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Fasting keeps finding its way into headlines, but what exactly is fasting, and why is it used? Fasting plays an important role in many religions, such as during Lent, Yom Kippur, and Ramadan, and it played an even larger role traditionally. You often have to fast before certain blood work, diagnostic tests, and surgery.

Then there is fasting for health benefits, which lately has become touted as the must-do action for wellness. Why is it so popular? Well, fasting has been linked to a range of benefits in studies, with improvements in blood sugar and insulin resistance, mood, cardiovascular health, metabolic health, cancer, aging and associated diseases, and cognitive health, as well as increased longevity.

Is this all hype, or is there merit to the claims? And are there different benefits to the different types of fasting? Let’s see what the evidence has to say.

Contraindications for Fasting

Before we get too far into the benefits, let’s take a moment to review the potential contraindications for fasting. Not everyone is a candidate for fasting, especially the more restrictive types. Everyone should talk with their doctor before doing any sort of long-term fast, but those at the greater risk of fasting include people:

• With type 1 diabetes
• With a history of eating disorders or disordered eating patterns
• Who are pregnant, trying to become pregnant, or are breast-feeding
• Taking certain medications
• With hypoglycemic tendencies
• With adrenal problems
• Who are underweight
• Who are severely ill or otherwise compromised

Generally, you need to be healthy enough to fast—physically, mentally, emotionally, and spiritually.

The Different Health-Related Fasting Categories

There is more than one category of fasting related to health and wellness. The main ones we’re going to review here are:

• Long-term medically supervised water fasts
• Intermittent fasting
• Time-restricted feeding
• Fasting-mimicking diet

Long-Term Medically Supervised Water Fast

Long-term medically supervised fasts are not as popular as they once were. This is when a person does not consume food for a long period under the close supervision of a medical team, frequently as an in-patient at a clinic. This is generally more popular for losing weight quickly or for severe health problems which longer fasts may be beneficial for.

Not as many studies focus on this type of fasting compared to the others, especially with the greater potential for harm with longer fasts. One randomized controlled trial on patients with type 2 diabetes found that a 7-day fasting program followed by dietary advice led to a greater weight loss after 4 months than a control group who had usual care and dietary advice. The trial group experienced 3.5 kg (7.7 lbs) weight loss compared to 2.0 kg (4.4 lbs) in the control group, and they also had a greater reduction in abdominal fat. Although this may be statistically significant, it was not clinically significant. There was also a significant decrease in blood pressure and an increase in quality of life. However, there were no differences in measurements for glucose control.

One systematic review on medically supervised water-only fasts demonstrated they may be safer than you might think, although there are still risks involved. The researchers looked at the adverse events corresponding to fasts that lasted at least 2 days and up to 41 days with supervised refeeding periods of half the fasting length. They found the adverse events were generally fatigue, nausea, insomnia, headache, hypertension, presyncope, dyspepsia, and back pain. Only two instances had serious adverse events requiring hospitalization, of which one was severe dehydration and the other was hyponatremia. About 6.5% of the visits had no adverse events, and the majority (65.8%) experienced mild to moderate adverse events.

Intermittent Fasting

Intermittent fasting is the fasting category currently making headlines due to its potential health benefits. As the name suggests, this method includes periods of eating normally and then fasting on a particular schedule. Common options include:

• 5 days eating, 2 non-consecutive days fasting
• Alternate-day fasting
• Modified alternate-day fasting (low-caloric consumption on fasting days, generally in one or two small meals, with mealtime not playing a significant role)

Some of the proposed mechanisms behind the benefits of fasting include hormesis (beneficial stress and adaptive changes), reduced oxidative stress, reduced inflammation, increased thermogenesis and browning of adipose tissue, circadian rhythm restoration, and beneficial hormonal changes. Metabolic flexibility, or the body moving back and forth from using glucose for fuel and using fat in the form of ketone bodies, may also be the reason for the efficacy of fasting. However, it may simply be that the total calories consumed over a longer period are less than the total caloric consumption of non-fasting individuals.

So, what are the main benefits demonstrated in studies? Most of the studies look at weight loss and metabolic markers, although some also look at cognitive function and the prevention or reversal of age-related diseases, including neurodegeneration and dementia.

