Intermittent fasting has been a buzzword for some time and numerous studies* show its positive impact on weight management, gut health, inflammation and various health conditions (*see further reading). As a PT, I see many clients who want to lose weight, and many with digestive concerns. Women are more prone to both weight gain and digestive issues, either caused by or made worse by our complex hormones, particularly in midlife. And for the record, when we speak about weight loss, we talk about fat loss specifically. When aiming for weight loss, we don't want to lose muscle mass, as the leaner we are, the better your metabolism (in simple terms)!
What is intermittent fasting?
Intermittent fasting can take shape as time-restricted feeding - e.g. 16/8 where you fast for 16 hours including overnight and eat within an 8-hour window (or a different fast/eat window), alternate day fasting, where you restrict your calorie intake to 500-800 calories every other day, and the 5:2 version, where you eat normally 5 days a week and have two days of 500-800 calories. I'm going to focus on time-restricted feeding, as I personally think this is a more manageable approach, for the simple fact that we are sleeping for large part of the fasting window.
With time-restricted feeding, you simply reduce the hours between your first and last meal of the day (including liquids, with the exception of water, black tea and coffee). This can be 16/8, 14/10, 20/4. 14/10 - a 14 hour fast and a 10-hour eating window, is a good place to start if you want to give it a go, and this is that the big IF Study by ZOE is testing at the moment.
Intermittent fasting and fat loss
There needs to be a calorie deficit to lose weight, and by restricting the number of hours we eat, we reduce our calorie intake in the process. It helps us snack less in the evenings and we probably have one less meal on a daily basis if breakfast and lunch turns into one meal. When we go a long time without food, we start to use stored sugars (glycogen) for energy, followed by tapping into stored fats for energy. Studies have shown that intermittent fasting reduces fat mass without reducing muscle mass, as long as protein intake remains high, though the research is not conclusive. However, if weight loss is the goal, this speaks for itself: A systematic review of 40 studies found that intermittent fasting was effective for weight loss, with a typical loss of 7-11 pounds (3-5kg) over 10 weeks. (Harvard.edu, link below).
Intermittent fasting and gut health
As we learn more about the gut microbiota, it seems that longer periods of fasting improves the balance of 'good' and 'bad' bacteria and reduces inflammation. This in itself has a plethora of health benefits, importantly reducing the risk of chronic illnesses, such as Type 2 diabetes and heart conditions. A longer fasting window allows your digestive system sufficient time to do its work, followed by a much-needed rest to restore itself. It seems that many bacteria follow our circadian rhythm and the microbiota is largely renewed within a 24-hour window, which means that it doesn't take long for the digestive system to be in a much better place. Our gut health is closely connected to what we eat and drink - we need to feed the good bacteria fibre, fermented foods and a good variety of nutrients (think: Eat The Rainbow) and reduce what the bad bacteria thrives on, i.e. sugar and highly processed foods.
Keen to give it a try? Just be aware that intermittent fasting is not advised in pregnancy and during breastfeeding, nor for those with diabetes or a history of disordered eating. It may help you lose weight and/or improve digestive issues. Start off by reducing your eating window to 12, then 10, then 8 hours over a few weeks to see what is a sustainable option for you. The easiest way to do the 16/8 fasting, is to have an earlier dinner and push your breakfast out in the morning, e.g. no food or snack after a 6pm dinner and breakfast at 10am the next day. And as with everything else fitness/nutrition-related, try a few different fasting options to get the right one for you. It also takes time to see results, patience is a virtue.
Further reading:
Why Intermittent Fasting Is Powerful For Weight Loss: https://www.dietvsdisease.org/intermittent-fasting-is-powerful-for-weight-loss/
Intermittent fasting and human metabolic health: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4516560/
Effects of Intermittent Fasting on Health, Aging, and Disease: https://www.nejm.org/doi/full/10.1056/nejmra1905136
Can intermittent fasting improve your gut health?: https://joinzoe.com/learn/intermittent-fasting-gut-health