What is intermittent fasting?
We’ve all heard of the term intermittent fasting, but what actually is it and how does it work? Despite climbing popularity in the use of intermittent fasting, it’s actually something that has potentially been around for years, way before our time. This is due to rolling back time where food was less accessible, and people ate when they could, not necessarily when they wanted (1).
Intermittent fasting is essentially the term that encompasses a variety of meal time scheduling. There are many health claims that intermittent fasting can aid weight loss and improve metabolic health (2). Despite different types of intermittent fasting (The 16/8 Diet, the 5:2 Diet, Alternative Day fasting etc.), the fundamental principles remain the same of cycling the times you eat to achieve weight loss and which cycle you choose is totally down to personal preference!
The aim of fasting is to control the times at which you consume meals, and this naturally helps calorie control. For intermittent fasting to be effective, you need to ensure you’re creating a calorie deficit. In other words, you’re burning out more than you’re putting in.
Is it okay to feel hungry?
Discussions around intermittent fasting often probe the question as to whether this type of plan will increase the feeling of hunger due to those windows of fasting. Naturally, adapting to this new regime might offset you a little, particularly if you’re used to eating during those times you’re now fasting. However, the hormone which triggers appetite, ghrelin, will rise during those times you’d usually eat and therefore it can take a little time to ‘retrain’ this hormone around your new eating times (3). Therefore, the longer you stick to the plan, the easier it should get, as your body adapts to the new expected times for consuming foods.
Weight loss – Many people take on intermittent fasting as a means of losing weight, backed up by many studies demonstrating the powerful results that intermittent fasting can have on weight loss, in particular, that stubborn tummy fat you’ve been struggling to lose!
Insulin levels versus human growth hormone – Whilst insulin levels in your blood decrease, growth hormone increases when you fast. Both of which contribute to fat burning (4).
Metabolic rate – Physiological changes that occur during fasting lead to an increase in your metabolic rate by 3.6-14% (5). This means you can burn extra calories!
Brain – Research shows that with fasting, we can increase the production and growth of new nerve cells, even those that help our brain function! (6)
Timing is key
Research has shown that restricting the eating window length of time, can have highly positive effects on insulin sensitivity, decreasing blood pressure and even reducing appetite. For example, having an 8-hour eating window (7am to 3pm) as opposed to a 12-hour eating window (7am to 7pm) (7). The ideal eating window would be starting early in the morning, to avoid eating late into the night.
Things to consider when fasting
Can I still exercise?
Absolutely. The benefits of exercise are extensively researched and exercising during intermittent fasting is no exception. In addition to the physiological and psychological benefits of exercise, it also contributes to the underlying goal of intermittent fasting – to burn out more calories than what you consume and help create that calorie deficit (8).
Take home message
Listen to your body! Your body will let you know what works for you and what doesn’t. The key is choosing a plan that works for you, makes you feel good and helps you reach those goals. This plan is a very popular choice for those seeking weight loss, although it’s important to remember it might not be suitable for everyone. For example, if you’re pregnant or breastfeeding, intermittent fasting is likely not to be the optimal choice due to restrictions on eating times.
1) Mattson, M. P., Longo, V. D., & Harvie, M. (2017). Impact of intermittent fasting on health and disease processes. Ageing research reviews, 39, 46-58.
2) Klempel, M. C., Kroeger, C. M., Bhutani, S., Trepanowski, J. F., & Varady, K. A. (2012). Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutrition journal, 11(1), 98.
3) Higgins, S. C., Gueorguiev, M., & Korbonits, M. (2007). Ghrelin, the peripheral hunger hormone. Annals of medicine, 39(2), 116-136.
4) Ho, K. Y., Veldhuis, J. D., Johnson, M. L., Furlanetto, R., Evans, W. S., Alberti, K. G., & Thorner, M. O. (1998). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. The Journal of clinical investigation, 81(4), 968-975.
5) Zauner, C., Schneeweiss, B., Kranz, A., Madl, C., Ratheiser, K., Kramer, L., . . . Lenz, K. (2000). Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. The American journal of clinical nutrition, 71(6), 1511-1515.
6) Martin, B., Mattson, M. P., & Maudsley, S. (2006). Caloric restriction and intermittent fasting: Two potential diets for successful brain aging. Ageing research reviews, 332-353.
7) Sutton, E. F., Beyl, R., Early, K. S., Cefalu, W. T., Ravussin, E., & Peterson, C. M. (2018). Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell metabolism, 1212-1221.
8) Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q. F., Battaglia, G., . . . Paoli, A. (2016). Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q. F., Battaglia, G., … & Paoli, A. (2016). Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk. Journal of translational medicine, 14(1), 290.