As a therapeutic intervention, fasting pre-dates all medical literature. Hippocrates wrote enthusiastically about the benefits of this approach, as did Plato and Paracelsus. Fasting centres were abundant in many countries in the early 20th century before pharmaceuticals became the order of the day. The benefits of fasting include improved autophagy (clearance of cellular debris) (1), increased growth hormone (2), enhanced resistance against oxidative stress (3), and higher levels of BNDF (4) a chemical known to help repair our brains. In other words, we exchange the stress of restricting food for a lot of health benefits.
In recent years, we have seen an explosion in the interest in fasting, primarily in the form of intermittent fasting and the 5-2 diet. Both are described as an easy way to better health and rapid weight loss.
Sounds good, right? There’s just one problem. Most people that I see who undertake fasting do themselves harm. By harm, I’m talking fainting, palpitations, wrecked sleep and weight gain. How can this be? How did such an approach become a problem?
The reality is that fasting has not changed, but the profile of the average person undertaking it has. Compared to just 100 years ago, when fasting was employed regularly in health centres (‘sanitoriums’) across Europe, we are now fatter, less active and with massively higher rates of chronic disease. As a society, we have been subjected to a stream of man-made hormones, antibiotics and untested food additives. We sleep for an average of two hours less each night. This means that we are more toxic, less able to burn fat and our stress responses is already pushed to the limit.
Removing food from such a population is simply a recipe for disaster. Health problems from fasting are well-documented by those working on the frontlines and in the medical literature (5, 6). So how can fasting help so many and yet cause harm in others? To put it simply, there are two big factors in whether or not fasting will help or harm you; our capacity for stress and our ability to burn fat. I will explain.
Physiology of Fasting
When we embark upon a fast, we see a number of hormonal changes in the body. The brain senses a drop in blood glucose and responds. Insulin levels drop, while glucagon, adrenaline and ghrelin increase. The combined effects of such hormonal shift is to release stored energy (fatty acids and glucose) into the bloodstream to help us survive. Our brains receive enough glucose to survive, although our performance is noticeably impaired (7). Our fat stores contain a huge amount of stored energy; our carbohydrates stores are much more modest. Most people store around 300g, or 1,125 kcals, worth of carbohydrates between their liver and muscle (in the form of glycogen). Depending the individual, we run low on carbohydrates between 24 and 72 hours of fasting. Once the glucose levels at the liver tank, we begin to rely heavily on fats.
In a responsive individual, this is where the magic of fasting begins. We will skip the complex metabolic explanations, suffice to say that the excess of fatty acids (relative to glucose) means that sees the liver change its focus from burning carbs and fats together in the ‘normal’ way, to processing fats into fatty acids and ketones. The muscles take the fatty acids, the brain takes the ketones.
The brain loves ketones and, as soon as the liver begins to produce enough of them, the brain becomes happy. It stops demanding more glucose, because it is no longer so important. Adrenaline therefore drops and the body can now relax. Mood improves and mental sharpness rockets. Yet, your body can still benefit from the benefits of fasting that I previously described.
The only problem is that ketone production is limited by the activity of three enzymes (8), and these enzymes can take up to three weeks to become optimized. However, in athletes (who periodically make use of ketones during intense exercise), ketone production is likely to be fairly efficient from day one.
However, what about individuals with adrenal issues? This subset invariably have raised levels of adrenaline, which maintains glucose levels in the bloodstream (and therefore the liver). This automatically blocks the enzymes required for ketone production. Ketones stay low, stress stays unbearably high.
How about overweight individuals? This group invariably have hormonal issues and take much longer to produce an efficient amount of ketones. This draws out the semi-starvation phase, loading their body with copious amounts of additional stress.
If your brain is forced to run on low carbohydrate availability and low ketone availability, the results are catastrophic. The breakdown of lean mass can leave a long-term legacy that effects both their health and their body composition. In this starvation state, the body will break down lean mass and, for every one kilogram of skeletal protein lost, we lost four kilos of water with it (9). “I know I look like shit and my brain is in shutdown, but I’ve lost 5kg in a week”… sound familiar?
Unfortunately, the demographic most likely to embark upon a fasting are overweight people and those with hormone issues, who have ‘tried everything’ to shed those extra pounds and are now willing to resort to more extreme methods to achieve their goal. It is important to remember that, 99% of the time, obesity is a hormonal disorder and is reliably resolved when it is approached as such.
Keto-adaption is the key
As you will see from the above explanation, there is nothing wrong with fasting. However, fasting without adequate production of ketones is a disaster.
I rarely recommended fasting because it is normally not necessary once we have taken sufficient steps to support the metabolic cascade involved in detoxification (such as methylation). Equally, I tend to deal with individuals who have experienced significant metabolic issues, which can lead to an accumulation of fat-soluble toxins. A full-on fast can therefore see a monumental release of biphenyls, dioxins and various other pollutants that were previously locked up in their fat cells, so toxic load should also be considered before such procedures.
However, the duration of the fast can be customized for each person and additional support can be provided (eg. saunas, charcoal, bile acid sequestrants) in all cases. I do make use of fasting, just primarily in those who stand to benefit from it. This is usually individuals who have already reached the finish line but wish to optimize their wellbeing further. I never use it as a weight-loss strategy.
On those rare occasions when I use fasting, I will always, always make sure that the individual is keto-adapted beforehand. This is the only way to ensure that the fuel supply to the brain is maintained and, thus, the initial insult avoided.
And I never recommend the 5-2 diet. It enhances all the negative aspects of fasting without providing the opportunity to get all the benefits.
– Fasting is stressful on the central nervous system but may provide a lot of benefits to the body as a whole
– The production of ketones minimizes this stress, and is the one factor that makes the net impact of fasting beneficial
– Unless you are on a ketogenic diet, all individuals will produce near-zero ketones in the first day
– Higher stress levels or adrenal dysfunction tends to produce higher adrenaline, which will make it more difficult to enter ketosis (and therefore make ketone production difficult)
– Anyone with hormonal issues may struggle to product ketones efficiently and most overweight/obese individuals fall into this category
– Fasting without a shift to ketosis is just starvation
– If you feel bad, STOP
Fasting is neither ‘good’ nor ‘bad’
Ignore the bloggers. In fact, ignore anyone who is impassioned one way or another about fasting. Ideological viewpoints and fasting have no place in the same room.
It pays to remember that fasting is a tool, and a powerful one at that. Used in the correct setting, it can be used for significant benefits. However, it can also be the single most stressful intervention on a person and this should be considered as this approach reaches fad levels.
9. Guyton, Arthur C. (1976). Textbook of Medical Physiology (5th ed.). Philadelphia: W.B. Saunders. pp. 284, 424. ISBN 0-7216-4393-0.