Whether it's the 5:2 diet, 16:8 or alternative day eating, many of us are turning to some form of fasting to help us manage our weight and improve our health. Adherents to the practice claim that it can help us to shed unwanted pounds, lower our cholesterol and even help us live longer. But what does the science say?
To explain this and much more we sat down with Prof Javier Gonzalez, an expert in nutrition and metabolism based at the University of Bath.
What happens in our bodies when we fast?
One of the main things that happens is a change in the fuels that we're using. When we wake up first thing in the morning, the main fuels supplying our tissues with energy are a mixture of carbohydrates and fats.
The carbohydrates are a key fuel. In this scenario, they're in the form of blood sugar and some cells like the brain and our red blood cells can't really use fat as a fuel. So running low on sugar can be a real issue.
But we've actually evolved an interesting way to circumvent that issue. First of all, our liver provides sugar to the bloodstream via the breakdown of stored carbohydrates and it can also convert other fuels into sugar for the brain to use.
However, if we continue to fast for about a day or so, then the stores of carbohydrates in the liver can become completely depleted. Luckily, we've got an alternative fuel that can kick in at this point. Our liver starts to produce extra fuels, called ketone bodies or ketones.
Ketones are neither carbohydrates nor fat, but they can be used by the brain and our muscles actually as an alternative fuel.
Is fasting a good way to lose weight?
It can be one of multiple strategies which are effective for losing weight. What's probably important to bear in mind here is that based on the evidence we have so far, it doesn't seem to be the case that fasting does anything special over and above any other form of calorie restriction.
What we might normally consider traditional calorie restriction is where we limit our food intake by about 30 per cent every day. So, we're eating slightly less than we normally would.
That would lead to a certain amount of weight loss proportional to the reduction in the amount of energy that we're eating.
With fasting, if we take, for example, alternate day fasting where you don't eat for one day and then you eat the next day, it just so happens that when people have eaten nothing on day one, on day two they don't eat 200 per cent of what they normally would.
In studies that have controlled for the amount of energy that's been eaten. It doesn't seem to be the case that people lose more weight than would be expected.
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Can fasting improve blood sugar levels?
There's a short-term and a longer-term effect on blood sugar levels. The immediate effect is that you would get a slight lowering of blood sugar, and that's because you're not eating anything, including carbohydrates.
But after a period of fasting for a few days, if you then eat a meal that contains carbohydrate, and because we've had this switch to burning fat as a fuel, our body isn't quite as adapted to using carbohydrate as effectively. This means we can actually get a rise in blood sugar that's more exaggerated, than we would have otherwise.
So there's a bit of a complex interplay there between short-term effects and longer term effects, and it might also be mediated by your current status. If you have prediabetes, for example, there could be a beneficial effect.
Can fasting help to lower cholesterol?
Yes, most markers of cardiovascular health will show an improvement even with relatively short-term fasting. Some of it will be down to physiological adaptation and some of it might be a true kind of health benefit.
With blood pressure, for example, part of the reduction might be because you're not eating any sodium, or salt. That will result in a reduction in the amount of blood circulating around the body and so can lead to our blood pressure can reducing.
Similarly, with cholesterol, if we're not eating any fat, including saturated fat, then we see a reduction in the amount of cholesterol in our blood, partly because our liver's using it up.
What effect does fasting have on our gut microbiomes?
We don't know very much from studies that can help us establish cause and effect but we do think that there's probably something going on.
One of the main things that our gut microbiome will respond to from a dietary perspective, is fibre. If we eat a lot of fibre, we're essentially providing food for our gut microbiome to feed off.
When we eat a lot of diverse fibres and fruits and vegetables we tend to have a diverse microbiome. So, if we're fasting we wouldn't be consuming any fibre and that could have implications for our microbiome. It could also have other implications, maybe it could change blood flow to the gut, for example. But that's really just speculation at this point.
What evidence is there to suggest that fasting could help to extend our lives?
At least in mice, there are many studies that have shown that any form of calorie restriction, including intermittent fasting, seems to extend lifespan. The most commonly studied form there is actually a 30 per cent calorie restriction. The mice basically eat 30 per cent fewer calories than they would choose to over their lifetimes and they tend to live longer.
We obviously can't ethically or practically have humans do that for their lifetimes. But there have been two studies in non-human primates. Unfortunately, the studies seem to conflict. One of them seems to show it extends lifespan while the other one suggests it doesn't.
Also, some people argue that the studies in mice are influenced by other factors. For example, as they are living in an environment that's germ-free, they don't need necessarily an immune system to fight off infections.
Some would argue that one of the trade-offs you get when you restrict calories is that your immune system could be suppressed so it could be that humans would trade any cardiovascular benefits with an immune system that might suffer.
About our expert, Prof Javier Gonzalez
Javier is professor of nutrition and metabolism based at the University of Bath.
His research has been published in The Journal of Clinical Endocrinology & Metabolism, The American Journal of Physiology: Endocrinology and Metabolism and The Journal of Physiology.
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