The lowdown on intermittent fasting
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Eat very little today, eat as much as you want tomorrow. Stop eating after tonight’s dinner and eat nothing until lunchtime tomorrow. Skip breakfast and lunch altogether, making sure to leave 24 hours between meals… Hmm. With so many intermittent fasting programmes out there, it’s no wonder we’re feeling bewildered. To add to the confusion, advocates are promoting it not only as a way to lose weight, but as the answer to all our health problems, yet others only highlight the dangers. Depending on what you read, intermittent fasting can increase longevity, reduce cholesterol levels, decrease inflammation, relieve asthma, decrease your chances of cancer and even reverse diabetes. But what exactly is it, how do you actually do it and are the health benefits really worth it?
Intermittent fasting involves cycling between periods of eating and periods of not eating. This can mean regularly abstaining from food during certain time periods, reducing your calorie intake for a couple of days each week or, for the stronger-willed, not eating for 24 hours once or twice a week.
1 Calorie-controlled intermittent fasting
Made popular by Dr Michael Mosley’s 5:2 Diet and Krista Varady’s Every Other Day Diet, this form of fasting does not require that you go without food. Rather, it allows you to eat every day, restricting your calorie intake on fasting days and eating normally on the others. The 5:2 Diet recommends that you restrict yourself to 500-600 calories for at least two days per week. The Every Other Day Diet recommends limiting calories to 500 calories on alternate days.
Accredited dietitian Gabrielle Maston explains that this type of fasting helps us to lose weight in two ways. “Firstly, by taking in less energy from food, the
If you’re having a fasting day, don’t waste most of your allowance on items like white bread and pasta that are full of empty calories. Instead go for whole foods and lots of veges to keep yourself feeling full and getting all the nutrients you need.
body needs to expend its fat stores, which leads to weight loss.”
The second and unexpected way that intermittent fasting helps lead to weight loss is by changing people’s psychology.
“When caloric intake is reduced,” says Maston, “you get used to eating less and your perception about how much you need to eat on the non-fasting days changes. What tends to happen is that people start to eat less on these days because they feel they don’t need to eat as much.”
She believes that intermittent fasting is easier to maintain than traditional diets.
“It’s more sustainable than a diet in which you forgo the things you enjoy eating on a daily basis. People can generally stick to this type of intermittent fasting for about three months – compared to one or two months for a typical daily calorie-restricted diet.”
In her studies on alternate-day fasting, Krista Varady, associate professor of nutrition at the University of Illinois in Chicago, found that people not only lost weight and decreased their body fat percentage, but also reduced their total cholesterol levels and improved their blood pressure.
The CSIRO reports similar findings. In their 16-week study, people took part in intermittent fasting that included three low-calorie fasting days per week, three calorie-controlled days and one day when they could eat whatever they wanted. On average, participants lost 11kg and saw improvements in cholesterol, insulin, blood pressure and blood glucose levels.
The benefits of intermittent fasting appear to extend beyond weight loss and improved biomarkers. In an eight-week study conducted by the US National Institute on Aging, overweight adults with moderate asthma reduced their diet to 20 per cent of their normal daily intake. Participants lost weight and saw a decrease in inflammation markers and a significant improvement in their asthma symptoms.
Despite these benefits, Maston warns that this form of fasting isn’t for everyone. “For some people, it encourages bingeing on the non-fasting days, which often takes the form of eating foods with little nutritional value.
It’s one thing to reduce calories, but people need to look at a combination of reducing calories and eating whole, nutrient-dense foods.”
2 24-hour fasting
Probably the most challenging form of intermittent fasting is the 24-hour fast. As the name suggests, it involves not consuming anything except water for 24 hours. The fast typically begins after you eat a meal such as dinner – you then ditch food until the same time the next day.
