Humans have been fasting for hundreds of years and have evolved to be able to deal with periods of feast and famine.
Today though we live in the thinking that we must eat every 2-3 hours with snacks in between. There is an ingrained belief in us now that if we leave large periods of time between eating that it may be harmful for us.
Over the last few decades the amount of time that we spend not eating, has dropped dramatically. In one study, it was shown that we are now consuming foods more frequently throughout the day than we did 30 years ago ( 1).
Recently, there has been an increase in the emerging scientific literature surrounding fasting. Particularly looking at the health benefits that this may have on both the body and the brain.
Below are some of the proposed evidence based health benefits of fasting:
1. Fasting Can Help the Aging Process And Increase Longevity
Although the process of ageing is complex, small genetic alterations can cause an increase in healthy lifespan.
One of the earliest studies looking at fasting compared to simple calorie restriction was carried out in 1945 in rats (2). They found that the fasted mice lived 20% longer than the control group.
These types of results have been shown in numerous other rat and mammal studies since then. Some of which identifying animals in the fasting group to have a 60% increase in their lifespan compared to control groups (3, 4).
Amongst the major effects of fasting relevant to aging are changes in the levels of Insulin Growth Factor-1 (IGF-1), glucose, insulin and human growth factor ( 5). These all play a role in the development of many chronic diseases and impact the physiological rate of aging.
Particularly that of IGF-1/insulin signaling pathway. In the body, IGF-1 has growth promoting effects on almost every cell in the body. Whilst having higher levels of it when we are younger and growing is important, when we age, too much can accelerate the aging process.
A randomized controlled trial looking at calorie restriction was carried out in 48 adults. After 6 months, they reported a significant reduction in both insulin levels and body temperature. Both of which, according to the authors, are biomarkers for longevity (6).
2. Fasting May Help Promote Weight Loss, Predominantly Fat Loss
One of the major reasons for the increase in traction around that of fasting is to do with its impact on weight and specifically fat loss.
There are many ways in which individuals can lose weight but the intermittent fasting can allow a lot more freedom whilst producing similar results.
A study carried out in 107 premenopausal women aged 30 to 45 placed them on either intermittent fasting (2 days a week fasted) or a standard calorie counting diet. They found that the intermittent fasting diet provided just as effective results in weight and fat loss as the standard calorie restriction approach ( 7).
A similar study examined the effects of alternative day fasting in 16 obese individuals (12 women, 4 men) for 10 weeks. They reported significant reductions in body fat percentage and no changes in fat free mass. This means that the weight loss was more associated with fat than muscle loss ( 8).
Fasting has also been looked at in non-obese or overweight individuals ( 9). Whilst they could report improvements in fat mass and fat oxidation, hunger levels were elevated and remained elevated throughout. This might indicate it being more challenging for those of a normal body weight to stick with extended periods of fasting. This may be due to the amount of fat that the body will have access to for energy.
As well as fasting causing you to eat less, the hormonal changes that are noted with fasting could also help with weight loss. Namely the reductions in insulin can allow the body to access its fat stores more easily to burn for energy.
3. Fasting Can Help Reduce the Risk of Developing Type 2 Diabetes
Insulin resistance is a well-defined disease associated with aging. Prolonged insulin resistance in the body can lead to the development of type 2 diabetes.
Pilot studies have shown that short term intermittent fasting is safe and tolerable as a dietary tool for those with type 2 diabetes ( 10).
Reductions in IGF-1 as well as glucose and insulin can help to promote insulin sensitivity. Thereby reducing the risk of developing diseases such as type 2 diabetes and the metabolic syndrome.
A study carried out in men found that after 2 weeks of intermittent fasting, there was a significant improvement in their insulin sensitivity ( 11). This has also been observed in women carrying out intermittent fasting ( 7). Greater reductions in both insulin and IGF-1 were reported and was more superior in the fasting group than the control group.
This does however need to be mirrored with some studies highlighting an adverse effect on glucose tolerance in women who carried out alternative day fasting for 22 days ( 12). These women were not overweight though and so the results may not be transferrable to those who do have a significant problem with insulin resistance.
4. Fasting Can Offer Protection Within the Brain
As well as having an impact within the body, fasting also appears to offer benefits to the brain. Two of the mechanisms that appear to offer the benefits within the brain are that of reduced oxidative damage and increased cellular stress response ( 13). Both of effects of fasting on the brain are similar to that produced during exercise ( 14).
