Fasting has great health benefits for your body, so in short, yes you should try fasting! Read on to see if it’s for you.
When you eat, your body builds up energy stores of glycogen within your muscles and liver. From there it’s all primed and ready to be used for energy (although you use glycogen, fat and protein for energy at all times, glycogen is the favoured fuel source for your body because it’s easier to convert into energy). But, during a long period of time without eating this changes. As you fast the levels of glycogen are drained like a battery and at around 12+ hours you switch to using primarily ketones for energy, which come from your fat stores.
This switch between energy stores has many health benefits including:
- Increased insulin sensitivity
- Weight loss
- Improved cholesterol levels
- Reduction of inflammation (1) resulting in disease prevention
- Anti-ageing through ramped-up autophagy (2) (replacing old cells for new)
- Improved memory, alertness and cognition
The time at which this switch occurs is about 12 hours or more, and this is why fasts are usually based on food restriction for a minimum of 12 hours. This time frame is unique from person to person and depends on a couple of components:
- How much food was consumed just before the fast began
- How much physical activity you engaged in during the fast
From an evolutionary standpoint, If you could survive well and function optimally without food for long periods of time, you would indeed thrive and go on to live another day. On the other hand if you became hangry and light-headed every time you missed a meal, you’d probably have trouble keeping up with the rest! Feeling focused and undisturbed when fasting is a good sign of metabolic flexibility between glucose and ketones and would ensure your survival if you were living out in the wild (3).
Brain health is everything, and as you know, fasting is excellent for this. BDNF (brain-derived neurotrophic factor) ramps up which helps to build new brain cells in a process called neurogenesis (4) . Exercise also increases BDNF levels and so to exercise in a fasted state would do wonders for your brain (5)! Other benefits of BDNF are:
- Reduction in depression (6)
- Improved cell quality (2)
- Preservation of brain cells
So how do you choose how long to fast for? A good way to begin is with time restricted feeding. This means to eat in a preplanned window of time in the day. For example, fast:eat:
- 12:12
- 16:8
- 18:6
Time restricted feeding can be done daily and your body will soon adapt and your metabolic function will begin to optimise (7). 16:8 is a personal favourite of mine because I can still comfortably eat 3 meals within the 8 hour window.
Once comfortable with time restricted feeding you can move on to 24 hour fasts which can be done once per week or once per fortnight depending on how you feel. You can then experiment with longer fasts from there.
Any fast that exceeds 24 hours requires extra preparation in regard to how you’ll break it. From around 36 hours your body begins to adapt to the abstinence of food by reducing digestive activity. So, the first food you eat should be easily digestible such as a soup or salad. Avoid inflammatory foods such as dairy, sugar and gluten as well as high fat foods such as red meat and nut butters.
Fasting is not for everyone. Even though it’s beneficial for our health it’s still indeed considered a stress. It’s a catabolic time meaning your body begins to break down structures in order to survive. Therefore if you require healing or growth then fasting should be avoided. The following groups should stay away:
- Pregnant or breastfeeding
- Recovering from trauma or surgery
- Children
- Difficulties in diabetes management
- A recent history of an eating disorder
Whatever fast you choose, take it slow. Hunger is normal but if you feel dizzy or unwell then break the fast and try again another time.
References
1. Anton, S.D. et al. (2017). ‘Flipping the metabolic switch: understanding and applying health benefits of fasting’, Obesity (Silver Sping), 26(2), pp. 254-268.
2. Stockman, M.C. et al. (2019). ‘Intermittent fasting: is the wait worth the weight?’, Curr Obes Rep, 7(2), pp.172-185.
3. Walker, A.K. (2010). ‘Conserved role of SIRT1 orthologs in fasting dependent inhibition of the lipid/cholesterol regulator SREBP’, 24(13), pp.1403-1417.
4. Baik, S.H. et al. (2020). ‘Intermittent fasting increases adult hippocampal neurogenesis’, Brain and Behaviour, 10(1), pp. 1444.
5. Mattson, M.P. et al. (2018). ‘Intermittent metabolic switching, neuroplasticity and brain health’, Nat Rev Neurosci, 19(2), pp. 62-80.
6. Pillai, A. et al. (2012). ‘Plasma BDNF levels vary in relation to body weight in females’, PLoS One, 7(7).
7. Patterson, R.E. et al. (2015). ‘Intermittent fasting and human metabolic health’, J Acad Nutr Diet, 115(8), pp:1203-1212.
