“Intermittent fasting is harmful to human health.”

Intermittent fasting is generally safe and produces few gastrointestinal, neurological, hormonal or metabolic adverse effects.

The current evidence provides some indication that intermittent fasting diets have similar benefits to continuous energy restriction for weight loss and cardiometabolic risk factors. … Of the 29 studies that described some adverse events, all but one was considered severe. Most trials reported mild side effects including constipation, nausea, hunger, diarrhoea, and dizziness. … Our network meta-analysis showed similar benefits for various intermittent fasting strategies and CER in cardiometabolic risk compared with ad-libitum diets.

Several meta-analyses of randomized clinical trials (RCTs) have demonstrated the many health benefits of intermittent fasting (IF). … In this umbrella review, we found beneficial associations of IF with anthropometric and cardiometabolic outcomes supported by moderate to high quality of evidence, which supports the role of IF, especially modified alternate-day fasting, as a weight loss approach for adults with overweight or obesity.

In adults with type 2 diabetes, diverse types of intermittent fasting, such as time-restricted eating, the 5:2 diet, and the fasting mimicking diet, reduce HbA1c (typically by 0·3–1·2%), fasting glucose, 24-h glucose, and bodyweight relative to usual eating patterns. … Moreover, intermittent fasting is as effective as calorie restriction for improving glycaemic control and does not increase the risk of hypoglycaemia when medications are adjusted using simple rules.

This meta-analysis suggested that IF was not associated with a greater risk of AEs in adult patients affected by overweight or obesity.

Our findings suggest that IF may have beneficial effects on a range of health outcomes for adults with overweight or obesity, compared to CER or non-intervention diet. … Specifically, IF may decreased WC, fat mass, LDL C, TG, TC, fasting insulin, and SBP, while increasing HDL-C and FFM.

In conclusion, the current systematic review and meta-analysis demonstrate that fasting regimens may help improve glycemic control by significantly reducing the FBS, HbA1c, and HOMA-IR levels. Similarly, IF was also successful in reducing the LDL-C levels.

Overall this systematic review and meta-analysis demonstrated that fasting state contributed to improved glycemic control including FBS, HbA1c, and HOMA-IR levels. In addition, fasting intervention caused to an improvement in lipid metabolism (LDL-c) and inflammatory state indicated by IL-6.

This meta-analysis suggested that IF was not associated with a greater risk of AEs in adult patients affected by overweight or obesity.

The current evidence provides some indication that intermittent fasting diets have similar benefits to continuous energy restriction for weight loss and cardiometabolic risk factors. … Assessment of adverse events were reported in 56 trials, of which 27 trials reported no harmful events in the intermittent fasting groups. Of the 29 studies that described some adverse events, all but one was considered severe. Most trials reported mild side effects including constipation, nausea, hunger, diarrhoea, and dizziness.

Intermittent fasting increased IMRS (0.78 ± 2.14 vs. controls: −0.61 ± 2.56; p = 0.010) but interacted with baseline IMRS (p-interaction = 0.010) to reduce HOMA-IR (but not MSS) more in subjects with higher baseline IMRS (median HOMA-IR change: fasters, −0.95; controls, +0.05) vs. lower baseline IMRS (−0.29 vs. −0.32, respectively). … Intermittent fasting increased 1-year IMRS mortality risk, but decreased 10-year chronic disease risk (PCRE and ICHRON). … The stress of fasting may be too great for some people, perhaps those who are already experiencing health challenges such as advanced comorbidities, frailty, or uncontrolled metabolic disease, and this raises important safety concerns regarding intermittent fasting.

Compared to regular dietary advice, intermittent fasting may result in little to no difference in weight loss or quality of life. Intermittent fasting may result in little to no difference in adverse events, but the evidence is very uncertain.

This meta-analysis suggested that IF was not associated with a greater risk of AEs in adult patients affected by overweight or obesity. … Overall, these data showed that IF regimens are not associated with higher risk of any major AEs when compared to a usual diet or other active comparators.

Our findings suggest that IF may have beneficial effects on a range of health outcomes for adults with overweight or obesity, compared to CER or non-intervention diet. Specifically, IF may decreased WC, fat mass, LDL-C, TG, TC, fasting insulin, and SBP, while increasing HDL-C and FFM.

Low-frequency 24-h water-only intermittent fasting did not reduce LDL-C or improve markers of cognitive function but did reduce both HOMA-IR and MSS. … As a genetically encoded response with few major side effects and a very low financial cost, intermittent fasting has the potential to be used by most people worldwide as a clinical therapeutic to treat cardiometabolic disease. … More importantly, low-frequency intermittent fasting may be an optimal preventive lifestyle that can produce life-long impact on many cardiometabolic risk factors and help inhibit the development of chronic disease.

No adverse effects were attributed to ADF, and 93% completed the 8-week ADF protocol. … ADF is a safe and tolerable approach to weight loss.

