Intermittent fasting: What is it and can it help with weight loss?

Everything you need to know about the food trend.
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  • Perhaps you've heard of 'intermittent fasting', but does it actually work?

    Intermittent fasting does exactly what it says on the tin – it involves periods of restricted and unrestricted calorie intake.

    A handful of celebrities have endorsed the diet, too.

    But is it a sustainable way to eat and, more importantly, is it safe?

    Here’s the low-down on one of the world’s most prominent health trends right now.

    What is intermittent fasting?

    Intermittent fasting is an eating pattern that cycles between periods of fasting and normal eating.

    This could mean everything from skipping a meal or two on certain days of the week, to carrying out monthly multi-day fasts.

    It’s not about restricting what foods you eat, rather when you should eat them – therefore it’s more commonly referred to as a healthy eating pattern than a diet.

    Sports Scientist Harry Aitken tells GoodtoKnow, “Intermittent fasting is a dietary technique in which all food is consumed within a relatively small window of time.

    “Fasting is going for a significant period of time without eating, and intermittent fasting simply brings in a small window of time where you are able to eat.

    “Intermittent fasting has been popularised by religious festivals such as Ramadan and Yom Kippur, where for religious regions people must fast during certain times. With studies confirming loss of bodyweight and fat.”

    These studies have shown that intermittent fasting can have benefits on both your body and mental health, as well as being a very powerful tool for weight loss.

    Intermittent fasting

    Depending on which method you take up, you can usually eat the same kinds of foods/ credit: Getty

    How does it work?

    Harry explains, “By reducing the amount of time you are allowed to eat in, this in theory, should reduce your caloric intake. If you are burning more calories than you are eating, then you will lose weight.”

    He adds, “Typically it begins with an 18-hour window of fasting. If you eat dinner at 7pm, you will fast until 1pm, whereupon you will eat food until 7pm, and the cycle repeats.

    “Many people also lengthen the time of fast, shortening the eating window, with some people eating all their food in just a 4-hour period.”

    What does intermittent fasting do to the body?

    Your body needs fuel to survive, and the preferred source of energy is glucose, derived mostly from the breakdown of carbs. Glucose that isn’t needed for your body to function is stored as glycogen in the liver and muscles. Anything left over is stored as fat, explains nutritionist Rob Hobson.

    “Being overweight is a cause of many diseases, such as heart disease, stroke and cancer,” says Eve Mayer, author of Life in the Fasting Lane. “Losing weight increases high-density lipoprotein and lowers triglyceride levels, which helps reduce the risk of those same diseases.”

    During fasting, the body’s reserves of glucose are used up. While this is an effective weight-loss intervention, it has also been shown to remove waste from cells, increase muscle and help our genes provide information for making proteins or other molecues, all of which are linked to longevity and disease prevention.

    Different intermittent fasting methods:

    There are a variety of ways in which you can carry out intermittent fasting.

    Different methods will provide better results for different people, and it’s important to choose a method that suits you and your lifestyle.

    Here are the most popular methods:

    1. The 16:8 method: this involves restricting your daily eating period to 8 hours, such as 10am-6pm, or 11am-7pm. There’s no specific foods that you need to eat within this time frame (although they should be healthy!) as long as you fast for the remainder of the day.
    2. The 5:2 method: by adopting this method, you eat normally for five days of the week, and then on two non-consecutive days you restrict your calorie intake to 500-600 per day.
    3. Eat-Stop-Eat: this involves fasting for 24 hours once or twice per week. During the 24 hour fast, no food is consumed, but you can drink low-calorie beverages. Some people chose to practice this method by abstaining from eating from dinner one day until dinner time the following day.
    4. Alternate-day fasting: A 24-hour fast, or eat only a few hundred calories, but eat whatever you want on non-fasting days.
    5. Warrior diet: Fast for 20 hours through the night and during the day, eat a huge meal at night, within a chosen four-hour window.

    Benefits of intermittent fasting

    Although the initial hunger as a result of fasting may make people feel weak, dizzy and suffer from headaches, many of these symptoms are only temporary, as your body adapts to new eating habits.

    Insulin resistance

    Intermittent fasting is known for having many health benefits, such as reducing the body’s insulin resistance, which helps control hunger and regulates blood sugar. For those at risk, this can also help to lower the risk of Type 2 diabetes.

