Fasting: who might NOT benefit?

Health, March 08, 2018

While there are many good things about incorporating fasting into your health routine, for some, it might need careful consideration before embarking.

While there are many people who benefit from intermittent fasting, and there is a myriad of health benefits to be had (as previously discussed), for some, it requires careful consideration and intuition as to whether it is a good approach for them. 

The potential for hormonal disturbance is the main consideration when undergoing a fasting regime. For example, in humans, TSH is suppressed with fasting duration, resulting in reduced circulating levels of thyroid hormones T3 and T4. In addition, the activity of enzymes that help convert T4 to T3 are also reduced, as are receptors that help other tissues take up the thyroid hormones for enzymatic and metabolic purposes. Another study found that a 16:8 protocol in healthy men lowered T3 and testosterone over an 8 week period. It is not known the extent to which this affects thyroid and overall health in the long term, though there are some who suggest that a lower T3 hormone, lowering metabolism, is one of the benefits of fasting as it lowers cell turnover, thus enhancing longevity. However we can’t ignore the fact that this points to an additional stress on the endocrine system that for some, may be too much. Therefore, if you have an element of thyroid dysfunction, speaking to a practitioner is the best option. It is worth pointing out that, though fasting can increase the production of stress hormones due to the deprivation of glucose to the brain, a review of a number of different studies found this may be an initial response to a fasting protocol, as cortisol tended to reduce to baseline levels after a number of weeks.

There have not been that many well designed trials to test the effects of fasting in women, however what we do know is that women’s hormones are much more sensitive to the effects of a reduced calorie consumption compared to men. In particular, a neuropeptide kisspeptin, plays a role in the regulation of reproduction and energy balance, and in order to keep a regular menstrual cycle. Rodent studies have revealed that during periods of fasting, kisspeptin levels are reduced. This could explain why women experience hormone imbalances that may lead to a disruption in the menstrual cycles. Indeed, a study investigating the effects of the 5:2 protocol amongst women reported a longer than average menstrual cycle compared to a calorie restricted regime  Another study found a 72 hour fast affected luteinising hormone but didn’t disrupt cycle length or follicle development amongst normal weight women. Different effects on menstrual cycle are likely to be related to the starting weight of the individual, overall energy balance and the number of consecutive days spent intermittent fasting.

One study investigating the effects of alternate day fasting found, while all participants had an increase in a gene that increases resilience to stress and enhances longevity (Sirt1) and men had an improvement in glucose tolerance, there was a detrimental effect on women’s glucose control. There are some considerations to take into account with this trial however. This change in glucose response may be a normal physiological adaption to fasting – increasing insulin resistance in skeletal muscle, thus sparing glucose for the brain and central nervous system. The participants were all healthy and had a body mass index (BMI) less than 30kg.m2 (therefore did not fall into the obese category). Those who may have excess body fat to lose could respond differently to the fasting protocol. Further, the study did not control for physical activity, and there were varying levels of activity among participants. If those that fell into the category of ‘active’ (three or more times per week) were women, and they exercised on the fasting days, this could increase the overall stress load on the body, thus impact on their hormonal response to fasting by pushing cortisol levels up and increase blood glucose levels in addition to the meal. These results are not automatically applicable to other groups in the population, so we can’t generalise to (for example) the older population as their hormonal response may be quite different. We also can’t generalise the results of this trial to fasting in general, as it is only applicable to an alternate-day fasting regime and not, for example, a 16:8 protocol or the 5:2 approach. Finally, it would be interesting to see the individual participant blood glucose responses to the fasting regime, rather than a group mean, as everyone is different to how they may respond, and within the group of participants there may have been those that responded favourably and those that did not respond well at all – these individual differences are diluted when grouped together to give an average.

Another study investigating eating behaviour among normal weight women on a fasting regime found that, after four weeks of a four-day per week fasting protocol of 600 Cal per day, there was a tendency to overeat on the days where eating was unrestricted. Measures of eating behaviour were assessed also, with participants reporting an increase in eating-related thoughts, feelings of hunger, and fear of loss of control on the fasting days.  They also reported worse mood, heightened irritability, difficulties concentrating, and increased fatigue. These effects are not what are generally reported and does potentially show that we can’t surmise that fasting, like any eating approach, is going to be the best approach for all.

Most of these detrimental effects may well be due to the restriction in calories rather than fasting per se – remember that calorie reduction is not a necessary element of fasting and, depending on the protocol used, it can be avoided for those who notice adverse effects. Many of the health benefits that occur in fasting aren’t necessarily related to calorie restriction, though obviously for significant weight loss, it IS a necessary element. The important thing is to listen to your body and assess the effects of your fasting protocol after undertaking it.

Recommendations for fasting:

  • Start with a more gentle approach, aiming first for a 12-13h fasting window (ie eating between 8am and 7pm)
  • Add in one fast day per week, which may be a 24h fast, where you consume nothing from dinner time one day to dinner time the next, or two 16:8 fasting days per week on non-consecutive days of the week
  • Avoid any excessive training or exercise that may be particularly demanding on a fast day; stick to light activity and walking
  • Add in 10g of branched chain amino acids (BCAA) to help fuel the central nervous system, thus reducing the overall stress response on the brain during a fast (this isn’t applicable to those undertaking a fast for circadian rhythm purposes).
  • Be intuitive with the fasting protocol whatever you do; if your sleep starts to become disturbed, if you start to notice a reduction in wellbeing, chipping nails or dry skin and hair, or weight gain – then more aggressive fasting is not the answer. Less (fasting) is more in this instance.


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