There are several types of intermittent fast, each with their own benefits and drawbacks.
In this article we cover the different types, their potential health benefits, and the common mistakes people make when fasting.
Important: This information is of a general nature and should not replace diagnosis, advice or treatment from your doctor or other healthcare/wellbeing professionals. For any individual concerns related to your health, diet or general wellbeing, or if you are considering changing your diet, supplements, medication or other health routines, please see a GP, pharmacist or other suitably qualified professional.
How do you do intermittent fasting?
An intermittent fast means you eat within a set amount of time and then limit or avoid food until the next eating window. Unlike typical diets that focus on calorie counting, intermittent fasting focuses more on when you eat rather than what you eat.
What hours are best for intermittent fasting?
The best hours for intermittent fasting are the ones that suit your health and lifestyle. Some people enjoy the 5:2 diet while others prefer alternate day fasting. If you’re a busy working parent, a 24-hour fast may be too stressful but a 16:8 or 14:10 plan could neatly fit your routine.
There’s no one-size fits all approach, and it’s fine to tailor the fast to your personal circumstances. Let’s look at the different types of intermittent fasting in more depth.
5:2 diets
On the 5:2 diet you limit your food intake to 500-600 calories for two separate days a week and eat a healthy, balanced diet the rest of the time. It’s a more flexible approach than alternate day fasting (see below) and you can choose which days to fast according to your schedule.
16:8 plan
The 16:8 plan is one of several Time Restricted Eating (TRE) plans. You eat your meals in an 8-hour eating window then fast for the remaining 16 hours of the day. For example, if you eat breakfast at 8am, your final meal of the day would be at 4pm so there’s a 16-hour fast before breakfast the next day.
During the fast you can have water, black coffee, black teas or herbal tea, but no food, milky drinks or soft drinks.
Other TRE plans include 14:10 (14 hour fast, 10 hour eating window) and 12:12, also known as an overnight fast. If you’re new to fasting, a TRE fast is perhaps the easiest one to start with. You can fast overnight and enjoy regular meals in the day as part of your daily routine.
Alternate day fasting
Rather than eating and fasting in the same day, alternate day fasting cycles between eating normally for one day and limiting food to around 500 calories or less on the next. It can be hard to maintain long-term and may be too extreme if you have a busy, demanding lifestyle.
24-hour fast
A 24-hour fast involves having no or very little light food such as fruit, for an entire day once a week. It’s important to include plenty of water and herbal teas, and to limit your activity while doing this kind of fast.
Intermittent fasting at a glance
Comparing types of intermittent fasting | |||
|---|---|---|---|
Fasting method | How it works | Best suited to | Main drawbacks / cautions |
5:2 diet | Eat a healthy, balanced diet on 5 days of the week and reduce intake on 2 days. | People who want flexibility and prefer not to fast every day. | Can trigger hunger or overeating on non-fasting days if meals are not planned well. It may be unsuitable for people who need regular food intake. |
16:8 plan (time-restricted eating) | Eat all meals within an 8-hour window and fast for the remaining 16 hours. Other TRE plans include 14:10 and 12:12 | Beginners and people who want a simple routine that fits daily life. | If you’re new to fasting, start with a 12:12 overnight fast before extending the fast any longer. |
Alternate day fasting | Alternate between a normal eating day and a fasting or very low-calorie day. | People who already have experience with fasting and can tolerate longer gaps between normal eating days. | Harder to maintain long-term and more likely to interfere with energy levels, exercise, work and social life. |
24-hour fast | Fast for a full day once a week or occasionally, with no food or only very light food. | People who prefer less frequent but longer fasts, and who can reduce activity on the fasting day. | May feel more intense than shorter fasting methods and can be difficult for beginners. Hydration is important, and this approach may not be practical on busy or physically demanding days. |
What can you eat and drink during a fast?
Most fasts encourage you to have no food at all. This can be hard to maintain, especially for longer periods like the alternate day fast or the 24-hour fast.
If you want to eat during a fast, opt for light foods like fruit or vegetable broth that are easy to digest. Be sure to stay well hydrated with water, herbal teas, black teas or black coffee.
Is intermittent fasting healthy?
When managed carefully, intermittent fasting can be safe and healthy. Research shows it can lower blood pressure, aid sleep, reduce inflammation, and improve cholesterol, triglyceride, glucose and insulin levels. It can also aid weight loss, although some studies show no additional weight loss benefits when compared to a calorie-controlled diet. However, the metabolic benefits can occur even without any weight loss.
Fasting for longer than 12 hours upregulates a process called autophagy in the body. This is when cells break down and recycle their old components so they can function more efficiently. Autophagy may play a key role in how we prevent and respond to disease, and research in this area is ongoing.
Intermittent fasting for women – is it different?
Yes, women often respond differently to fasting than men. For years, men’s health research has been extrapolated to women, but scientists now understand that women’s hormonal shifts can significantly influence their response to fasting.
