Fasting is an abstinence or reduction from some or all food and/or drink, for a period of time.
An abstinence from all food and liquid usally for 1 or more days is called dry fasting (absolute fasting).
Water fasting is an restriction of food when only water is allowed .
Fasting has been known for ages in different civilisations, cultures and religions. Many wise men in history treated fasting as a remedy:
„I fast for greater physical and mental efficiency” - Plato
„Fasting is the greatest remedy – the physician within” - Phillippus Parcelsus (one of the father of Western Medicine)
„Everyone has a doctor in him or her; we just have to help it in its work. The natural healing force within each one of us is the greatest force in getting well. Our food should be our medicine. Our medicine should be our food. But to eat when you are sick, is to feed your sickness” - Hipocrates
The aim of fasting
There are also other types of fasts, sometimes just partially restrictive, excluding only particular foods (i.e. not eating meat on Friday by Christians).
Medical fasting might be used to clean up the digestive tract prior to colonoscopy or gastroscopy tests.
Diagnostic fasts (8-72 hours) might be also useful once examining a hypoglycemia, when a level of insulin and C-peptide is measured, or checking the fasting glucose level after abstinence of food by 10-12 hours .
Fasting can be used as a tool to make a political statement, to protest and bring awareness to a cause.
Fasts are also present in many cultures and religions worldwide such as Hindu, Islam, Christianity, Buddism, Judaism, Sikhism and others.
From evolutionary point of view, the abstinence of food is a natural part of our life, as our ancestors (hunter-gatherers) were having a periods of feast/fast interchangably.
This is why, our genes, equipped us (with not always beloved) ability of storing the excess of the energy (as a fat mass) for the periods of famine, allowing us to survive.
What is intermittent fasting?
It is a very broad term for various eating models which are basically periods of calorie restrictions (not eating caloric food or eating up to 600 kcal) and non fasting periods appearing periodically.
The most popular forms of intermittent fasting are:
- alternate day fasting (ADF) – 24 hour fast followed by 24-hour non fasting period
- 16:8 – every day fast which basically mean shortening the duration of the eating window - the time of the day when eating is allowed – i.e. after last meal on Monday at 8 pm, the next meal will be 12 am on Tueday
- 5:2 diet - two non-consecutive days of the week requires restriction in calories to 400-500 kcal for women and 500-600 kcal for men, during the remaining five days of the week there is no restricions with calories
For most of those method fasting means restriction in food, but not in non-caloric beverages such as water, tea and coffee without additions such as sugar/milk.
Even if you have not fasted in your life, you might realise that actually you do it every day, once you are sleeping (non-eating period).
What happen in the body during a fast?
During non-fasting period, ingested food is digested and turned thanks to enzymes and bile into nutrients which cause the metabolic response.
Insulin, increased by the pancreas allows us to push glucose from blood stream into cells (increase lipogenesis) and inhibits the fat burning.
During the fasting periods, there is a significant drop in insulin levels resulting in using fat as a source of energy.
However, on short term fasting, the proteins coming from breakdown of muscles are being used as a source of energy in the process called gluconeogenesis.
Muscle mass has an adverse effect during fasting from our point of view (people want to have a lean muscle mass and not to much fat mass), however from evolutionary point of view our body burned the muscles as one of the most calories-consuming tissue which allowed to slow down metabolism and survive during the famine periods.
Nowdays, the negative effect of muscle mass loss can be alleviated by appropriate proteins and/or BCAA intake.
Health benefits of intermittent fasting
The findings suggest that intermittent fasting has the potential to improve health and indirectly reduce the risk of chronic diseases.
Below you will find what we currently know about intermittent fasting, however the health benefits presented in this article should be interpreted with caution as they have not been reproduced yet in long-term human studies.
Studies on rodents have shown that intermittent fasting can extend lifespan similarily as continous calorie restriction [3,4].
Calories restriction on animals without malnutrition has been suggested that in yeast, fish, rodents and dogs slows down aging, resulting in increase in both median and maximum lifespan .
However it does not mean that these finding can be extrapolated on every species.
The main studies on non-human primates (rhesus monkeys) have failed to demonstrate the increasing median lifespan, however the group with calories restrictions were only 36.4% as likely to die from age related causes and only 56.2% rate of death from any cause in comparison to placebo group .
In humans the long-term effect of moderate calories restriction are unknown yet.
