Charlotte Faure Green used the 5:2 diet to keep her weight down, but when her periods became irregular and her mood erratic, she took a serious look at her eating patterns. Now a nutritionist, this is what she wants you to know about when to fast and when to take the pressure off

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Whether it's  5:2,  16:8 or any other form of  time-restricted eating,   intermittent fasting  (IF) has become one of the most popular ways to keep weight down, increase fat loss, lower  cholesterol,  improve  blood pressure,  insulin sensitivity and even  gut health . Indeed certain gut bugs can only do their vital clean-up work in the absence of food. Some biohackers even swear by  fasted training  as the best way to burn fat.

But like anything, there's a time and a place for fasting and if you're stressed or anxious, on your period or going through perimenopause, it's worth taking a long hard look at whether your otherwise healthy eating regime could be affecting your health. It's more common than you think, says registered nutritionist Charlotte Faure Green (pictured above) and there are some tell-tale signs.

"Often clients come to me and they are intermittent fasting (IF), they’re feeling 'really great'. But their sleep is screwed, periods irregular and they have unexplained or increased anxiety," she says.

"IF can be great for some people, but not so much for others. How we react is very individual. Fasting is something I would rarely recommend in my clinic for uber stressed or clients with anxiety. Yes, it very much depends on the client before me, but mostly I want to see them nourished," she says.

It's something she knows from personal experience. In her fast-paced mid-twenties working as a music tech PR, sleep was scarce,  caffeine  was plentiful and she relied on the 5:2 diet (eating only 500-800 calories daily for two days a week) to manage her weight. "[It] felt good. On starve days, I felt buzzy and I was in control of my dress size (I now know this to be cortisol). Life was like this for a good two years and fasting seemed to suit me. But eventually, my periods became irregular, I found it harder to cope with pressures at work and mood fluctuations came fast."

She saw her GP who ran tests and diagnosed hypothyroidism (underactive thyroid). "Fasting may not have been the sole cause of my thyroid disorder (autoimmunity and other factors play a part)," reflects Charlotte. "The role of fasting in reducing the conversion of thyroid hormones is well-studied, and stress is a prime suspect in thyroid disease and impacting our metabolism."

She was "medicated but still felt wonky," and saw a nutritionist who pulled her off the 5:2 and introduced her to a much more hormone-friendly time-restricted eating, within a 12-hour window, "Nothing too restrictive - I soon got the 'old me' back. This was the first time I truly saw the impact of what and how we eat on the body and mind, and incidentally when I decided to retrain as a nutritionist!"

Why is intermittent fasting a bad idea if you are stressed?

Intermittent fasting causes us to produce  cortisol , and if we’re already over-producing the stress hormone as a result of being anxious, stressed or burning the candle at both ends, it's simply too much. The result? Sleeplessness, anxiety and disrupted periods.

Why does it cause these symptoms? Our body doesn’t know why there’s a lack of food, it just knows that it has to prioritise the search for it. “Scarcity of food is a stressor for our body. Our ancient brains can’t determine if we’ve stepped into a vast expanse of desert, or we just haven’t popped to Sainsbury’s, or if we’re doing this by design. It just knows there isn’t any fuel coming.”

What too much stress also does is put on the back burner all the 'non-essential' functions such as hair growth, libido, our reproductive system and even sleep because our body doesn’t know it’s truly safe to conceive or sleep under all this threat, Charlotte explains.

What's more, when we fast, our body has to create energy from our fat and protein stores because all the energy from food has been used up, Charlotte explains. This process is called gluconeogenesis and a by-product of this process is cortisol – the stress hormone.

The stress caused by fasting isn’t necessarily a bad thing if your stress levels are normal. “Stress keeps us alive and helps us adapt," says Charlotte. "It can motivate us to make changes, to thrive and execute quick reactions when faced with a challenge.”

Good stress is called 'eustress'; it is something that encourages growth in the body and happens when we feel safe, Charlotte explains. “There’s a healthy dose of challenge, but we perceive ourselves to be safe and ultimately it feels pleasurable. Examples of physical eustress include a hike, a sweaty sauna session, or intermittent fasting. However, we all perceive stress in different ways and too much 'eustress' can quickly switch to distress for some.”

When should you avoid intermittent fasting?

1. If you're stressed or anxious

Fasting increases the amount of stress hormone cortisol in your body. If you're already stressed or anxious, you'll likely have high er levels of stress hormones in your body which may not only increase anxiety but interfere with the hormones that rule your periods.

2. If you're on your period

Most women experience a decline in energy, focus and sometimes mood when they’re on their period. Charlotte points out it’s also often a time of reflection and vulnerability.

“Our periods should be a time to retreat and practice self-care. This includes good nourishment, maybe a touch of chocolate (dark for the  magnesium ), and not piling on additional life stressors,” says Charlotte. “Stress is metabolically and nutritionally expensive, meaning it requires a heavy supply of good minerals and vitamins through diet, and there are minerals and nutrients already depleted during your bleed that need restocking – your period is not the time to deny your body nourishment.”

3. If you have an eating disorder

“Being on any kind of restrictive diet (and IF is a diet!) may negatively impact your relationship with food as it encourages black and white thinking,” says Charlotte.

“It prevents us from seeing food as nourishment and we may start to see it as the enemy. Spending an inordinate amount of time thinking about your food can lead to an eating disorder called  orthorexia . Intermittent fasting can also lead to  binge eating  on the days following a fast day and has been recognised as a risk factor for bulimia nervosa.”

4. If you have thyroid issues

Stress is a prime suspect in thyroid disease, as Charlotte found from her own hypothyroidism diagnosis, so if you have a family history of thyroid problems it’s best to avoid fasting diets that can cause more stress on the body.

5. If you are in perimenopause

Some time-restricted eating (such as giving yourself a 12-hour window to eat during the day and fasting for 12 hours overnight, for example) can be useful during perimenopause as it helps to improve insulin resistance, says Charlotte. "However 5:2 or extended fasting is not recommended in menopause as it restricts the nutrients that we can take in through diet," she says, "The nutrients it restricts are the ones we need most at this time, such as protein (due to reducing muscle mass) or calcium (due to increasing osteoporosis risk)."

7. If you're pregnant or breastfeeding

"During pregnancy, the placenta passes nutrients to the growing baby - nutrients such as iron , zinc , vitamin B12, vitamin B9, iodine , selenium, omega 3 fats and proteins. The very same nutrients are vital to produce breast milk," Charlotte explains.

"Nature is smart and will always prioritise the baby, so will zap the mother's stores. Go a little too far and the milk supply may be affected. Pregnancy and breastfeeding is a time for nourishment and unapologetic eating, not restriction!"

Charlotte Faure Green is a BANT Registered Nutritionist, speaker, writer and brand nutritional advisor. Find her  @charlottefauregreennutrition   and at