Animal studies conducted by the US National Institute on Aging have shown an increase in longevity and improved cognitive function with this type of intermittent fasting. In one study, rats who were starved every second day lived 83 per cent longer on average than rats who weren’t fasted. In a similar study, it was concluded that intermittent fasting can improve age-related decline in brain function in rats. However, the researchers stress that human studies are required before claims about longevity and cognitive function can be made. Perminder Sachdev – scientia professor of neuropsychiatry at the Centre for Healthy Brain Ageing at the University of NSW – agrees but points out that many studies of centenarians show that restricting calories
If you plan to work out on fasting days, it’s important to listen to your body and stop if you feel faint or lightheaded. Try to eat a carb-rich snack after high-intensity workouts, to help your glycogen-depleted
leads to an extended and healthier lifespan.
Maston argues that this form of fasting is dangerous as it carries a risk of dehydration and electrolyte imbalances, among other things. “Fasting for 24 hours stops people listening to what their bodies are telling them. Our bodies are very good at sending us hunger cues. Twenty-fourhour fasts override these cues and are the opposite of intuitive eating, which is how we should be eating,” she says.
3 Time-restricted eating
Another popular variation of intermittent fasting is time-restricted eating. For most people, this means eating their last meal at night and not eating again until lunchtime the following day.
The theory is it takes about six hours without food before the body enters a fasted state, when it begins to break down fat for energy. For most people, the only time their bodies reach a fasted state is when they’re asleep. Advocates of timerestricted eating argue that by skipping breakfast, you remain in a fasted state for longer, which results in increased fat loss.
Dr Nick Fuller, one of Australia’s leading obesity researchers and author of
Interval Weight Loss, disagrees.
“By skipping meals, you’re setting yourself up for failure,” he says. “Whenever energy intake is reduced, whether by intermittent fasting or other forms of dieting, your body undergoes the same metabolic response. First, your metabolism decreases; second, it causes an increase in the appetite hormones that tell you to eat more. You will lose weight, but your body will do all it can to get back to its set point.”
Fuller believes our bodies are tuned to a set point – the natural weight they will work to try to maintain. “Once you stop fasting and start eating normally, your body will drive your weight back to where you started, or even higher.”
One major benefit of intermittent fasting, shown across multiple studies, is that it reduces blood glucose levels. Given high blood glucose levels are a major risk factor in developing type 2 diabetes, this is good news.
Sachdev, of the Centre for Healthy Brain Ageing, explains that reducing blood glucose levels leads to a cascade of health benefits, including a reduced risk of atherosclerosis and stroke, reduced cell damage and better immune function.
“It therefore slows the age-related decline in brain function and improves vascular brain health, both of which are important for cognitive disorders in later life, including dementia,” he says.
Fuller’s not convinced. He says these results are due to weight loss and not fasting. “Fasting causes short-term weight loss and, as a result, you can see an improvement in blood glucose levels.”
Indeed fuller argues that fasting isn’t sustainable in the long term, so once you reintroduce foods and regain the weight, you’ll be worse off than when you began.
Skipping meals is particularly dangerous for people with type 2 diabetes, especially those who are insulin dependent, he warns. “Skipping meals puts those who have diabetes and are taking insulin at huge risk of harm, as they need small, regular meals to regulate their blood glucose levels.”
A less immediate danger, says Maston, is overeating. “When people restrict themselves to eating within a short timeframe, their primal instinct kicks in.
For many people, if they feel they’re going to have to go without, they’ll try to get in as much food as they can during that period.”
This can cause problems such as reflux and feelings of nausea due to the increased volume of food consumed in one sitting.
4 But wait, there’s more…
More extreme forms of fasting include multi-day fasts of 10 days or more, during which nothing but water is consumed – although some full-on fasts even discourage drinking water.
Maston warns there are serious medical dangers associated with extended fasts. “Not eating for long periods results in malnutrition and electrolyte imbalances such as potassium deficiency, which can lead to heart attack and even death,” she says.
Muscle wasting is also common as the body goes into starvation mode and begins to break down muscle to protect the vital organs. In women, long-term fasts can result in bone loss and the reproductive system shutting down.
As well as those with type 2 diabetes, intermittent fasting is not advised for those with a history of eating disorders, or in the case of pregnancy. To be on the safe side, if you’re considering fasting, talk first to a health practitioner.