Intermittent fasting and weight loss

Share This Post

Intermittent fasting seems to be everywhere at the moment.  My clients often ask me if they should follow this way of eating.  The reason for its growing popularity is that there are no specific foods to include or remove from your current diet.  It is all about adjusting when you eat your meals to follow a certain eating window [1].  Therefore, it’s attractive to many because you don’t have to count calories, weigh out food, cut out carbs, sugar or fat, so it seems the perfect option to lose weight.  But is it actually different to any other diet?

What is Intermittent Fasting (IF)?

This is an umbrella term and there are different versions that have emerged over the years.  Sometimes it is referred to as ‘chrononutrition’, which is an area of nutritional science that acknowledges the influence of meal timing on health outcomes, independent of the quality or quantity of food eaten.

The 5:2 diet.

This is where you eat normally on 5 days of the week and on the other two (non-consecutive) days you limit your intake to 500-600 calories per day [3].  Whilst this may work for some people, this particular pattern is generally found to be quite challenging to stick to as it really limits your activities on those fasting days.  Sometimes unexpected plans come up, and no diet should be stopping you enjoying your food or life.

Alternate Day Fasting.

This variation involves eating what you like one day, fasting (or eating up to 500 calories) for 24-hours the next. 

The 16:8 diet or Time Restricted Eating (TRE).

This version varies from the 5:2 as it refers to the same day rather than the week.  The aim here is to fast for 16 hours (including overnight) and then consume your meals during an 8-hour window.  Depending on your schedule and lifestyle, you can choose your 8-hour eating window to suit you.  For example, popular eating windows are between 12pm-8pm or 8am-4pm.  The 16:8 is popular because essentially you are skipping one meal (usually breakfast) and then you have a fairly normal day of eating, so it tends to fit in well with many lifestyles.

Other Ways to Fast.

More recently, there has been research around the fasting window and what could be beneficial.  As a result, there is some evidence to suggest that any fasting window such as 12 hours overnight is enough to yield some health benefits, including weight loss [4].  In light of this, there are other versions of the 16:8 including the 18:6, the 14:10 and the 12:12.

They are all designed to restrict your overall food intake to achieve a steady weight loss over time, but you may find you lose more in the first few weeks as your body loses fluid and adjusts [2]. 

Are there Health Benefits with Intermittent Fasting?

Intermittent fasting has grown in popularity as an alternative way to encourage weight loss.  Some research has taken place, which suggests intermittent fasting can reduce blood pressure, blood sugar, insulin resistance and cholesterol [5,6]. It’s worth noting that many of the studies taken place are in animals.  The human studies that have occurred are fairly small, of short duration and of mixed results, so we do not have enough reliable information to recommend it to all.  As with all these different approaches, we need to consider the pros, cons, and our own personal circumstances.

The Side Effects of Intermittent Fasting.

As with any diet with the intention of weight loss, there are certain risks involved that are worth considering.  Whichever approach is taken, ultimately there is a restriction in calories.  Your body needs to adjust to this new way of eating, which may result in common, short-term symptoms such as brain fog, headaches, dizziness, feeling tired, mood swings, constipation and hunger pangs [7,8].  Long-term effects of this way of eating are not yet fully understood.

It’s not just physical side effects, following any sort of plan can lead to stress.  Especially when it doesn’t fit with your work or social life patterns or fit with other family members and their eating habits.  Fasting itself, creates stress on the body and depending on the level of stress experienced in your life, this could hinder weight loss over time.  Stress can also be a trigger for cravings and periods of overeating, creating an ‘all or nothing’ approach to weight loss.  This way of eating, then just becomes another restrictive diet.

Is There a Best Way to Fast?

There is no right or wrong way to do this and there are so many variations out there.  It is worth noting, that pursuing weight loss at all costs, doesn’t automatically equal good health. 

Whilst it is true that you can eat what you like whilst fasting, some foods may not be optimal to consume during your eating window.  You’re probably making a change, to improve your health and maybe to lose weight, therefore, it makes sense to nourish your body and make the meals and snacks you do eat really count.  By this we mean try to focus on nutrient dense foods and well-balanced meals, so that you are not missing out on vital nutrition.  However, there is still a lot of research needed on how best to optimise food during the eating window [9]. 

Who Should Not Fast.

If you are currently pregnant, breastfeeding, under the age of 18, or have any health condition where restricting food should not occur, fasting is not for you.