More specifically, fasting has been shown to increase the signaling of brain-derived neurotrophic factor (BDNF; 15). BDNF plays a vital role in the brain by influencing a variety of functions such as: promoting the growth of new neurons, preventing the death of existing neurons and supports overall cognitive function. A lack of BDNF in the brain has been linked with depressive conditions ( 16).
In animal models of ageing neurodegenerative disorders (such as Alzheimer’s disease and Parkinson’s Disease), BDNF has been shown to increase the resistance of neurons in the brain to the dysfunction and degeneration caused by these diseases ( 15).
In an animal model of Huntington’s disease, whereby reduced levels of BDNF are a part of the aetiology of the disease, intermittent fasting was shown to significantly raise the level ( 17). Therefore causing a protection against the symptoms of the condition.
Other areas where it appears to offer protection from is within dementia and Alzheimer’s disease. In rats predisposed to Alzheimer’s disease, when fed a conventional control diet, they developed symptoms of the disease through memory and learning problems. Those rats put on intermittent fasting though, rarely appeared to show any signs of dementia and only deteriorated towards their end of life ( 18).
5. Fasting May Offer a Promising Therapeutic Potential Within Multiple Sclerosis
Multiple Sclerosis (MS) is an autoimmune disorder characterized by neurodegeneration of the nervous system. Currently there is no cure for the disorder and a clear need for other novel strategies are needed.
One emerging strategy to help in this area is that of fasting. In mice, a 3-day fasting diet given every 7 days (for a total of three cycles), was shown to ameliorate MS type symptoms in all and reversed disease progression in a portion of the mice ( 19).
The same research group also carried out a randomized, 3 arm pilot trial ( 19). They looked to assess the safety and feasibility of a fasting diet compared to a ketogenic diet in 60 patients with relapsing-remitting MS. Both diets reported significant improvements in health-related quality of life parameters and mild improvements in MS specific scales.
Of course, many more studies are needed to be carried out before this could ever be deemed as an appropriate tool to use. Also more is needed to be understood around its mechanism of action in MS but one proposed action is the anti-inflammatory effects of fasting.
6. Fasting Can Switch on Specific Repair Genes: Autophagy
As well as being able to reduce levels of insulin and IGF-1, fasting has also been shown to switch on certain repair genes and processes in the body.
When we first begin to fast, we send our body into an initial state of stress. The brain then tries to signal to you to get food. But when this is not carried out, our bodies will start to go more into a survival mode to help conserve the body until the next time it gets food. This means that the body is way more interested in processes that will protect the body, rather than growing or reproducing.
Although the exact mechanisms are not fully understood, one proposed way is through a process known as autophagy. Autophagy literally means to ‘self-eat’ and it is the body’s way of being able to break down and remove old cells. It has been recognized as a key defense mechanism in many ageing diseases ( 20).
Autophagy has been consistently shown to be activated when the body is in starvation mode, or deprived of nutrients ( 21). Short-term fasting of 24-48 hours in mice revealed a significant up-regulation of autophagy in the neurons within the brain ( 22).
One major factor that has also been shown to further inhibit autophagy in the body is that of insulin/IGF-1. When the insulin/IGF-1 pathway is down regulated, the process of autophagy can be switched on in the body. Fasting again has been shown to be able to cause this down regulation of the IGF-1 pathway ( 23).
7. Fasting Can Offer Protection from Certain Cancers and Help Mitigate the Side Effects of Standard Treatment
Cancer is a disease characterized by abnormal cell division and growth, that affects millions of people worldwide.
Although it is a very complicated disease, fasting has been shown to have several metabolic benefits within the body that may offer a reduced risk of cancer.
At present, most of the data that exists around this is still within animal and cell models ( 24, 25, 26). But human observational data has shown links to the initiation of cancer formulation (26). More longer-term human studies are needed before this statement can be made definitive.
What is interesting is the role that fasting appears to play in the side effects associated with cancer therapy. The problem with radiotherapy and chemotherapy is although it can kill of cancer cells, the treatment options are not selective. This means that although it kills cancer cells, it can also kill off normal, healthy cells.
In animal and human cancer cell lines, short term fasting (up to 48 hours), has been shown to protect normal but not cancer cells during standard treatment (27, 28).
Ten patients with different types of cancers, voluntarily fasted for a total of 48 to 10 hours prior to treatment and/or 5 to 56 hours after treatment. The results showed that whilst fasting, 6 patients reported a reduction in fatigue, weakness and gastrointestinal side effects when undergoing treatment ( 29).
This of course is a case study report, with a small sample size. Many more well controlled randomized clinical trials are needed to determine the exact clinical effects and safety.
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