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10 Intermittent Fasting Benefits + Who Shouldn’t Try It

Written by Carlos Tello, PhD (Molecular Biology) | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Carlos Tello, PhD (Molecular Biology) | Last updated:

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Intermittent Fasting
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Dieting can be a difficult and frustrating way to lose weight. Research suggests that intermittent fasting is a simple and effective strategy for getting lean while maintaining muscle mass. Read this article to learn about the potential benefits of intermittent fasting and whether you should try it.

What Is Intermittent Fasting?

Intermittent fasting is a technique primarily used for losing weight that may have several other health benefits. Because it only requires changes in eating patterns but allows to maintain the same diet and exercise level, some people may find it easier to follow this weight-loss strategy.

There are many different ways to fast. Common practices include:

  • Alternate day eating: one day on and one day off
  • Time-restricted eating: typically involves skipping breakfast
  • 5:2 fasting protocol: restricting calories to 500-700 per day for two days a week

Proponents of intermittent fasting claim that it allows the body to enter its highest fat-burning state, which occurs about 8-12 hours after eating a meal, and eliminate fat without sacrificing muscle mass. A typical eating schedule does not normally allow the body to reach this point.

Snapshot

Proponents

  • May help lose weight
  • May reduce the risk of heart disease
  • May help lower blood sugar
  • May help reduce inflammation and boost immunity
  • May improve mood
  • May help prevent breast cancer
  • May help with IBS

Skeptics

  • Insufficient evidence for most benefits
  • May cause hunger, headaches, and lightheadedness
  • Uncontrolled fasting is extremely unsafe
  • Not recommended in multiple groups of people
  • May reduce exercise performance
  • May disturb sleep patterns
  • Unclear effects on cognition

Health Benefits

Possibly Effective for:

1) Weight Loss

Overweight People

Studies have found the 5:2 fasting protocol most effective for losing weight. A trial on 115 overweight women found that those who steadily reduced their daily calorie intake became more insulin-sensitive and lost more weight than those following a regular eating pattern [1].

Combining a low-calorie diet with one day of severe calorie restriction (120 kcal) per week reduced body weight and blood cholesterol (total and LDL) levels in a clinical trial on 54 obese women, especially when combined with a liquid diet [2].

In another trial on 56 slightly overweight but otherwise healthy men, 5 days of a low-calorie (300-500 kcal below their usual intake) combined with 2 days of Muslin fasting per week reduced body weight, body fat, blood pressure, and blood cholesterol [3].

Importantly, 2 of the 3 studies didn’t include a control with a steady low-calorie diet without intermittent fasting (which is proven to help lose weight), making it difficult to estimate the contribution of intermittent fasting to the effects observed.

In another trial on 40 obese people, a low-calorie, protein-paced (6 meals per day and >30% proteins) diet with 1 fasting day per week reduced body weight and body fat. In 24 people who completed the weight-maintenance phase, the diet prevented weight regain. Again, a control without the fasting day is needed to assess the contribution of intermittent fasting [4].

A more recent trial on 112 people comparing intermittent fasting and steady-eating with calorie restriction found that both strategies were similarly effective to lose weight, but intermittent fasting increased feelings of hunger [5].

Similarly, both intermittent fasting and calorie restriction helped lose weight and improve emotional well-being in a clinical trial on 36 people with multiple sclerosis [6].

A study in mice genetically prone to high blood cholesterol found that intermittent fasting induced obesity, diabetes, and artery-clogging in this case, suggesting it may not be recommended in people with high blood cholesterol levels [7].

Normal-Weight People

In a clinical trial on 34 resistance-trained men, consuming all the daily calories in 3 meals within a time range of only 8 hours for 8 weeks reduced fat mass when compared to eating 3 meals at normal times [8].

In another trial on 18 lean people, severe calorie restriction during 24 hours increased subjective appetite but didn’t increase the levels of the appetite-promoting hormone ghrelin accordingly, suggesting it may help reduce food intake and weight gain [9].

Another trial on 36 healthy people found that fasting for 1 day/week for 8 weeks reduced body fat mass without changing any other metabolic parameters [10].

Although limited, the evidence suggests that intermittent fasting may help lose weight. However, doing more exercise and improving your diet may be a safer and more effective strategy. Consult it with your doctor before starting an intermittent fasting regime and carefully follow their recommendations.

2) Preventing Heart Disease

In 6 clinical trials on over 300 people (most of them overweight or obese), intermittent fasting lowered blood pressure, heart rate, and blood fat levels (total cholesterol, LDL cholesterol, and triglycerides). However, it had no effect on the risk parameters of 34 resistance-trained men [11, 2, 12, 5, 3, 13, 8].

In another trial on 30 healthy people, intermittent fasting reduced the production of an oxidant compound associated with heart disease (TMAO) by the gut bacteria [14].

Although no studies have directly investigated the incidence of heart disease in people doing intermittent fasting, several found that this practice reduced risk factors such as blood pressure and fat levels. You may discuss with your doctor if you may implement it as a complementary approach. However, intermittent fasting should never replace any medications or lifestyle interventions prescribed or recommended by your doctor.

