you did before the fast.
As you fast, your body will adjust by reducing your appetite, so you will initially feel less hungry. However, once you have stopped fasting your appetite hormones will return full force and you may actually feel hungrier and be more likely to binge.
When you fast, your body is forced to dip into energy stores to get the fuel it needs to keep going, so you will definitely lose weight.
The big question is how long you will keep that weight off. Because food was often scarce for our ancestors, our bodies have been genetically programmed to combat the effects of fasting.
When you eat less food, your metabolism slows down to conserve energy. Then, when you go back to your usual diet, your lowered metabolism may cause you to store more energy, meaning that you will probably gain back the weight you lost and possibly even put on more weight when eating the same calories you did before the fast.
As you fast, your body will adjust by reducing your appetite, so you will initially feel less hungry. However, once you have stopped fasting your appetite hormones will return full force and you may actually feel hungrier and be more likely to binge.
Alternative day fasting
Alternative day fasting (ADF) is a bit different. It involves eating what you want one day, then a very restricted diet (fewer than 600 calories for men 500 for women) the next, and most surprisingly, it does not seem to matter that much what you eat on non-fast days.
Dr Krista Varady of the University of Illinois at Chicago carried out an 10-week trial comparing two groups of overweight patients on ADF.
Her findings concluded that ADF is a viable diet option to help obese people lose weight and to decrease their coronary artery disease risk.
"If you were sticking to your fast days, then in terms of cardiovascular disease risk, it didn't seem to matter if you were eating a high-fat or low-fat diet on your feed (non-fast) days," she said.
Dr Mosley couldn't manage alternate day fasting but did a five:two ratio, five days normal eating and two days of under 600 calories.
He stuck to this diet for five weeks, during which time he lost nearly a stone and hisblood markers, like IGF-1, glucose and cholesterol, improved.
He said: "If I can sustain that, it will greatly reduce my risk of contracting age-related diseases like cancer and diabetes."
Dr Krista Varady of the University of Illinois at Chicago carried out an 10-week trial comparing two groups of overweight patients on ADF.
Her findings concluded that ADF is a viable diet option to help obese people lose weight and to decrease their coronary artery disease risk.
"If you were sticking to your fast days, then in terms of cardiovascular disease risk, it didn't seem to matter if you were eating a high-fat or low-fat diet on your feed (non-fast) days," she said.
Dr Mosley couldn't manage alternate day fasting but did a five:two ratio, five days normal eating and two days of under 600 calories.
He stuck to this diet for five weeks, during which time he lost nearly a stone and hisblood markers, like IGF-1, glucose and cholesterol, improved.
He said: "If I can sustain that, it will greatly reduce my risk of contracting age-related diseases like cancer and diabetes."
"Alternate day fasting is another way of reducing the overall amount of calories eaten to help with weight loss," according to Emer Delaney from the British Dietetic Association.
"There has been some debate recently that this 'new' way of eating can offer major health benefits, however there simply isn't the evidence to back this up."
"There has been some debate recently that this 'new' way of eating can offer major health benefits, however there simply isn't the evidence to back this up."
He says unsurprisingly, reducing the overall intake of calories, whether it's every day or on alternate days, will result in weight loss.
"Whilst it may work for some people, they need to ensure their diet on 'non fast' days is packed with fruit and vegetables, wholegrains, lean protein rich foods such as chicken, fish, turkey and low fat dairy products."
"Whilst it may work for some people, they need to ensure their diet on 'non fast' days is packed with fruit and vegetables, wholegrains, lean protein rich foods such as chicken, fish, turkey and low fat dairy products."
Do you live longer fasting?
Studies of fasting in both rodents and humans appear to indicate a connection between calorie restriction and longevity. In one study of overweight men and women, a calorie-restricted diet improved markers of ageing such as insulin level and body temperature.
Fasting might also improve longevity by delaying the onset of age-related diseases including Alzheimer's, heart disease and diabetes.
