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"Our food should be our medicine. Our medicine should be our food. But to eat when you are sick is to feed your sickness."
- Hippocrates
Fasting is one of the most ancient and widespread healing traditions in human history. This solution has been practiced by virtually every culture and religion on earth. Hippocrates of Cos (c460 – c370 BC) is widely considered the father of modern medicine. Among the treatments that he prescribed and championed was the practice of fasting. The ancient Greek writer and historian Plutarch (c46 AD– c120 AD) also echoed these sentiments. He famously wrote, “Instead of using medicine, better fast today”. Ancient Greek thinkers Plato and his student Aristotle were also strong supporters of fasting.
The ancient Greeks believed that medical treatment could be observed from nature. Humans, like most animals, do not eat when they become sick. For this reason, fasting has been called the ‘physician within’. This fasting ‘instinct’ that makes dogs, cats and humans anorexic when sick. This sensation is certainly familiar to everybody. Consider the last time you were sick with the flu. Probably the last thing you wanted to do was eat. So, fasting seems to be a universal human instinct to multiple forms of illnesses. Thus fasting is ingrained into human heritage, and as old as mankind itself. The ancient Greeks also believed that fasting improves cognitive abilities. Think about the last time you ate a huge meal. Did you feel more energetic and mentally alert afterwards? Or, instead did you feel sleepy and a little dopey? More likely the latter. Blood is shunted to your digestive system to cope with the huge influx of food, leaving less blood going to the brain. Result – food coma.
Fasting is also widely practiced for spiritual purposes and remains part of virtually every major religion in the world. In spiritual terms, it is often called cleansing or purification, but practically, it amounts to the same thing. The practice of fasting developed independently among different religions and cultures, not as something that was harmful, but something that was deeply, intrinsically beneficial to the human body and spirit.
So fasting is truly an idea that has withstood the test of time. But what is exactly fasting and what does science say about it?
Fasting involves controlled, voluntary abstinence from caloric intake to achieve a physical, mental, or spiritual outcome.
Our ancestors would regularly go days or even weeks without food. As a result, humans have evolved specific adaptations to survive, and even thrive, during periods of famine. So, in reality, the body only exists in one of two states – the fed (high insulin) state or the fasted (low insulin) state. Either we are storing food energy (increasing stores) or we are burning stored energy (decreasing stores). It is one or the other, but not both. More analytically :
Feeding – During meals, insulin levels are raised. This allows uptake of glucose into tissues such as the muscle or brain to be used directly for energy. Excess glucose is stored as glycogen in the liver.
The post-absorptive phase – 6-24 hours after last meal . Insulin levels start to fall. Breakdown of glycogen releases glucose for energy. Glycogen stores last for roughly 24 hours.
Gluconeogenesis – 24 hours to 2 days – The liver manufactures new glucose from lactate and amino acids in a process called “gluconeogenesis”. Literally, this is translated as “making new glucose”. In non-diabetic persons, glucose levels fall but stay within the normal range.
Ketosis – 2-3 days after beginning fasting – This is when interesting things start to happen for the body. The low levels of insulin reached during fasting, stimulate lipolysis, the breakdown of fat for energy. The storage form of fat, known as triglycerides, is broken into the glycerol backbone and three fatty acid chains. Glycerol is used also for gluconeogenesis. Fatty acids may be used for directly for energy by many tissues in the body, but not the brain. Ketone bodies instead , which are produced from fatty acids during ketosis , are capable of crossing the blood-brain barrier for use by the brain. After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta hydroxybutyrate and acetoacetate, which can increase over 70 fold during fasting.
Protein conservation phase – >5 days – High levels of growth hormone maintain muscle mass and lean tissues. The energy for maintenance of basal metabolism is almost entirely met by the use of free fatty acids and ketones. Increased norepinephrine (adrenalin) levels prevent the decrease in metabolic rate.
We see that the human body has well developed mechanisms for dealing with periods of low food availability. In essence, what is happening while fasting is a process of switching from burning glucose to burning fat . Fat is simply the body’s stored food energy. In times of low food availability, stored food is naturally released to fill the void. So no, the body does not ‘burn muscle’ in an effort to feed itself, at least until all the fat stores are used.
Lets have a look on the effects of fasting on Hormonal Adaptation
- Insulin
Insulin and insulin resistance are major drivers of obesity. Fasting on the other hand , is the most efficient and consistent strategy to decrease insulin levels. This was first noted decades ago, and widely demonstrated scientifically afterwards. It is quite simple and obvious. All foods raise insulin, so the most effective method of reducing insulin is to avoid all foods. Blood glucose levels remain normal, as the body begins to switch over to burning fat for energy. This effect can be observed in fasting periods as short as 24-36 hours. Longer duration fasts reduce insulin even more dramatically. More recently, alternate daily fasting has been studied as an acceptable technique of reducing insulin.