Whereas the few randomized controlled trials and observational clinical outcomes studies support the existence of a health benefit from fasting, substantial further research in humans is needed before the use of fasting as a health intervention can be recommended.

In adults with type 2 diabetes, strong recommendation against intermittent fasting (IF) due to the lack of evidence on its benefits and harms, and risk of hypoglycemia [Strong recommendation against, low quality of evidence].

In adults with overweight or obesity, IF and CR are comparably effective for reducing body weight and adiposity, as well as for improving cardiometabolic health markers.

Research shows fasting for a certain number of hours each day or eating just one meal a couple days a week may have health benefits. … Overall, research continues to indicate that it is a safe way to reduce weight. However, it’s important to check with your doctor before trying intermittent fasting.

Most IF diet regimens induced significant weight loss that was comparable with that induced by CER diet regimens with a similar absolute energy restriction, based on low- to high-certainty evidence. … Similar results were noticed for body measurements, blood pressure, blood lipids, and glycemic profiles.

Modified alternate-day fasting, and time-restricted eating appear to be promising approaches for reducing most cardiovascular risk factors. … These intermittent fasting methods may be considered as potential components of lifestyle interventions aimed at managing cardiovascular disease risk factors.

Accordingly, the available data do not allow us to unequivocally assert a link between IF and depression, overeating, disruption of the HPO axis, and physical activity. At the cellular level, IF may increase resistance to oxidative stress, reduce the severity of inflammation, and contribute to an increase in lifespan. However, at present, most of the results have been obtained in animal experiments and, accordingly, are predominantly speculative in nature.

The current evidence provides some indication that intermittent fasting diets have similar benefits to continuous energy restriction for weight loss and cardiometabolic risk factors.

"In really good studies," Dr. Schriever continued, "where they compare people who are doing intermittent fasting and those doing caloric restriction, there really isn't any significant difference in weight loss. There's no significant difference in heart health or other metabolic markers. … "Anyone with a history of eating disorders really should be cautious about considering it," Dr. Schriever said. "Anyone with diabetes should do it cautiously and not without medical supervision. Anyone who is older … I'd be worried about because we want to watch their nutritional intake. … By not eating a meal or only eating during a limited time frame, "you have a shorter time frame to get your nutrients in," Dr. Schriever continued. "You might become vitamin deficient or might struggle to get enough protein in to keep your muscle mass up. … Depending on how your insulin response is and blood sugar response is, you might have lower blood sugar and just really feel foggy or not well. And also, that can disrupt sleep."

Research shows that intermittent fasting may improve some signs of health in the short term. … While intermittent fasting is safe for many people, it may not be a healthy pattern for people who: - Have an eating disorder. - Are pregnant or breastfeeding. - Are at high risk of bone loss and falls.

Several randomized trials have found that intermittent fasting leads to modest weight loss and may reduce risk factors linked to heart disease, including diabetes, high blood pressure, unhealthy blood lipid levels, and inflammation. … Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes. Some people who take medications for blood pressure or heart disease also may be more prone to imbalances of sodium, potassium, and other minerals during longer-than-normal periods without food.

Despite the promising outcomes of intermittent fasting, it does not come without side effects. Unfortunately, evidence documenting the ill effects of intermittent fasting regimens is sparse, primarily because the duration of assessing intermittent fasting regimens is weeks to months. Some of the commonly reported adverse effects include hypoglycemia, dizziness, and weakness.

It is unclear that intermittent fasting is superior to other weight loss methods in regards to amount of weight loss, biological changes, compliance rates, and decreased appetite. … More high-quality studies including randomized controlled trials with follow-up of greater than one year are needed to show a direct effect and the possible benefits of intermittent fasting. Strong recommendations on intermittent fasting for weight loss cannot be made at this time.

Even with net energy intake restricted to that of daily dieters, alternate-day fasting less effectively reduced body fat content and offered no additional short-term improvements in metabolic or cardiovascular health compared to daily energy restriction. … Alternate-day fasting less effectively reduces body fat mass than a matched degree of daily energy restriction and without evidence of fasting-specific effects on metabolic regulation or cardiovascular health.

Studies on intermittent fasting have found that this approach to eating may carry some risks along with its potential benefits. They can range from simply making you feel lousy while you’re fasting to more serious threats to your health. … A large study reported at an American Heart Association conference in 2024 found that people who followed a time-restricted eating plan were 91% more likely to die of cardiovascular disease than others who ate a typical diet.

Before weighing intermittent fasting side effects, it's important to know that there are several forms of intermittent fasting, and the evidence of their long-term effectiveness and safety is not yet known. … But we do know intermittent fasting could be risky in some cases. If you're already marginal as far as body weight goes, you could lose too much weight, which can affect your bones, overall immune system, and energy level. … Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes.

Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses of randomised controlled trials.