Intermittent Fasting: What You Need To Know

By Sinead Berry, Registered Nutritionist (MSc, mBANT, rCNHC)

By Sinead Berry, Registered Nutritionist (MSc, mBANT, rCNHC)

Intermittent fasting, time-restricted eating, 16:8, 5:2, the Fast Diet, whatever you want to call it, you’ve likely heard the buzz around the potential health benefits of fasting. Unlike traditional ‘diets’, intermittent fasting promises that by simply changing when you eat, instead of what you eat, you can achieve weight loss, reduced risk of disease and a longer lifespan. But does intermittent fasting actually live up to the hype? What exactly are the benefits and potential risks and what does the evidence say? 

What is fasting? 

Voluntary fasting has been practised across the globe for centuries and forms an important part of many religions and cultures. The latest buzz around fasting for health is focused on intermittent regimens, particularly for weight loss. ‘Intermittent’ basically means stopping and starting at intervals, and for fasting, this involves abstaining from food or drink (other than water) for a set period of time. 

What are the most popular ways to fast?

Three key types of fasting have emerged: whole day, alternate day and time-restricted eating. With whole day fasting, the fasting period is — you guessed it! — 24 hours. Alternate-day fasting, as the name suggests, involves alternating regular eating days with days where you consume a reduced amount of energy. But it’s time-restricted fasting that’s really having a moment right now. 

Time-restricted eating narrows the time that you’re eating, limiting it to a certain number of hours per day. For example, the popular 16:8 method involves fasting for 16 hours and then refuelling within an 8-hour window (e.g. having no calories between 7pm and 11am; water and calorie-free drinks such as black tea and coffee are allowed).

How exactly does fasting work?

When you eat food, any carbohydrates you’ve eaten will provide you with a supply of glucose, your body’s primary source of fuel. Blood sugar levels increase and higher levels of insulin are secreted — the hormone responsible for transporting glucose from the bloodstream to the cells to be used for energy. Any extra glucose is stored in the liver and muscles as glycogen. This ‘fed state’ lasts for a few hours, blood sugar and insulin decline and then your body switches to start breaking down glycogen into glucose for energy. 

Between 12 and 24 hours after eating, when glycogen stores have depleted, your body starts breaking down protein and fat stores for energy, which produces ketone bodies — a compound made in the liver from fatty acids. This stage is called ketosis and it’s when the body uses fat as its primary source of energy, although you may not actually reach this state through intermittent fasting. 

By giving your body a break from focusing on digesting food, intermittent fasting may also promote a process called autophagy, where the body degrades and removes damaged cells, which is essential for maintaining cell health. Some evidence suggests that autophagy may be linked with reduced inflammation and a decreased risk of some types of cancer and chronic diseases (1). 

Does intermittent fasting lead to weight loss?

While it’s still early days for research into intermittent fasting, there are studies showing that it may lead to short-term weight loss (2), although some studies concluded that weight loss was no greater than on a regular low calorie diet (3). This is likely because with intermittent fasting, rather than it being some sort of magic bullet, you’re simply consuming fewer calories overall. However, when it comes to weight loss, for some people fasting may have the opposite effect. When you skip meals, your body produces more of the hunger hormone ghrelin, which may lead to overeating when the fast is broken. So ensuring that you eat healthy balanced meals when refuelling is really important. 

What are the other health benefits of fasting?

It may improve heart health

Aside from weight loss, intermittent fasting has the potential to limit the risk of heart disease by reducing LDL (‘the ‘bad’) cholesterol, increasing HDL (the ‘good’) cholesterol (4), reducing inflammation and lowering blood pressure (5).

It supports better blood sugar management

Recent research has also shown that intermittent fasting (with a 6-hour window), reduces the risk of type 2 diabetes in men by improving insulin sensitivity and lowering blood sugar levels (6). However, there is some (although limited) research that found, with alternate day fasting, that women experienced an impaired response to blood sugar (7). 

It may improve brain health

Animal research shows that intermittent fasting may benefit brain health with the prevention of age-related diseases, such as stroke and Alzheimer’s (8).

It may increase lifespan

As well as improving the overall health of mice, alternate-day fasting resulted in a 13% increase in lifespan (9), although signs of ageing were not significantly reduced. 