One study on a 24-hour water-only fast found a reduction in TMAO (trimethylamine N-oxide, a biomarker from intestinal microbiota linked to cardiovascular disease) levels in a similar manner whether the participants fasted for 24 hours and then ate ad-libitum for 24 hours or vice versa. However, the TMAO levels returned to baseline 24 hours after refeeding. Although this study was short, the researchers postulate that more regular intermittent fasting may have a longer-lasting beneficial impact on TMAO levels through more sustainable changes to the gut microbiota.

One clinical trial on alternate-day fasting (ADF) for 4 months with metabolic syndrome patients found a greater reduction in BMI, body weight, and fat mass compared to those who were on a calorie-restricted (CR) regimen. Additionally, the participants on the ADF diet demonstrated a greater reduction in hs-CRP concentrations, prothrombin time, and fibrinogen levels (markers of inflammation). One review article found that alternate-day fasting that lasted 3 to 12 weeks led to a reduction in body weight, body fat, total cholesterol, and triglyceride levels. Whole-day fasting for 12 to 24 weeks also improved body weight and composition as well as lipids. They did not find sufficient research on time-restricted feeding for a consensus. One meta-analysis and systemic review including 7 randomized control trials evaluating the effects of alternate-day fasting on adults found it to be an effective method of reducing weight, BMI, total cholesterol, LDL, triglycerides, fat mass, lean mass, and blood pressure compared to control groups. It should be noted that the control groups in this review maintained their current lifestyle, or were directed to eat without any time restriction.

In a rat study, intermittent fasting protected against Alzheimer’s disease and improved memory function. However, in estrogen-deficient rats, the glucose levels and HOMA-IR were elevated during fasting, showing that it may be contraindicated in postmenopausal females with impaired glucose metabolism.

In a case study on 3 patients with type 2 diabetes, alternate-day fasting and 3-day-a-week fasts, in which participants consumed only dinner on fasting days and all 3 meals on non-fasting days, the clients all were able to go off their insulin medication. Additionally, they experienced weight loss of at least 10% and improved HbA1c levels.

Many studies on intermittent fasting are done on subjects undergoing fasting for Ramadan. During Ramadan, it is traditional to fast between dawn and dusk, including no drinking or eating during this time. One study on healthy, lean men found no changes in glucose metabolism, cognitive function, or body composition during Ramadan fasting, which was 14 hours of abstinence from food and drink. A study of healthy, college-aged males found no significant changes in body composition but did find during Ramadan fasting both systolic blood pressure and fasting glucose levels elevated significantly.

Another study on patients with psoriatic arthritis undergoing Ramadan fasting found a statistically significant reduction in CRP levels as well as an improvement in markers for disease activity independent of any weight change. Another Ramadan study on healthy volunteers found that inflammatory markers were significantly lower after fasting during Ramadan. A meta-analysis on Ramadan fasting in type II diabetic patients found that Ramadan fasting reduced  BMI, fasting plasma glucose, HbA1c., and body mass index. The paper does note that the meta-analysis was not able to review for incidences of hypoglycemia and longevity of the results since only a small number of studies included this data. Some people also find that fasting triggers migraines.

Participants in a randomized control trial on healthy men and women aged 20–45 years were split into groups following a 12-hour eating window or a 6-hour eating window. Participants following in the 6hr group had a reduction in mean plasma glucose, and the majority of the difference was seen in the evening and while sleeping. The control group’s glucose levels remained elevated into sleep periods. The time-restricted feeding group also demonstrated lowered fasting glucose, insulin, and Akt2 indicating that there may have been an improvement in insulin signaling in the morning.  There was an increase in HDL and LDL cholesterol and β-hydroxybutyrate in the morning In the time-restricted feeding group, which may indicate a higher level of circulating ketones possibly reducing oxidative stress and hunger levels. Lastly, the time-restricted group demonstrated improved cortisol rhythms to promote a healthy sleep-wake cycle and an increase in circadian genes that drive protein synthesis and regulate cell growth, differentiation, and metabolism. While this study looked at both men and women, the majority of studies on fasting primarily focus on men.

Not all studies demonstrate benefit to intermittent fasting. In one rat study, an intermittent fasting meal pattern of one meal per day led to higher glycogen storage in liver and skeletal muscle as well as an increase in markers for insulin resistance despite a reduction in fat and increase in brown fat. In one human study on 14 adults with type 2 diabetes split into two groups, one 10 hour time-restricted feeding group and one 14-hour feeding group, the time restricted feeding group demonstrated 24-hour glucose homeostasis but no changes in insulin sensitivity or hepatic glycogen. Another randomized human trial on 116 overweight, obese adults looking at time-restricted feeding compared to consistent meal timing concluded that time-restricted feeding was not more effective for weight loss than consistent eating, and neither group demonstrated significant differences in changes in fasting insulin, fasting glucose, hemoglobin A1c levels, estimated energy intake, total energy expenditure, and resting energy expenditure.