One study showed that missing meals once a month, as members of the Mormon religious group do, reduces the risk of clogged arteries (the build-up of plaque that can lead to heart attacks and strokes). However, it is not clear from this research whether fasting alone or the Mormons' generally healthier lifestyle (they also abstain from coffee, alcohol and smoking) is responsible for the improved heart health.
A study which examined 100 overweight women from Greater Manchester found that women who followed a strict 650 calories- a- day diet for just two days a week (and ate what they wanted on the other days) lowered their risk of breast cancer by 40%. Researchers found the calorie-controlled regime almost halved cancer-causing hormones in women at high risk of the disease.
Researchers do not yet know whether the effects of fasting translate into an actual increase in lifespan, because they have not followed people for long enough periods of time.
Registered dietitian and food coach Sasha Watkins says: "Caloric restriction does seem to have some link with living longer. The population of Okinawa in Japan eat 20% less than the Japanese national average and have many more centenarians in their midst. However, the research is not conclusive. A recent study found that monkeys eating 30% less calories did not live any longer than the control monkeys but they did have lower cholesterol levels."
Fasting might also improve longevity by delaying the onset of age-related diseases including Alzheimer's, heart disease and diabetes.
One study showed that missing meals once a month, as members of the Mormon religious group do, reduces the risk of clogged arteries (the build-up of plaque that can lead to heart attacks and strokes). However, it is not clear from this research whether fasting alone or the Mormons' generally healthier lifestyle (they also abstain from coffee, alcohol and smoking) is responsible for the improved heart health.
A study which examined 100 overweight women from Greater Manchester found that women who followed a strict 650 calories- a- day diet for just two days a week (and ate what they wanted on the other days) lowered their risk of breast cancer by 40%. Researchers found the calorie-controlled regime almost halved cancer-causing hormones in women at high risk of the disease.
Researchers do not yet know whether the effects of fasting translate into an actual increase in lifespan, because they have not followed people for long enough periods of time.
Registered dietitian and food coach Sasha Watkins says: "Caloric restriction does seem to have some link with living longer. The population of Okinawa in Japan eat 20% less than the Japanese national average and have many more centenarians in their midst. However, the research is not conclusive. A recent study found that monkeys eating 30% less calories did not live any longer than the control monkeys but they did have lower cholesterol levels."
She says more studies are definitely needed in this area to understand if and how fasting may be good for our health.
Current medical advice
As it stands medical opinion at the moment is that the benefits of fasting are unproven and until there are more human studies it's better to eat at around 2000 calories a day.
If you really want to fast then you should do it in a proper clinic or under medical supervision, because there are many people, such as pregnant women or diabetics on medication, for whom it could be dangerous.
The British Dietetic Association says that rapid weight loss occurs when fasting or severely restricting dietary intake, but this weight loss is mainly water, glycogen (the body's carbohydrate stores) and muscle, rather than body fat.
If you really want to fast then you should do it in a proper clinic or under medical supervision, because there are many people, such as pregnant women or diabetics on medication, for whom it could be dangerous.
The British Dietetic Association says that rapid weight loss occurs when fasting or severely restricting dietary intake, but this weight loss is mainly water, glycogen (the body's carbohydrate stores) and muscle, rather than body fat.
"Routine fasting is practiced successfully by many cultures for religious purposes and may have some health benefits, says BDA spokesperson Rick Miller, "However, the clinical evidence for fasting as a treatment in healthcare is not clear.
"Fasting could potentially be unsafe in some individuals without medical supervision or lead to the development of poor eating patterns. Dietitians would always recommend a well-planned, healthy diet in the first instance for health and longevity."
Dietitian Sian Porter says: "A lot of people have a five days and two days eating pattern.
"Make sure you eat foods that make you feel full and keep your fluid intake up."
Dietitian Sian Porter says: "A lot of people have a five days and two days eating pattern.
"Make sure you eat foods that make you feel full and keep your fluid intake up."
http://www.webmd.boots.com/diet/features/is-fasting-good-for-your-health#
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