It supports gut health

The impact of your body taking a break from digestion during the fasting window also seems to improve the diversity and number of beneficial bacteria in the gut. This has been shown to positively affect weight change and metabolism (10).

Is fasting safe?

Although intermittent fasting may be safe for healthy adults, there’s a shortage of large long-term studies in humans to determine the long-term effects of adopting this type of eating pattern. Possible downsides include reduced energy levels, low mood and irritability due to lack of food (11). Research has also found that intermittent fasters may not be consuming enough of certain nutrients, including calcium, zinc, magnesium and potassium (12). This form of intermittent eating may also be difficult to sustain, in particular as part of a lifestyle where enjoying food is very much a social occasion. The potentially negative impact on an individual’s relationship with food is also a possible trigger for disordered eating (13). 

Why is fasting different for men and women?

Given the physiological differences between men and women, it’s understandable that intermittent fasting may affect women differently to men. Many of the benefits of fasting happen due to the body’s response to the perceived short-term stress of not eating. In some instances, short-term stressors can make us more resilient to future challenges. But when your body is chronically stressed, high levels of the main stress hormone, cortisol may impact the balance of other hormones (14). 

The female body is particularly sensitive to stress. When under stress, the hypothalamus in the brain suppresses the production of the hormones needed for healthy ovulation and menstrual cycle, as well as metabolism, insulin sensitivity, body fat, bone formation and mood. Raised cortisol levels may cause a surge in insulin, leading to high blood sugar levels and weight gain (15). The timing of your menstrual cycle may also impact your body’s response to fasting — in the week leading up to your period, oestrogen levels drop and your sensitivity to cortisol increases. If you’re overly-stressed, anxious or already struggling with hormone-related issues (such as thyroid problems), your body may not be able to handle the additional stress of fasting. 

There are some circumstances in particular where intermittent fasting isn’t suitable:

1. If you’re pregnant, breastfeeding or trying to conceive

2. If you’re an older adult

3. If you have a chronic illness of existing health condition (such as, type 1 diabetes) and have to take medication at mealtimes with food 

4. If you have a history of eating disorders

What should you be eating between fasting? 

If you’re thinking that intermittent fasting could be for you, you’re probably wondering what exactly you should be eating when you do break your fast. As a general guide, the best foods to eat are the same as in a regular healthy diet — wholegrains, fruit, vegetables, legumes, nuts and seeds. Keep in mind that when you fast, you’re giving your body a break from digestion, so try to avoid jumping straight in with foods that are more difficult to digest (avoid the raw vegetables and opt instead for cooked). 

It’s important to refuel with nutrient-dense foods that will sustain your energy throughout the day. Rather than loading up on a highly processed carb-heavy first meal that may disrupt your blood sugar balance, choose something high in protein, such as tofu scramble, quinoa cashew porridge or a chocolatey peanut protein smoothie. A nourishing high-protein first meal will provide you with long-lasting energy to keep you feeling fuller for longer and avoid making less healthy choices later in the day. 

Fibre should also be a priority when refuelling as not only does it help with sustaining energy when combined with protein and healthy fats, high fibre foods are an excellent source of vitamins, minerals and food for the beneficial bacteria that form your gut microbiome. Think leafy greens, legumes, nuts and seeds; choose options such as a high protein tofu bolognese, a crispy chickpea & lentil pasta, or a roasted vegetable & butter bean stew for a nutritious, balanced way to fuel your body.   

The bottom line on fasting (so far)?

So although research into intermittent fasting is in its early stages, the potential benefits to weight loss, heart disease and type 2 diabetes make it an interesting area to follow. 

If you’re thinking of trying intermittent fasting, it’s important to make it work for your personal lifestyle and health goals, and always prioritise your mental wellbeing.

Given that fasting may affect women and men differently, easing in with small gradual changes may be the best approach for women. Start slowly to get your body used to your new eating pattern, trying different timings and make sure it’s realistic and achievable. When you do come to refuel, make sure you’re eating enough to give you the energy you need. Prioritise balanced, nutritious and enjoyable meals — and stay hydrated. 

Please note: if you’re considering any form of diet, please consult your doctor or healthcare provider first to ensure you can do so without risk to your health.

Sinéad Berry, Registered Nutritionist (MSc, mBANT, rCNHC)



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