A systematic review and meta-analysis found that after short-term severe caloric restriction and fasting, there was an increase in serum cortisol levels. However, this increase was in the initial period and tended to decrease after several weeks. It is important to take note that the stress of fasting or caloric restriction may impact the HPA axis and cortisol levels which in turn may lead to some negative outcomes in the short-term although it does not necessarily cause similar effects in the long-term. Some people also find that fasting triggers migraines.

Another important consideration is that fasting may alter the hedonistic quality of food or how much you like it. One study found that people had a stronger liking and desire regarding their favorite foods, including just images, after 24 hours of fasting compared to those in a fed state.

The benefits of intermittent fasting may not be any superior to that of long-term caloric restriction. In a study with 16 healthy individuals, researchers assigned participants either a 5:2 style intermittent fasting diet (fasting for two days and eating normally for the other 5 days) or a continuous energy restriction diet. Between groups, there were no significant differences between post-prandial insulin, glucose, or appetite ratings, but the intermittent fasting pattern group did demonstrate improvements in fasting glucose levels.  A similar randomized controlled trial found a similar but not superior difference between a 5:2 intermittent fasting pattern and continuous caloric restriction in overweight and obese individuals for a 12-week period. The weight change was 7.1% reduction in intermittent fasting and 5.2% reduction in caloric restriction, while the control group experienced a reduction in 3.3%. The intermittent group may have had a greater weight loss during the 12 weeks; there was no difference in the genetic tests that are linked to obesity and chronic diseases nor in the metabolic biomarkers tested. Additionally, at the 26-week follow-up, the weight loss was 5.2% in intermittent fasting, 4.9% in the caloric restriction diets, and 1.7% in the control.

One study on insulin-resistant participants analyzed the difference between alternate-day fasting versus daily calorie restriction and found no difference in weight loss, BMI, and fat mass reduction. However, alternate-day fasting did produce greater decreases in fasting insulin and HOMA-IR levels. Neither group demonstrated changes in body composition, lipid profiles, blood pressure, or markers of inflammation.

In general, the studies pointing to fasting show potential for stress resistance, enhanced cell and DNA repair, autophagy induction, and enhanced mitochondrial function. These effects impact metabolic health, weight loss, and circadian rhythms. It may be too early to make a strong case for efficacy above other options or for long-term benefits. Many of the studies analyze men, post-menopausal women, and those who are already obese and/or at risk of metabolic diseases rather than healthy individuals. More research is needed to determine the appropriateness and benefits of intermittent fasting for different health circumstances.

Time-Restricted Eating

Frequently lumped in with intermittent fasting, time-restricted eating (also known as time-restricted feeding or TRF) may also be recognized as its own category of fasting. This is where you eat only during a particular period of the day, generally reducing eating time to an 8-hour window, although this may be more or less. For example, if you eat dinner at 6:00 pm, you would not eat again until 10:00 am the next day.

An evaluation of time-restricted feeding reviewed human studies and found an association between TRF and reduced body weight and several markers associated with metabolic diseases, such as insulin sensitivity.

In one study looking at data from the 2005 – 2016 NHANES found that for every one-hour increase in nighttime fasting, there was a 7% increase in the odds of abnormal insulin and a 1% increase in the odds for abnormal HDL cholesterol.. The timing of meals matters, as the same study found that participants with the latest meal first mealtime compared to the earliest first mealtime had statistically significantly greater insulin levels. With every one-hour increase in the time of the first meal, there were increases in CRP, HbA1c% and glucose, and decreases in HDL cholesterol.

In one pilot study on humans, time-restricted eating of 8-hour eating window led to a reduction in weight and systolic blood pressure, although other markers of obesity and metabolic syndrome were not significantly different from controls after 12 weeks.

One mouse study found that when eating a high-fat diet for 8 weeks, restricting food consumption to 6 hours led a reduction in weight gain and blood lipid levels. Those who consumed more food earlier in the feeding time also had more pronounced benefits. Although some inflammatory markers were better, there were still some inflammatory markers that were increased in both groups.