The rise and fall of oestrogen and progesterone over a monthly cycle affects everything from appetite and digestion to sleep and energy. Women are often more resilient to the physical stress of fasting in the first half of their cycle leading up to ovulation, and more sensitive to it in the second half, before their period. During perimenopause and menopause, longer fasts can aggravate hormonal symptoms.
A 12-14 hour overnight fast with a 7am – 7pm or 9am – 7pm eating window is widely recommended for women. This type of 12:12 or 14:10 plan means you can fuel properly during the day to support hormone function and still get the benefits of fasting while you sleep.
Does intermittent fasting lead to weight loss?
Yes, intermittent fasting can help you lose weight but only if you’re mindful of what you eat when you’re not fasting. Because fasting can make you feel hungry, it’s important not to overeat when the fast ends. Healthy food choices after each fast will support better weight loss outcomes than if you eat high-sugar, highly processed foods.
Is intermittent fasting safe?
Most healthy adults can safely practice a 12-14 hour overnight fast without any difficulty. Longer fasts and intermittent fasts are not recommended for:
Anyone with an eating disorder or disordered eating.
Type 1 diabetics.
Young children and teenagers.
People with underactive or overactive thyroid, chronic kidney disease, heart disease, chronic fatigue syndrome, or immunodeficiency.
Pregnancy and breastfeeding.
Those who are underweight or at risk of becoming underweight.
People taking medications that require regular food intake.
In the right circumstances, fasting can be safe and effective. If you have a pre-existing health condition or feel uncertain about fasting, speak to a suitably qualified healthcare professional.
How to start intermittent fasting
If you’re new to intermittent fasting, start with a simple 12-hour overnight fast. If you normally eat breakfast at 8am, make sure you’ve finished your evening meal by 8pm. Once you’re comfortable with this, bring your evening meal forward to 6pm and extend the fast to 14-hours.
If an overnight fast doesn’t work for you, try the 5:2 plan instead. Choose light foods like vegetable broth or fruit for the two fasting days and eat a healthy balanced diet for the rest of the week.
Common mistakes people make with intermittent fasting
Here are the most common mistakes people make when starting fasting:
Jumping straight into a 24-hour fast or alternate day fasting. These fasts can be hard to maintain, and as a beginner you’re likely to feel tired, irritable, and lethargic.
Not hydrating enough. It’s important to keep your fluids levels up if you’re doing a daytime fast.
Eating too much after the fast ends. Don’t undo all your hard work! Plan healthy meals that support your energy, weight balance and wellbeing.
Not planning the fast properly. Always plan an intermittent fast around your lifestyle, not the other way around. If you have a busy job, lots of daytime activities, or caring responsibilities, choose a simple overnight fast that still lets you fuel properly during the day when you need it most.
Ignoring your body’s cues. There are times when you’ll feel unwell or simply too tired to fast. Listen to what your body needs and adjust your fasting plans accordingly.
What else should I consider?
Before starting an intermittent fast, make sure you’ve chosen a plan that suits your health and lifestyle, and decide in advance which foods you’ll eat once the fast is finished.
If the fast feels too long to begin with, make it shorter until you feel comfortable with it. Build your meals around vegetables, wholegrains, healthy fats, and protein, and limit sugary foods to the occasional treat.
FAQs
Can I exercise while intermittent fasting?
You can exercise as normal during a TRE plan like 14:10 but during a longer daytime fast such as a 24-hour fast, it’s advisable to rest as much as possible.
Will intermittent fasting slow down my metabolism?
No, done properly intermittent fasting supports a healthy metabolism and can improve metabolic markers such as glucose and insulin levels.
How long does it take to see results from intermittent fasting?
This varies widely, depending on the type of fasting and your individual health needs. Studies have shown changes in mood, energy and metabolic health within 4-12 weeks of starting an intermittent fast.
Can intermittent fasting affect your hormones?
Yes, done properly, intermittent fasting can positively affect insulin levels and insulin sensitivity. However, longer fasts can be stressful for the body and increase stress hormones like cortisol.
The effects on reproductive hormones vary, according to your sex, weight, and health. There’s very little research in this area which makes it hard to draw firm conclusions.
Is intermittent fasting suitable for older adults?
Intermittent fasting can be suitable for older adults although there are some points to note:
Older adults may be more prone to side effects like dehydration and light-headedness when changing their eating habits.
Many older adults naturally have a smaller appetite so make sure that meals are providing enough nutrition.
Always check with your doctor first before fasting while taking any form of medication.
Done carefully, intermittent fasting can be as beneficial for older people as it is for younger adults.
What's the difference between intermittent fasting and a calorie-controlled diet?
Intermittent fasting focuses on when you eat rather than what you eat. A calorie-controlled diet shapes meals around their calorie content and you can eat whenever you choose.