Insufficient Evidence for:

1) Lowering Blood Sugar

In a small trial on 8 healthy men, intermittent fasting (for 20 hours every second day during 15 days) increased sugar uptake from the blood in response to insulin [11].

In another trial on 63 obese or overweight people, intermittent fasting (5:2) was as effective as continuous energy restriction at improving blood sugar control (measured as glycated hemoglobin levels) [15].

In a clinical trial on 12 men at risk of developing diabetes, early-time fasting improved insulin sensitivity and reduced blood pressure and oxidative damage [12].

In a clinical trial on 41 people with type 2 diabetes, intermittent fasting (5:2) improved sugar control and blood sugar levels. The effects were the same regardless if the fasting days were consecutive or not [16].

However, a one-week fasting period reduced weight and blood pressure but had only non-significant effects on blood sugar control and insulin resistance parameters in another trial on 32 diabetics [17].

In a recent trial on 137 people with type 2 diabetes, intermittent fasting even increased glycated hemoglobin in the long term (24 months) [18].

A few small trials with mixed results cannot be considered sufficient evidence to support the use of intermittent fasting to improve blood sugar control. Larger, more robust clinical trials are needed to shed some light on this potential benefit.

2) Antioxidant

In a clinical trial on 43 obese people, a low-calorie, protein-paced (6 meals per day and >25% proteins) diet with 1 fasting day per week improved antioxidant status [19].

In another trial on 24 healthy people, intermittent fasting increased the production of a protein that protects from oxidative stress (SIRT3) [20].

Reducing oxidative damage may increase lifespan by protecting from diseases and delaying aging. Indeed, intermittent fasting and calorie restriction increased lifespan by up to 30% in several animal studies [21, 22, 23].

Two small clinical trials and some animal research cannot be considered sufficient evidence that intermittent fasting has antioxidant effects in humans. Further clinical research is needed.

3) Boosting Immunity

In mice with multiple sclerosis, intermittent fasting ameliorated the symptoms, improved the composition of the gut microbiota, and reduced Th17 cells while increasing Tregs. In a small trial on 17 people with this condition, intermittent fasting caused similar changes in the gut microbiota and immune function [24].

In a study in mice infected with salmonella, intermittent fasting lowered intestinal and systemic bacteria by boosting their intestinal immune system (IgA production) [25].

White blood cells obtained from asthmatic people after intermittent fasting produced fewer proteins involved in the development of Th2 cells and prevented lung lining cells from producing inflammatory cytokines in response to dust mites [26].

Again, the evidence is insufficient to attest to the role of intermittent fasting in boosting immunity. More clinical trials on larger populations are needed.

4) Mood

In a clinical trial on 31 aging men, intermittent fasting with calorie restriction improved vigor and reduced tension, anger, confusion, and mood disturbances. However, it had no effect on depressive mood [27].

In another trial on 36 healthy volunteers, intermittent fasting for 8 months improved general well-being during the 6-month follow-up period [10].

Once again, 2 small clinical trials cannot be considered sufficient evidence that intermittent fasting improves mood. More clinical research is needed to confirm these preliminary findings.

5) Preventing Breast Cancer

An observational study on over 2,600 associated restricted energy intake and fasting for longer intervals during the night with a reduced incidence of breast cancer [28].

However, remember that a cause-and-effect relationship hasn’t been established. In addition, dietary, environmental, and genetic factors could have had a greater influence on the incidence of cancer in several cases.

6) Inflammation

In a pilot trial on 18 mild asthmatic people and another trial on 19 healthy people, fasting for 24 hours reduced the activation of a pro-inflammatory protein involved in the innate immune system (NLRP3) [26, 29].

In animal and cell-based studies, fasting reduced pro-inflammatory cytokines and immune cells, while increasing ghrelin (which suppresses inflammation) [30].

Two small clinical trials with modest results and some animal and cell-based research cannot be considered conclusive to claim that intermittent fasting may reduce inflammation. The results are very preliminary and should be replicated in further clinical trials on larger populations.

7) Irritable Bowel Syndrome

Fasting therapy improved stomach pain, diarrhea, nausea, and anxiety in a clinical trial on 58 people with IBS [31].

It may do so by increasing the production of motilin and ghrelin, two hormones that promote gastrointestinal motility and gastric acid secretion [32].

More clinical research is needed to confirm the potential benefits of intermittent fasting in people with IBS.

8) Cognitive Function

In 2 clinical trials on 38 Muslims during Ramadan fasting, reaction times to stimuli increased. However, a trial on 8 healthy Muslim volunteers found that Ramadan fasting didn’t cause drowsiness or increased reaction times under controlled conditions of calorie intake, circadian rhythm, and sleep quality [33, 34, 35].

In a clinical trial on 50 elderly subjects, calorie restriction increased verbal memory [36].

In animal studies, intermittent fasting promoted the growth of brain cells (especially in the hippocampus) and reduced their death by making them more resistant to oxidative damage, repeated overactivation, and inflammation in animal studies [37].

