What is fasting

Fasting is a form of calorie restriction. We have naturally fasted for millenia due to our previous nomadic lifestyles and reliance on seasonal foods so our bodies are adapted to cope with fasting. 

Fasting is also used as a part of religious practice as a form of spiritual discipline.  It has only been recently that fasting has been popular for health reasons. In fact some of the early studies on fasting were conducted on those who fasted for religious reasons and health benefits in particular reduced body mass and blood fats and cholesterol.

People in the Blue Zones (areas of the world where there is a higher percentage of people who are over 100 years old and thriving) all fast in one way or another. 

There are different types of fasting  – some people fast for days without any food – just water, some people fast with only soups and broths, some give up red meat and refined carbohydrates, some people just skip specific meals and then there is Intermittent Fasting which is where you cycle between fasting and eating. 

There are also varying ways to intermittently fast – the most popular is eating within a an 8 hour window and fasting overnight for 16 hours. This is also known as time restricted eating because you are restricting your eating to a specific time frame – anywhere between 8 and 11 hours has shown benefits.   My personal favourite is eating within 10 hours and fasting overnight for 14 – which for me is more doable. I have a later breakfast/brunch at about 10am  and have my dinner before 8pm.

Why is it good for us?

  • Studies have shown it can promote significant weight loss inc. belly fat without losing muscle
  • May reduce insulin resistance
  • May reduces inflammation
  • Helps reduce risk factors for heart disease (blood fats, insulin, glucose)
  • May prevent cancer
  • May protect from dementia and Alzheimer’s
  • May promote longevity by ‘switching on’ specific genes
  • Can increase energy and productivity by supporting mitochondria function and production

How does it work?

  • It appears to increase human growth hormone (which helps to increase fat loss and muscle gain),
  • improves sensitivity to insulin,
  • improves cell function (programmed death and repair), – changes gene expression (ageing and protection against diseases)
  • Time Restricted Eating in particular has been shown to improve liver function, and reduces blood fat and improves the gut microbiome diversity and gut function. This form of fasting is also thought to switch on circadian genes that are specifically responsible for increasing our metabolism during daylight hours. 
  • by putting the body under stress it adapts and becomes more resistant
  • ensures stores of fat are used for energy due to calorie restriction

Health Benefits

Unfortunately these are still largely based on animal studies!!!   One study showed that rats lived up to 83% longer than rats who did not fast. Over the last 15 years has seen research grow and include human studies on weight loss, gut health, brain health, diabetes and cardiovascular health.

One very recent study suggested that Time Restricted Eating was no better than caloric restriction when it came to weight loss. This was a long term human study analysing the effects of these type of diets on 139 people with obesity. Researchers foud that both groups lost weight. The media reported that time restricted eating was therefore unnecessary for weight loss. Unfortunately researchers did not look at those other parameters of health including the gut microbiome, mitochondrial function or whether the weight lost was actually muscle or fat.


Is it safe? 

Yes but… I would recommend you always check with your health practitioner first (GP, nutritionist, naturopath) to see if it is safe for you to fast. 


Who should not fast?

  • Pregnant or breastfeeding women due to higher caloric and nutritional needs. 
  • Women trying to conceive.
  • Non-menopausal women experiencing amenorrhea (not menstruating)
  • If you are a woman??? – hormonal imbalances, anaemia, reproductive conditions can all be affected by fasting, so again, please check with your health practitioner first.  In some animal studies, female rats became infertile, masculinised, and in some instances their ovaries shrank. This is still an area that needs more research. Calorie restriction can disrupt reproductive hormones which then disrupts ovulation causing irregular cycles or lack of menses). It is more of a problem if you are already lean or underweight. Skipping breakfast can exacerbate anaemia in women (particularly athletes). 
  • Some evidence that intermittent fasting worsened blood sugar control in women compared to men, although testosterone has been shown to be reduced in men.
  • Anyone underweight
  • Anyone who has a history or are currently experiencing an eating disorder
  • Those experiencing extreme stress/burnout (fasting can put more stress on the body – you need nourishing and nurturing). 
  • Children who have higher caloric needs
  • Elderly
  • Athletes (high energy needs) – check 1st with sports nutritionist
  • If you are on medication (check with doctor 1st) as fasting can be contraindicated in some cases. 
  • If you have a medical condition (diabetes, low blood pressure, cardiovascular disease, anaemia – must check with doctor 1st) and be monitored. 

What do I need to know?

  1. Intermittent Fasting does not specify WHAT to eat just when to eat – so is really just one piece of a bigger puzzle. If you eat crap during your non-fasting days or times you will put on weight and you will not see the health benefits 
  2. It is really important to focus on what you eat as much as when you eat:
    • You still need 20-30g protein per meal or 90g protein/day·
    • Fibre is important – veggies, fruit, wholegrains, legumes – constipation can be common. 
    • Stay hydrated by drinking sufficient water.
    • Eat a Mediterranean type diet – 5-7 servings of veggies, 2 servings of fruit, whole grains, nuts, seeds, legumes, healthy fats (avocado, olive oil, fish, nuts, and seeds), and lots of herbs and asian spices.
    • Be careful with “bulletproof” coffee – replaces breakfast – calorie-dense not necessary nutrient dense.
    • Don’t overcompensate – if you miss a meal or fast on a day don’t binge on crap on your eating days.
  3. Get support – check first- start slowly – skip a meal…make sure that your next 2 meals are super nutrient-dense.
  4. Be realistic – plan ahead but be flexible
  5. Use an app to check nutrients and calories
  6. Stop fasting if your period’s stops
  7. If in doubt please get advice.  I can check to see if what you eat in your ‘eating window’ is nutrient dense. Book a call to see how I can help. Links below. 


  1. The Circadian Code by Satchin Panda (TRE /16:8)    https://www.google.co.nz/books/edition/The_Circadian_Code/sb5jDwAAQBAjhl=en&gbpv=1&printsec=frontcover
  2. What the Fast by Professor Grant Schofield (missing breakfast & Lunch on Mon/Tues)     https://whatthefatbook.com/product/what-the-fast/
  3. The Fast 800 by Dr. Michael Mosley  https://thefast800.com/ (5 days of eating : 2 days of fasting)


  1. https://nutritionj.biomedcentral.com/articles/10.1186/1475-2891-9-57
  2. https://www.nejm.org/doi/full/10.1056/NEJMoa2114833
  3. https://www.frontiersin.org/articles/10.3389/fncel.2019.00196/full
  4. https://www.tandfonline.com/doi/full/10.1080/10942912.2018.1560312
  5. https://www.sciencedirect.com/science/article/abs/pii/S193152441400200X
  6. https://www.cell.com/cell-metabolism/fulltext/S1550-4131(15)00224-7

There is a YouTube video of this blog. Head over to Sarah’s You Tube Channel if you are interested in watching rather than reading.

If you want to get to know me or work with me then go to the links page  https://sarahbrenchleynaturopathy.com/links/ to find out more or book a free 30 minute call for a chat

Email me at sarahb@sarahbrenchleynaturopathy.com 

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