Alzheimer's Diseases (AD) is the most common neurodegenerative disease, and recent estimate suggest that AD has become the third leading cause of death in United Stanes .
Currently there is no cure available for AD, so preventing it from showing up is the critical point to tackle.
Study in rats has shown that intermittent fasting can ameliorate age-related deficits in cognitive function, delaying the onset of AD or reducing its severity .
Other studies on animals suggests neuroprotective properties of intermittent fasting in Alzheimer's and Parkinson's diseases [8,9].
A Small study on humans affected by Alzheimer's Disease has shown that the therapeutic system which included fasting 12 hours each night has improved AD sympthoms in 9 out of 10 patients.
The results should be taken with caution as the fasting was just one component of a larger therapeutic system, therefore studies on larger populations focused on calories restrictions or intermittent fasting are needed .
Diabetes type 2
Diabetes type 2 is a metabolic disorder, caused primarily by obesity (especially visceral fat mass) and lack of physical exercise.
The main symptoms include, high blood sugar level and insulin resistance which can be improved but taking actions such as diet modification and introducing exercise are needed.
Caloric restriction (CR) and intermittent fasting (IF) are known to improve glucose homeostasis and insulin resistance in several studies on animals including humans.
Study on mice has shown that CR and IF can modulate the lipid droplet composition and reduce intracellular diacyloglycerols species (DAGs) which indirectly reduce the risk of type 2 diabetes .
Study on humans has suggested that intermittent fasting and alternate day fasting can in overweight and obese populations be an alternative to calorie restricion for weight loss and type 2 diabetes reduction, but more studies are needed to confirm those findings .
Obesity and body composition
Obesity is one of the biggest health problem worldwide. Reduction in energy intake has been show to lower the risk of coronary disaese in obese subjects .
Skipping meals in your fasting periods, unless compensated by extra calories in non-fasting periods will leads to fewer calories intake.
Moreover, it enhances the weight lose by favorable effects on endocrine system such as: dropping insulin level, increase levels of adrenaline and human growth hormone (hGH) .
Some studies suggest that short-term fasting as opose to continuous calories restriction, can actually increase the metabolic rate .
Intermittent fasting improve weight and other risk related outcomes, and was associated with lower prevalence of coronary artery disease or diabetes.
However further research in humans is needed to confirm those findings [11,16].
Studies on overweight women has shown intermittent evergy intake being as effective as continuous calorie restriction (CER) in reducing weight, improving insulin sensitivity and other biomarkers and may be offered as an alternative to CER for weight loss and reducing disease risk .
Some trials have shown that besides it’s favorable effect on weight loss and fat oxidation, hunger was not decreased, and suggested that adding one small meal on a fasting day can make this approach to dietary restriction more acceptable .
Growth Hormone (GH)
Growth hormone (somatotropin) is a stress hormone which raises concentration of glucose and free fatty acids.
It can stimulates growth, cell reproduction and cell regeneration in humans and other animals.
Studies has shown that release of growth hormone from pituitary gland is infrequent and erratic.
Higher levels of GH can be feasible for fat burning, gaining lean muscle and improving bone density .
Fast induces enhancement of GH release through combined frequency (discrete pulses) and amplitude (sinusoidal periodicities) modulation.
These alteration may play an inmportant role in substrate homeostatis during starvation .
Just two-days fast in normal man trial, resulted in 5-fold increase in the 24-h endogenous GH production rate .
Inflammation and oxaditive stress
Oxidative damage and free radicals have been suggested as a cause of aging, cancer and chronic diseases.
Some of the studies have shown that fasting can reduce the inflammation and oxidative stress in the body.
Study on overweight adults with asthma have shown that alternative day calorie restriction (ADCR) reduce markers of oxidative stress (8-isoprostane, nutrotyrosine, protein carbonyls and 4 hydroxynonenal adducts) and indicators of inflammation including tumor necrosis factor-alpha were decreased by ADCR .
Trials on fasting was examined during the Ramadan had a positive effect on inflammatory status of the body reducing inflammation markers including IL-1, IL-6, tumor necrosis factor-alpha, homocysteine, CRP, and TC/HDL ratio [20,21].
Other study has shown that intermittent fasting reduced oxidative damage and improved cellular stress resistance, and suggested that the influence on cardiovascular system and brain can be similar to those on regular physical exercise and probably sharing the same mechanisms .
Cardiovascular disease (CVD) is the single biggest cause of death worldwide.
The risk of disease can me reduced with appropriate diet, lifestyle and physical exercise.
