Sounds like a simple question with a simple answer. Fasting must just be the physical act of not eating. As soon as you’re not eating you must be fasting, right? Wrong. If you really want to tap into the reported benefits of fasting then there are a few things to consider.
As soon as you have a meal and finish eating, you enter what’s called the postprandial state, which lasts from your last bite until about 4 hours afterwards; hence why the general recommendation is to wait at least 4 hours between meals. During these 4 hours your body goes about its vital work of digesting the food you’ve eaten, breaking it down into its component parts, absorbing what nutrients it needs from that food and either utilising it as fuel, if needed, or storing it away to draw upon later.
After this 4 hours is up you enter another phase called the post-absorptive state, i.e. what happens after the nutrients in your food have been absorbed. In this period, which lasts 6 hours, you start to break down glycogen, i.e. stored carbohydrates, and also begin to break down stored fat which can be used for fuel too.
So, during these 10 hours your body has a constant supply of fuel and doesn’t need anything extra. It has the initial injection of calories from your meal, it digests it for 4 hours and then, when no more food comes in, it mobilises stored energy to keep you going too. When the initial food comes in the glycogen it contains is stored in the liver, which is one way that our bodies regulate our blood sugar balance. During the post-absorptive state it’s still able to use this stored glucose and so it is only after this phase has finished that you are technically in a ‘fasted’ state and truly ‘empty.’ We can characterise a fasted state as the body needing to look elsewhere for fuel, i.e. it is unable to use glucose. And this is where burning fat for fuel comes in which, as is quite clear, contributes to weight loss, but is also when the other processes like better detoxification and cellular repair, in fact bodily repair in general, come into play.
A seminal study in the 1930s showed that restricting the calorie intake of rodents lead to a longer lifespan compared to those that were able to eat freely and in traditional and ancient forms of medicine, such as Traditional Chinese Medicine, the recommendation is to only eat until you’re 70% full. Historically, as humans, we’ve never before had such an abundance of food and many argue that intermittent fasting, or sporadic periods of longer fasting is actually just a regression to the way we used to eat - eating our fill in times of plenty but consuming significantly less at other times.
Is a juice cleanse the same as a fast?
Technically, no. To be in a properly fasted state you have to not spike your blood glucose, which any form of eating or drinking will do (this is slightly disputed but essentially, something like a black coffee is fine but if you were to add any milk to it then it wouldn’t be fine, for example). However, one of the main benefits of fasting is the rest for your digestive system. In terms of gut health in particular we can really benefit from not being so “on” all the time. True, our gut microbes need fibre to ferment but if you are constantly eating, or overeating, then digestion can get sluggish and sub-optimal, your energy production will be reduced and you can wind up feeling pretty down in the dumps. Cleansing with organic and cold pressed juices which are low in fibre, so need less work to digest, can give your digestion a bit of a break in the same way whilst still providing some nutrients, vitamins and minerals from the organic fruits and vegetables.