At the end of 2019 fasting for weight loss as well as the benefit of fasted workouts hit the headlines thanks to two celebrity Jennifers. As a fan myself, I thought I’d take a look at the research.
Both Jenifer Aniston and Jenifer Lopez have revealed themselves as fans of fasting, but in very different ways. While Aniston does the intermittent fasting diet plan 16/8 (only eating for eight hours in the day), Lopez does fasted exercise first thing in the morning under the instruction of her trainer Dodd Romero.
I’ve spoken to the experts and found out all you need to know: from fasted workout benefits and whether fasted cardio burns more fat, to the side effects of intermittent fasting and what you can drink.
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Intermittent fasting diets
The 5:2 diet kicked off the popularity of intermittent fasting for weight loss in 2012, when the book the Fast Diet by Michael Mosley and Mimi Spencer was published. In this fasting schedule, on two days a week you can only consume up to 500-600 calories. More recently, the 16:8 weight loss diet has become increasingly popular (I’m a fan myself). It instructs you to each for eight hours of the day, and fast for 16.
Other, less popular, intermittent fasting diets for weight loss include Eat-Stop-Eat, where you completely fast for two days a week; alternate-day fasting, where you fast every other day by fasting completely or eating less than 500 calories; The Warrior Diet, where you eat small amounts of fruit and vegetables and have one large meal around 4pm.
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Intermittent fasting benefits
The main reason people fast in this way is for weight loss, but fasting for short periods of time also triggers a lot of healing responses throughout the body.
Intermittent fasting can help regulate hormone levels and allow the body respond better to insulin, which together can aid fat loss. It also improves cell function and repair which is important for exercise recovery. It can also lower the levels of bad cholesterol, boost energy and improve cognitive function.
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How to do intermittent fasting
At first, it can make you feel tired and maybe even weak. It may be best to start with a larger eating window and shave off an hour a week. Something to be wary of, especially if you’re trying it for the first time, is overindulging outside of fasting times. If you find yourself overeating, or even binging on food, in your eating hours or days, then it probably isn’t for you and it may lead to unhealthy eating habits.
What can you drink during intermittent fasting?
You can drink water, black coffee, tea (without milk), and zero-calorie drinks like Diet Coke. Anything above 50 calories will trigger your metabolism and so break the fast.
Is intermittent fasting safe for everyone?
While it has been proven to help prevent diabetes, for those already diagnosed it may not be suitable and you should always consult their doctor — this goes for most pre-existing health conditions.
Another big problem with intermittent fasting is that almost all of the research has been done on cis-men, and female biology is different, with a hormonal cycle every month that has a big impact on the body. One study done with cis-women showed a negative effect on blood sugar levels, and some health professionals question the effect on female reproduction.
This is a huge issue in most health and fitness research, and it needs to change. But that’s another blog post for another day.
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What is a fasted workout?
Fasted exercise — usually undertaken in the morning before breakfast — is said to force the body to use stored fat for energy, which in time it will become accustomed to doing more often throughout the day causing it to be generally more efficient at burning fat stores. Fasted exercise should be restricted to cardio, because your body is unlikely to perform well at weight lifting and strength training in a fasted state.
You need to have gone roughly eight hours without food for it to count as fasted — hence why people usually do it in the morning — but it varies quite a lot depending on a person’s metabolism. For some it could be as little as a three or four hours, for others with a slow metabolism it could be up to 12.
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Fasted cardio benefits
Studies also show it is likely to reduce blood sugar levels. Researchers from the University of Bath in England also found exercising on an empty stomach had the tendency to stop people overeating later in the day — so this could be a good alternative to those who struggle with overeating when intermittent fasting for weight loss.
The side effects of fasted exercise
While burning the body’s stored fat may sound appealing, low carbohydrate levels in the body can make you feel tired and less motivated to train in the first place. While one study showed an increased fat loss of 20% in fasted cardio, other research has shown that eating before will enable you to exercise for longer, therefore equalling the results.
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It is also possible that you will struggle to train at a high intensity, so it is a good idea to do it on days when you’re exercising at a low intensity or not trying something new. And, again, much of the research has only been done on men.
Another risk of fasted cardio is losing muscle mass. The body is indiscriminate in its search for energy — so if you’re trying to build muscle it may not be right for your regime. As a general rule, keep the intensity low — so HIIT training is not recommended — and consume lean protein and wholegrain carbohydrates afterward.