If you take medications and supplements that need to be taken with food, then this is something to check to see whether you can alter the timings of these tablets to fit with a shorter eating window.  Some medications taken on an empty stomach can cause nasty side effects [10] – always double check with your GP if unsure.

If you have a history of an eating disorder or struggle with disordered eating patterns, then following a restrictive diet, such as intermittent fasting is not recommended.  There is some evidence that those who follow an earlier eating window during the day, may be triggered to experience overeating in the evening.  This then defeats the object and worsens people’s relationship with food.

Key Things to Remember.

If you decide to try this way of eating, there are some things to bear in mind:

  • Assess your current eating habits – write down everything you eat and drink for a day.  Look critically at how adjusting this will fit with your daily routine.  Start simple, maybe the 12:12 approach and check in with how you feel and how it works for you. You can then adjust over time.
  • Stay hydrated – during your fasting window, make sure you drink enough water as water does NOT break the fast.
  • If you are exercising that day, try to make sure that you do this close to or within your eating window so that you can properly refuel after your workout.  Or choose a less intense form of activity.
  • Adjusting to periods of no eating can be harder on some people than others, if you feel faint, nauseous or unwell at any point, have something to eat and reconsider your fasting window for next time.
  • Whilst you may notice some weight loss initially, after a month or so following this way of eating your weight loss will slow down or even plateau as your body adjusts to this eating pattern.

Bottom line

If you have tried every so-called ‘weight loss miracle’ diet or never quite found a way of eating that works well for you and your goals, you may be tempted to try intermittent fasting.  Especially, as there are some potential health benefits which go beyond weight loss.  

This way of eating will work for some but as will with all weight loss approaches, it doesn’t address the reasons why we eat.  If this is something that you struggle with, then seeking support to manage your relationship with food can help you reach your long-term goals. 

For more information on the above or for ways on how to work with me get in touch, I’d love to hear from you.

  1. Regmi P, Heilbronn LK. Time-Restricted Eating: Benefits, Mechanisms, and Challenges in Translation. iScience. 2020 Jun 26;23(6):101161
  2. Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Cardiometabolic Benefits of Intermittent Fasting. Annu Rev Nutr. 2021 Oct 11;41:333-361
  3. Healthline, 2022. Beginner’s Guide to the 5:2 Diet. Accessed 16th April 2023, Online, available at:
  4. Regmi P, Heilbronn LK. Time-Restricted Eating: Benefits, Mechanisms, and Challenges in Translation. iScience. 2020 Jun 26;23(6):101161.
  5. Jamshed H, Steger FL, Bryan DR, Richman JS, Warriner AH, Hanick CJ, Martin CK, Salvy SJ, Peterson CM. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med. 2022 Sep 1;182(9):953-962
  6. Vasim I, Majeed CN, DeBoer MD. Intermittent Fasting and Metabolic Health. Nutrients. 2022 Jan 31;14(3):631
  7. Shalabi H, Hassan AS 4th, Al-Zahrani FA, Alarbeidi AH, Mesawa M, Rizk H, Aljubayri AA. Intermittent Fasting: Benefits, Side Effects, Quality of Life, and Knowledge of the Saudi Population. Cureus. 2023 Feb 7;15(2):e34722.
  8. Wang Y, Wu R. The Effect of Fasting on Human Metabolism and Psychological Health. Dis Markers. 2022 Jan 5;2022:5653739
  9. Parr EB, Devlin BL, Hawley JA. Perspective: Time-Restricted Eating-Integrating the What with the When. Adv Nutr. 2022 Jun 1;13(3):699-711
  10. Simple, 2021. Can you fast while taking medications? Accessed 16th April 2023. Online, available at:

More to Explore

What is a Dietitian?

Dietitians are experts in the complex science of food and nutrition. They are the only qualified and legally regulated nutrition professionals. Their role is to assess, diagnose and treat dietary and nutritional problems using scientific evidence.

Read more

What is a non-diet approach?

Over the last few years, the way that we approach weight loss and diets has started to change as more and more evidence has emerged on how fad diets and quick fix weight loss gimmicks don’t actually work at all, long-term.

Read more

Menopause: Is weight gain inevitable?

Many women I work with have struggled with their weight and relationship with food for many years. When we reach our time for the menopause these struggles can be amplified and it can be really disheartening to feel your body changing, and not necessarily in the way you’d like.

Read more