When people want to lose weight, the potential benefits of fasting can seem very appealing, because only having a certain window to eat means you potentially consume fewer calories.
But what is fasting? According to registered dietitian Marcela Fiuza, a spokesperson for the British Dietetic Association, fasting means consuming no calories in a certain time frame. It can be ‘intermittent’, meaning you switch between eating and fasting, or ‘prolonged’, which generally means fasting from two days onwards.
“Intermittent fasting, in particular time-restricted eating like the 5:2 or 16:8, has become popular in recent years,” she adds. “It involves eating within a time-restricted window each day, usually eight to ten hours.”
As well as weight loss, lots of people do fasting purely for the potential benefits, including better gut and heart health as well as lower blood pressure. But it’s important to note that fasting doesn’t guarantee these results, and it’s not suitable for everyone — particularly anyone with disordered eating, pregnant women, diabetics, seniors and children.
In this article we talk more to Fiuza about the potential benefits of fasting, and take a look at some of the cons too.
What does fasting do to the body?
Before we look at the potential benefits of fasting, it’s important to know what it actually does to the body.
Fiuza explains: “During fasting the body makes a number of metabolic adaptations to keep functioning optimally in the absence of external fuel (food). In the first few hours of fasting, the body resorts to its glycogen stores for energy. Once these are depleted, there is a metabolic switch, in which the body starts breaking down fatty acids into ketones that are then used as a source of energy.
Marcela Fiuza is an award-winning registered dietitian based in London, U.K. She has a decade of experience working in the National Health Service, private practice, and the commercial sector. She holds an MSc in Nutrition and a Postgraduate Diploma in Dietetics from King’s College London. She is a member and media spokesperson for the British Dietetic Association.
“The timing for this metabolic switch depends on what your last meal was, how much energy you use and the amount of glycogen stored in your liver. On average it can take 12-26 hours without food.”
Of course, not all benefits are guaranteed. “Plenty of studies, mainly in animal models, suggest benefits from being in a fasted state, and there is increasing evidence emerging from human trials, too,” says Fiuza.
“But more research is needed until we can fully understand the long-term impacts of fasting in human health.”
One of the possible benefits of fasting is that it can trigger a process called autophagy — your body’s cellular recycling system. Acting as a sort of quality control for your cells, autophagy allows the body to break down and reuse old cell parts so they can work more efficiently.
Put simply, it’s the body’s way of housekeeping and getting rid of mutated cells that could develop into cancer or neurological conditions like Alzheimer’s or Parkinson’s, according to a study in the EMBO journal.
The process of autophagy begins after a period of fasting and could be an evolutionary throwback to our hunter-gatherer days, where people would go longer without eating due to the labor-intensive nature of finding food.
Researchers are studying autophagy’s role in potentially preventing and fighting disease, says Fiuza. “Evidence from a study published in Science Direct suggests that fasting may be able to enhance autophagy.” Another study, published in Autophagy Journal, revealed that regular fasting could ‘reset’ the body and help it run more efficiently by clearing out cellular debris.
2. Improved gut health
There is evidence to suggest that a radical change in diet, such as fasting, could alter the gut’s microbial make-up and change what gut bacteria is doing.
Fiuza told Live Science: ”Some forms of fasting may be beneficial to the gut microbiome, which has been linked to a range of health benefits from improved metabolic health, reduced risk of cancer, heart disease and obesity.”
Other research found that alternate-day fasting (24 hours of eating normally followed by 24 hours of fasting) promoted ‘bacterial clearance’ that could support the health of the gut microbiome.
3. Healthy heart and blood sugar levels
Fasting may also improve the body’s response to the hormone insulin, which controls blood sugar levels. When your blood sugars are regulated it reduces the risk of weight gain and diabetes, which are two risk factors for cardiovascular disease and other heart-related health problems.
Fiuza adds that intermittent fasting could also improve heart health by reducing low-density lipoprotein (LDL), or bad, cholesterol as well as “exerting positive effects on blood pressure regulation and by reducing inflammation”, but more research is still needed in this area.
4. Weight loss
Fasting has gained in popularity as many people see it as a way to lose weight. “It can potentially help some people to lose weight in the short term,” admits Fiuza. “Although it doesn’t appear to be superior to other types of calorie-restricted diets for this purpose.” And ultimately to lose weight, you need to be in a calorie deficit.
A systematic review in peer-reviewed journal Canadian Family Physician found that in all 27 trials examined, intermittent fasting resulted in weight loss, ranging from 0.8% to 13.0% of baseline body weight.
But as with all extreme eating plans, there are some cons to consider, says Fiuza.
“There are potential side effects to fasting, but most usually subside over time. The main ones are lethargy, irritability and headaches, but there is also a risk of disordered eating for those with a predisposition to eating disorders.
“Prolonged fasting is much more intense than intermittent fasting and anyone considering it should speak to their healthcare professional beforehand. Intermittent fasting may also not be appropriate for everyone.
“People who are pregnant or have type 1 diabetes, suffer from an eating disorder or take medicines with food, as well as children and older adults, should avoid fasting.”
If fasting doesn’t sound appealing, check out our guide to the Mediterranean diet instead.
This article is for informational purposes only and is not meant to offer medical advice.
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Maddy has been a writer and editor for 25 years, and has worked for some of the UK's bestselling newspapers and women’s magazines, including Marie Claire, The Sunday Times and Women's Health. Maddy is also a fully qualified Level 3 Personal Trainer, specializing in helping busy women over 40 navigate menopause.