Intermittent fasting (IF) and alternate day fasting (ADF) is considered as an alternative to calorie restriction to reduce fat mass, insulin resistance and inflammation which can reduce the risk of CVD [10,14].
It has been also reported that IF can change lipid profie, change serum adipokine levels and change the expression of genes related to inflammatory response [24,25].
Brain and nervous system
The mechanisms which can play an important role in healthy brain functioning and are improved when affected by IF are: insulin-like signaling, FoxO transcription factors, sirtuins and peroxisome proliferator-activated receptors.
These pathways stimulate the production of protein chaperons, neurothropic factors and antioxidant enzymes, which can help neurons to cope with stress and resist disease .
Studies in humans with usage of blood-oxygenation-level dependent (BOLD)-functional magnetic resonance image (fMRI) confirmed the influence of IF on brain plasticity and oxidative stress .
The positive effect of IF has also been shown in athletic success.
Moreover, the immunological which are indirectly involved in cognitive functioning, through IF can impact the relationship cytokine have between brain and cognitive deficits, including memory deficits .
On the biochemical level IF can increase BDNF – brain derived neurothrophic factor, which increases the resistance neurons to stressors nad degeneration, and also mediate beneficial effects of IF on glucose regulation and cardiovascular function .
Another mechanisms of IF – ketogenic effect proposed to be neuroprotective. Increasement in neural levels of hypoxia-inducible factor-1alpha (HIF-1alpha) suggested to inhibit the proxyl hydoxylase activity (enzyme which has been targeted in treatment of many diseases i.e. stroke and ischemia).
The Sirt1 in neurons can be activated as well as reduced activity of mTORC1 in hippocampus which can induce neuronal macroautophagy (the process which has been suggested as a rational strategy in prevention of neurodegenerative disorders, elimination of damaged mitochondria that increases oxidative stress) .
Who might benefit of intermittent fasting ?
- young, healthy males – the ability to cope with calories restriction works extremely well, and energy balance could be sustained
- experienced exercisers – people who are not beginners in terms of incorporating physical exercise into their life
- experienced in monitioring calories and food choices – people who observe their body and know how it reacts under different nutrition factors, people who know how to make healthy nutrition choices and know that nutrient dense food (as periods of fasting require being efficient) provide nutrients and calories in non-fasting periods (instead of binge eating junk food)
- single or have no children – you don't want to be asked by your son/daughter: „Why is mummy/daddy not eating with us anymore?” and also the eating patterns learned at home can affect the future nutrition actions of our children. Another thing is when you having a baby from a young age, fasting will be just another stressor which gives you more harm than good
- low stress levels – intermittent fasting can be another stressor in life
- want to be athletic/lean not to be huge – the impact on anabolic hormones (insulin and IGF-1) can affect the gaining mass and losing weight
When intermittent fasting is probably not a good idea ?
It seems to be a bad idea to be introduced to intermittent fasting in the following cases:
- beginner in diet and exercise - they cannot make the right nutritional choices to ensure safety of fasting, making bad food choices in non-fasting days
- have performance-oriented jobs – if your job hugely depends of your physical/mental performance, social interactions
- individuals (couples) with children
- children – require well balanced diet to ensure the correct physical and psychological development, fasting can be very dangerous and affect their future life
- females - (especially in reproductive age, who want children, pregnant and nursing) – women seem to be more sensitive in keeping an energy balance, also fast can affect hormonal balance, menstruation cycle, amenorrhea and decline in sex steroids. It can be extremely dangerous especially when without supervision
- eldery and underweight – calorie restriction in those individuals lower the chance of staying with life and detoriate health state
- chronically ill, cancer patients – in those cases the health professional supervision is essential
Intermittent fasting can be an alternative option to the recommended 5-6 meals including breakfast, but definately not for everyone.
Certain individuals: women (especially pregnant, nursing, in reproductive age), children, underweight, beginners to exercise and diet, individuals with family lifes should not practice this nutritional model (especially without medical supervision).
In other cases, it might be an attractive way to go crazy with a diet and cutting calories down.
The studies on animals and humans have shown positive effect of intermittent fasting on: diabetes type 2, obesity and fat loss, brain function, cardiovascular health, growth hormone, oxidative stress and aging, however making strong statements about intermittent fasting should not be discouraged and the findings (even the most positive and promising) should be interpreted with caution and require confirmation in larger and longer-lasting studies on humans.
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