This may enhance cognitive function and protect the brain from low blood supply or from the neurodegenerative processes occurring during aging and diseases such as Alzheimer’s and Parkinson’s, as seen in rats and mice [38, 39, 40, 41, 42].

Although the results from animal studies are promising, the evidence from clinical research is limited and contradictory. Further studies are needed to shed some light on this topic.

Animal and Cell Research (Lack of Evidence)

No clinical evidence supports the use of intermittent fasting for any of the conditions listed in this section. Below is a summary of the existing animal and cell-based research, which should guide further investigational efforts. However, the studies should not be interpreted as supportive of any health benefit.

Autoimmune Disease

Fasting every three days reduced multiple sclerosis and lupus erythematosus symptoms in mice [43, 24, 44].

Wounds

In mice, intermittent fasting improved wound healing [45].

Chronic Pain

Intermittent fasting enhanced neuroplasticity and learning in animal studies, which can aid in healing chronic pain [46].

Possibly Ineffective for:

Improving Sleep Quality

Three clinical trials on over 600 people found that Ramadan fasting delayed bedtime and wake time [47, 48, 49].

In 4 clinical trials on over 600 people, Ramadan fasting reduced sleeping time. However, this parameter remained unchanged in another study on 41 people [50, 48, 51, 49, 47].

Ramadan fasting had no effect on sleep architecture, except for reducing REM sleep, in 3 small trials on 27 people. Intermittent fasting also reduced REM sleep in animal studies [52, 53, 54, 55].

While 3 clinical trials on over 300 people found increased daytime drowsiness during Ramadan fasting, 5 studies on over 400 people failed to observe this effect [56, 49, 57, 47, 48, 58, 53, 35].

Taken together, intermittent fasting doesn’t seem to improve sleep quality. Its most clear effects are a delay in bedtime and waking time and a reduction in REM sleep. Research also suggests that it reduces sleeping time, although the results are mixed and the quality of some studies was low. Concerning daytime drowsiness, larger, more robust clinical trials are needed before drawing any conclusions.

Exercise Performance

In a clinical trial on 10 Muslim soccer players, Ramadan fasting modified the circadian rhythm of muscle power and fatigue by lowering peak and mean power in the evening while increasing perceived fatigue [59].

This effect could be counteracted by reducing resistance training volume and maintaining a normal body fluid balance, daily energy intake, and sleep duration during Ramadan in 2 trials on 36 [60, 61].

In a trial on 21 active men, 3-day intermittent fasting had positive effects on the blood fat profile but reduced speed and power performance [13].

However, it didn’t reduce muscle gain or cause muscle loss in 2 clinical trials on 52 active men [8, 62].

Another trial on 20 cyclers found that intermittent fasting initially reduced performance but the body adapts to the new dietary regime after 10 days [63].

To sum up, intermittent fasting may have some negative effects on exercise performance that may be prevented by controlling the sleep cycle and food and fluid intake.

Potential Side Effects

This list does not cover all possible side effects. Contact your doctor if you notice any other side effects.

Call your doctor for medical advice about side effects. In the US, you may report side effects to the FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada, you may report side effects to Health Canada at 1-866-234-2345.

Controlled, mild intermittent fasting has generally been shown tolerable and safe, being the most common adverse effects reported hunger, lightheadedness, and headaches [64].

In contrast, fasting for excessive periods or restricting calorie intake too much may have severe adverse effects such as [64]:

  • Excessive weight loss
  • Malnutrition
  • Anemia
  • Diarrhea
  • Eating disorders
  • Organ damage
  • Poor immune function

For this reason, it’s important that you discuss it with your doctor before implementing any drastic dietary interventions such as intermittent fasting and always follow their recommendations carefully.

In a study in mice prone to high blood cholesterol (due to the lack of LDL receptors), intermittent fasting increased insulin resistance, and total and LDL cholesterol compared to those following a normal eating pattern [65].

In rats, intermittent fasting reduced the ability of the heart to pump blood (cardiac reserve), which increases the risk of heart failure [64].

Fasting also lowered thyroid hormones in animal studies [66].

Who Shouldn’t Do Intermittent Fasting

In general, people who should be excluded from intermittent fasting interventions are [64, 67]:

  • Children
  • Pregnant women or people trying to have a child
  • People who are severely underweight
  • People with diabetes or who get hypoglycemic
  • People with eating disorders
  • People with a history of cardiovascular disease (heart attacks, circadian rhythm issues, peripheral vascular disease, stroke)
  • People who received an organ transplant in the past year
  • People with mental health issues or substance abuse disorders
  • People who have problems related to the limbic system
  • People with hypothyroidism
  • People with high blood cholesterol

About the Author

Carlos Tello

Carlos Tello

PhD (Molecular Biology)
Carlos received his PhD and MS from the Universidad de Sevilla.
Carlos spent 9 years in the laboratory investigating mineral transport in plants. He then started working as a freelancer, mainly in science writing, editing, and consulting. Carlos is passionate about learning the mechanisms behind biological processes and communicating science to both academic and non-academic audiences. He strongly believes that scientific literacy is crucial to maintain a healthy lifestyle and avoid falling for scams.

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