In a rat study, time-restricted feeding for 9 hours during the normal awake phase on weekdays but not on weekends still led to a reduction in weight gain. The rats consumed a similar high-fat diet ad libitum during the hours allowed to eat for 9 weeks, and they were their own control, and the reduction in weight gain occurred despite consuming similar total caloric intake to rats not in the time-restricted feeding groups.

One randomized control trial on 82 non-obese human participants compared the effects of time restricted eating between 6:00am-3:00pm, time restricted eating between 11:00am-8:00pm, and a control group with no restriction on meal time for 5 weeks. The group with restricted eating starting earlier in the day demonstrated improvements in fasting plasma glucose, body mass, adiposity, inflammation, and gut microbiota diversity compared to the control group and the group starting meals later in the day.

Some of the benefits of time-restricted feeding may come from its influence on circadian rhythms if the feeding window is during the normal awake phase (daytime for humans). However, there is limited data on whether time-restricted feeding during non-awake time would provide the same benefits. Phase shifting, or eating outside the natural circadian rhythm, has been linked to harm, such as an increased risk of diabetes and obesity.

Fasting-Mimicking Diet

 A diet known as the fasting-mimicking diet (FMD) is an eating style that makes the body think it was fasting but still allowed for individuals to consume foods.  It is low-calorie, high-fat, and low-carb and uses whole-food and plant-based foods. In mouse models, one study on the fasting-mimicking diet was able to restore insulin generation in the pancreatic cells in those with type 1 diabetes, which is an autoimmune condition whereby the body attacks the cells that create insulin. Another mouse study found that in a model of type 2 diabetes, the FMD led to normalization of blood glucose levels and increased insulin sensitivity. Beta cell function was also improved. Additionally, there were beneficial changes to the gut microbiome, reducing the levels of certain problematic species and increasing commensal ones.

In another mouse model, the fasting-mimicking diet reduced MS symptoms and did so better than a ketogenic diet. In 21.7% of one group, the FMD reversed the severity score to 0 and below 0.5 in half of the mice. There were also beneficial changes in the underlying causes of MS symptoms, including inflammation and the activity of immune cells. Another mouse study also found benefits to Parkinson’s disease, increasing brain-derived neurotrophic factor levels and decreasing the number of glial cells and certain inflammatory cytokines, reducing neural inflammation overall.

In a mouse model for IBD, the fasting-mimicking diet for four cycles (five days on, nine days off) led to a reduction in several symptoms of IBD as well as some of the pathological causes of the disease. There was a reduction in systematic as well as intestinal inflammation levels as well. There was also regeneration of the intestinal lining cells and benefits to the makeup of the gut microbiome.

In a human study, 44 omnivore participants were asked to either continue their unrestricted diet supplemented with 4 servings of vegetables while the other group consumed the FMD for five days.  Seventy-five percent of the participants who followed the FMD experienced a reduction in trimethylamine N-oxide (a microbiota metabolite associated with a higher risk incidence of adverse cardiovascular outcomes), a reduction in body weight, and an improved HOMA-IR index.

Conclusion 

There are other fasts not covered in this blog, including juice fasts or detox fasts. Many of these are not well-researched and are too frequently promoted by those not well-informed of the benefits or potential risks or harms.

Despite the potential benefits found in many of the studies, there remain some areas of caution and concern. For one, many of the studies remain in animal, mechanistic, or early-phase human studies. They also are short-term and do not look into the impact on long-term adoption of this lifestyle, especially in humans.

Additionally, in many of the studies on fasting, the participants or the animals were able to eat ad libitum during the non-fasting hours, which may have included high-fat, processed foods and/or energy-dense foods compared to nutrient-dense foods. Exercise and other lifestyle factors were likewise not always accounted for during the study.

Studies on meal patterns that are plant-based, such as the traditional Mediterranean diet, demonstrate similar benefits in terms of weight loss and metabolic diseases, as well as a reduction in mortality. For example, one meta-analysis found that those who had a higher adherence to the Mediterranean diet had a 18% less chance of developing type 2 diabetes. Thus, there are many ways to achieve the same results, and while fasting may have demonstrated some benefits, it may not be the right avenue for you to pursue to achieve the results.

If you choose to undertake any kind of fasting, make sure to discuss it with your doctor and/or another healthcare practitioner to ensure you are an appropriate candidate—and which type of fasting might be most appropriate for your situation. It may take some time to find the right level of fasting for you that provides you with the purported benefits.