Regular fasting, in addition to lowering insulin levels, has also been shown to improve insulin sensitivity significantly. Many argue that this is the missing link in the weight loss puzzle. Most diets reduce highly insulin-secreting foods, but do not address the insulin resistance issue which is crucial in diabetics. Weight is initially lost, but insulin resistance keeps insulin levels and body weight high. Fasting is an efficient method of reducing insulin resistance.
Lowering insulin also rids the body of excess salt and water. Insulin causes salt and water retention in the kidney. Very low-carb diets often cause diuresis, the loss of excess water, leading to the contention that much of the initial weight loss is water. While true, diuresis is beneficial in reducing bloating, and feeling ‘lighter’. Some may also note a slightly lower blood pressure. Fasting has also been noted to have an early period of rapid weight loss. For the first five days, weight loss averages 0.9 kg/ day, far exceeding the caloric restriction and likely due to a diuresis of salt and water.
- Growth Hormone
Growth hormone is known to increase the availability and utility of fats for fuel. It also helps to preserve muscle mass and bone density. Secretion is known to be pulsatile, making accurate measurement difficult. Growth hormone secretion decreases steadily with age. One of the most potent stimuli to growth hormone secretion is fasting. Over a five-day fasting period growth hormone secretion is more than doubled. The net physiologic effect is to maintain muscle and bone tissue mass over the fasting period.
- Adrenalin
Adrenalin levels are increased so that we have plenty of energy to go get more food. For example, 48 hours of fasting produces a 3.6% increase in metabolic rate, not the dreaded metabolic ‘shut-down’. In response to a 4 day fast, resting energy expenditure increased up to 14%. Rather than slowing the metabolism, instead the body revives it up. Additionally, studies show that the adrenalin-induced fat-burning does not depend upon lowering blood sugar. Presumably, this is done so that we have energy to go out and find more food.
And what about vitamins ,minerals and electrolytes?
Concerns about malnutrition during fasting are misplaced. Insufficient calories are not a major worry, if the fat stores are quite ample. The main concern is the development of micronutrient deficiency. However, if the fasting regime is accompanied by the use of a multi-vitamin and mineral supplementation that will provide the recommended daily allowance of micronutrients , there should’t be any issue. It is worth noting, that in 1973 a therapeutic fast of 382 days that resulted in loss of 125 kilos for a patient, was maintained with only a multivitamin potion and had no harmful effect on health . Actually, this man maintained that he had felt terrific during this entire period. The only concern may be a slight elevation in uric acid that has been described in fasting and can be solved by increased water consumption.
Additionally, evidence suggests four brain health effects linked to fasting:
- Brain cell re-generation
- Cognitive and psychological benefits
- Resilience to neurological conditions
- Slowing the effects of aging.
This research on brain health is focused on the use of ketones, molecules that as we saw before, are being produced and used by the body as a source of fuel while fasting. Administration of ketones is a well established therapy since decades for intractable epilepsy and seizures. It should be considered early in the treatment of Dravet syndrome and myoclonic-astatic epilepsy (Doose syndrome). A growing body of literature suggests also that the use of ketones may be beneficial in certain neurodegenerative diseases, including Alzheimer disease, Parkinson’s disease, and amyotrophic lateral sclerosis. In these disorders, ketones appears to be neuroprotective, promoting enhanced mitochondrial function and rescuing adenosine triphosphate production. Ultimately , dietary therapy is a promising intervention for cancer, given that it may target the relative inefficiency of tumors in using ketone bodies as an alternative fuel source.
So, let’s summarise.
Fasting, but not low calorie diets, results in numerous physiological and hormonal adaptations that all appear to be highly beneficial on many levels. The main benefits of fasting are metabolic flexibility and weight management. In essence, fasting transitions the body from burning sugar to burning fat. Resting metabolism is NOT decreased but instead increased. We are, effectively, feeding our bodies through our own fat. We are ‘eating’ our own fat. This makes total sense since fat is, in essence, stored food. Fat is food stored away for the long term, like money in the bank. Short term food is stored as glycogen, like money in the wallet. The problem we have, is how to access the money in the bank. As our wallet depletes, we become nervous and go out working to fill it again. This prevents us from getting access to our stored money in the bank. In the same manner, as our glycogen ‘wallet’ depletes, we get hungry and want to eat. That makes us look for food, despite the fact that there is more than enough food stored as fat in the body ‘bank’’. How do we get to that fat to burn it? Fasting provides an easy way in.
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