    Insulin resistance occurs when insulin tells your cells that fuel is coming, but they don’t open up to receive the glucose. So, sugar remains in the bloodstream and, eventually, it is stored as fat. During intermittent fasting, you’re consuming food in a shorter window, which reduces your ability to over-eat, therefore increasing insulin sensitivity.


    Fasting increases autophagy – the process of recycling and renovating our cells. As we age, our ability to regenerate new cells is decreased. By fasting, we can increase this process, which may slow down the rate of ageing.

    Heart Health

    A study by New England Journal of Medicine has shown that regular fasting is linked with a longer life and lower incidence of heart failure in heart patients. Even just one day of fasting a month – when done over a lifetime – can have a profound effect on heart health.

    Circadian Rhythm

    Your internal body clock regulates tiredness and alertness over a 24-hour period. Research suggests intermittent fasting may strengthen our body’s circadian rhythm, as overeating can cause interrupted sleep, therefore helping us to sleep better.

    Increased mental sharpness

    Some worry that fasting might tire them out and dull their senses, but in fact, it can have an energising effect on the mind. Fans of fasting have said that it has increased their mental sharpness and clarity.

    Intermittent fasting

    Credit: Getty

    Mistakes with intermittent fasting

    Harry says it’s important not to eat dinner too late – this can make you feel even more hungry the next morning.

    He says, “If you eat a big meal right before bed, your stomach does not have the chance to shrink overnight. It is busy digesting food as you sleep, and as your metabolism slows down overnight it digests slower – meaning this process takes longer.

    ‘Typically your stomach digests all the food, then shrinks (as it has nothing in it) and you wake up with a small stomach, not feeling hungry. Whereas by eating a big meal, it doesn’t shrink overnight, and you wake up with a big stomach groaning for food.”

    Harry also says to avoid jumping in at the deep end – instead start out small.

    He adds, “Start with a small fast, maybe just push your breakfast back by an hour, or eat dinner an hour earlier. Then continue doing this.”

    It’s also important to remain active – this will keep your body burning calories.

    Is it safe?

    Intermittent fasting isn’t safe for everyone, so it’s important to be aware if it’s right for you.

    “Fasting can mean we don’t meet some of our basic nutritional needs, so it’s important the meals you are eating are well balanced,” explains consultant dietician Sophie Medlin. Fasting isn’t for everyone. If you feel light-headed, get a headache or can’t concentrate, fasting might not be for you. “Some people feel hungry in the morning, so they might want to experiment with skipping dinner,” suggests Sophie.

    Due to its restrictive nature, the diet isn’t suitable for children, the elderly, or those under the healthy weight range. If you have any health concerns surrounding the diet, it’s best to check with your GP before taking it up.

    Is intermittent fasting good for weight loss?

    Short term fasting can boost your metabolism whilst helping you eat less calories, making it a very effective fat burning and weight loss tool.

    Celebrities who have seen success with intermittent fasting

    Last year, Jennifer Aniston told Radio Times (reported by CNBC) she fasts every day but keeps Sunday as her “cheat day.”

    She said, “I do intermittent fasting, so there’s no food in the morning.

    “I noticed a big difference in going without solid food for 16 hours.”

    Actor Chris Pratt also praised the intermittent fasting diet in one of his Instagram Stories, back in 2018.

    He said, “So I’m doing this intermittent fasting thing, don’t eat till noon, try to get my cardio in in the morning. It’s super exciting actor stuff.”

    He encouraged his followers to do the same, following his quick success.

    Tips for starting your first fast:

    • Drink plenty of water throughout the day.
    • Keep your schedule busy during the fasting window, if you’re busy you won’t be as likely to crave boredom snacks.
    • Incorporate your sleep into your chosen fasting time, it’ll make the whole process much more manageable – and the time still counts!
    • Work your way up to it – if that means adopting the 16:8 method at first, or even just delaying your breakfast by an hour in the morning. Every little change counts, and it’s supposed to be for your benefit so don’t push yourself too hard at first.
    • Eat more protein – A high protein diet will reduce your appetite and helps you keep you full and satisfied for longer.
    • Add magnesium to the menu – This mineral helps your body convert food into energy. Magnesium-rich foods include cheddar cheese, cooked salmon, cooked brown rice and raw spinach.
    • Don’t shop when hungry – If you’re hungry in the supermarket, you’re more likely to choose junk food.

    Fasting might not be many people’s choice for a way to lose weight, but with a combination of fasting and healthy amounts of exercise, you’re bound to shed any unwanted pounds.