Intermittent fasting for weight loss – should you try it?

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Fasting as a ‘body cleansing’ tool has been around for millennia but how about intermittent fasting to achieve weight loss? Does it work and is it safe?

There are many types of fasting, which is the voluntary withholding of food (unlike starvation) and usually lasts hours or days. Intermittent fasting is, in a sense, a normal part of our lives – for example between dinner and breakfast. However, ‘proper’ fasting means not eating for a period of time longer than it takes your body to merely digest the food you had just eaten. During true fasting, your body uses its own stored energy, first from glycogen (carbohydrate stored in the liver and muscles) and other molecules, and eventually from body fat if the fasting continues for longer or if you’re physically active.

One advantage of fasting is that it improves your body’s sensitivity to insulin (blood sugar-regulating hormone); your body gets a chance to not be constantly digesting and storing food and you burn up some extra energy. It’s a natural state for us because during our evolution we didn’t always have food readily available so we’re well adapted to going without food for a while – and it can make you feel good.

The downside is that you’ll most likely feel hungry, may experience constipation, headaches and there’s a risk you’ll end up bingeing afterwards and eat more energy than you just burned up.

Daily fasting

There are two main patterns of fasting – either restricting your food intake to certain hours in the day or having certain days in a week when you fast or eat very little. The main idea is that by not restricting your food intake all the time, only some of the time, you’re more likely to stick with the regime, achieve weight loss and eat less in general. Giving yourself rules about when you can and cannot eat gives you a schedule to follow and many people truly thrive on that. However, it works on the premise that you won’t overeat every time you can eat to make up for the fasting time.

If you’d like to try it, the daily patterns of fasting, such as 16:8 or 20:4 (16 or 20 hours of fasting a day with an eight or four-hour window in which you eat) are the best ones to start with in order to test how your body responds. Make sure you have a wholesome, varied diet when you do eat and do not restrict your food intake too much. There is a risk that having a daily fasting pattern may drive your food intake too low – if you’re not sure how you’re doing, try using one of the food tracking apps to check how much you’re eating. At the other end of the spectrum, eating too much or choosing a daily eating period of more than eight hours likely won’t bring the desired results because it’s too close to no restriction.

Weekly fasting

Among the weekly fasting patterns, the 5:2 is very popular as it’s flexible, manageable in the long-term and produces good results. It means eating as usual five days a week and fasting or limiting your calorie intake to 500 calories on the remaining two days. You don’t have to stick to the same fasting days each week so it can be adapted to suit your needs. It’s a good idea to have some light foods on your fasting days to help tide you over – fruit and vegetables, soup, crispbread. Some people don’t have any food on fasting days and it’s up to you whether you eat a little or not at all but pay attention to your body. If it starts sending warning signals – feeling like fainting or dizziness – have a small snack.

If you’d like to do fasts that last 24-36 hours, plan ahead so you don’t do too much exercise on those days. It’s perfectly fine to fast and exercise but if you’re not used to it, go easy on yourself. If you do intensive training, you’ll need to replenish electrolytes with an isotonic drink – mix one part fruit juice (fresh or cold-pressed) with one part water, a tiny pinch of salt and you can always add extra ingredients, such as slices of fresh ginger, lemon juice or cinnamon. One teaspoon of brown sugar or agave/date/maple syrup is fine to add as well because your body burns up more than that through exercise.

Fasting for more than a few days at a time can be dangerous in terms of nutrient deficiencies and is only recommended under medical supervision.

Not for everyone

There are several scenarios in which any kind of fasting should be approached with caution – if you have diabetes, migraines, are on medication that has to be taken with food or have other serious medical conditions. It’s impossible to predict how your body will react – it could welcome the change or it might make you suffer unnecessarily. If you have diabetes, it may be too dangerous to attempt fasting but if you want to try, ask your doctor and they may adjust your medication for the duration of the fast.

There are, however,  situations when you absolutely shouldn’t fast at all – if you are underweight, have a history of eating disorders, are pregnant, breastfeeding or under 18 (your body is still growing).

If you try intermittent fasting and discover you like it a bit too much, be kind to yourself and stop. There’s a risk that you may be prone to eating disorders and fasting may trigger a dangerous behaviour pattern.

How does it compare to calorie-restricted diets?

For some people it works wonderfully, for others not. Studies show that intermittent fasting is about as efficient at achieving weight-loss as a daily calorie-restricted diet. Some of us work better on schedule (intermittent fasting) and some of us on structure (restricted diet) – although we all tend to struggle with limiting our food intake. Whether intermittent fasting is for you also depends on your work patterns and how fasting affects you – it makes some people so ‘hangry’ that it’s not sustainable!

In any case, whatever eating regime you adopt, it’s always good to give your body time to wake up and not eat as soon as you get out of bed. And to make sure your body has time to rest and recover, wind down your eating at least two hours before going to bed.


PS If you’d like to try the 5:2 version, our Vegan Recipe Club has some smashing meal ideas to offer!

About the author
Veronika Prošek Charvátová
Veronika Prošek Charvátová MSc is a biologist and Viva! Health researcher. Veronika has spent years uncovering the links between nutrition and good health and is an expert on plant-based diets.

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