Written by Deep Shukla — Fact checked by Hannah Flynn
Intermittent fasting is an eating pattern that involves alternating periods of fasting and eating. In other words, it restricts food intake to specific times.
There are multiple different approaches to intermittent fasting. Certain schedules only allow food intake during 8 hours of the day, whereas others restrict or reduce food intake to certain days of the week.
Multiple randomized clinical trials have shown that intermittent fasting can help reduce weight and improve metabolic and cardiovascular health.
Furthermore, researchers have conducted meta-analyses to summarize the findings of these clinical trials on the health benefits of the various forms of intermittent fasting. These meta-analyses have generally focused on the association between specific intermittent fasting methods and health outcomes.
An umbrella review inspected previous meta-analyses examining the impact of various intermittent fasting regimens on metabolic and cardiovascular health. It focused on investigating the health benefits of four intermittent fasting regimens.
What is an umbrella review?An umbrella review can make the data from a large number of meta-analyses more accessible to clinicians, allowing them to make recommendations to their patients. Umbrella reviews can also help researchers and clinicians to compare and contrast the merits of the different forms of an intervention, such as intermittent fasting.
The study found that intermittent fasting has associations with weight loss and improvements in metabolic and cardiovascular health.
The researchers also found that certain intermittent fasting methods were more effective in producing weight loss.
The study’s co-author Krista Varady, a professor of nutrition at the University of Illinois, Chicago, told Medical News Today, “This study is essentially a review of review articles. It demonstrates that the different forms of intermittent fasting, i.e., alternate day fasting, the 5:2 diet, and time-restricted feeding, are all effective weight loss interventions for people with obesity.”
“This review also shows that intermittent fasting may be an effective means of lowering heart disease risk by decreasing blood pressure, low-density lipoprotein (LDL) cholesterol, and triglycerides. The article also shows that these diets may help prevent type 2 diabetes by lowering insulin resistance and fasting insulin,” added Dr. Varady.
The study appears in the journal JAMA Network Open.
Intermittent fasting involves fasting for long enough durations to ensure the depletion of the sugar stores in the body, leading to the burning of fat to meet the body’s energy requirements.
The body converts excess carbohydrates consumed during a meal into glucose and subsequently stores them as glycogen in the liver in the presence of insulin. Insulin also promotes the storage of excess glucose in the form of fat.
As insulin levels start to decline between meals, glycogen in the liver breaks down to maintain blood glucose levels. More extended periods of fasting, such as those prescribed for intermittent fasting, lead to depleted glycogen stores in the liver. As a result, the body resorts to breaking down fat to sustain the normal functioning of the body and the brain.
Besides fat loss, intermittent fasting also promotes beneficial adaptive responses, such as increased insulin sensitivity and lower inflammation levels.
Intermittent fasting schedules can vary considerably. Commonly followed intermittent fasting regimens include alternate-day fasting, the 5:2 diet, and time-restricted eating.
There are two forms of alternate-day fasting: zero-calorie alternate-day fasting and modified alternate-day fasting.
Zero-calorie alternate-day fasting involves fasting on alternate days, with no calorie intake on these fasting days. In contrast, modified alternate-day fasting allows limited calorie intake on the fasting days.
The 5:2 diet involves fasting for 24 hours once or twice a week, whereas time-restricted eating involves fasting for at least 12 hours during the day.
Fasting’s effects on obesity, insulin
The present study reviewed 11 meta-analyses summarizing the results from randomized clinical trials involving intermittent fasting. Specifically, these meta-analyses included results from 130 clinical trials examining the impact of the four aforementioned intermittent fasting regimens on metabolic and cardiovascular health.
The researchers also analyzed the impact of bias and inconsistencies on the results of these studies. Accordingly, they graded these results as low, moderate, or high quality.
The researchers found that intermittent fasting has associations with weight loss, lower body mass index (BMI), and improvements in blood glucose levels, blood pressure, insulin sensitivity, and cholesterol levels.
The modified alternate fasting diet and 5:2 diet produced a weight loss of more than 5% in people with overweight or obesity. The zero-calorie alternate fasting diet and time-restricted diet did not have the same effect.
The analysis also showed that most of these studies were likely to be of low quality.
However, the researchers found more reliable evidence to support seven associations between intermittent fasting and health outcomes.
This included high- and moderate-quality data showing:
- Modified alternate-day fasting had associations with a greater decline in BMI or body weight in adults who were healthy, overweight, and obese than a regular diet.
- Zero-calorie alternate-day fasting and 5:2 diet had associations with a greater reduction in fat mass and fasting insulin levels, respectively, in adults with overweight or obesity than continuous energy restriction.
Long-term safety of fasting unknown
The authors conceded that there were certain limitations to their analyses.
The randomized clinical trials included in the umbrella review have generally focused on healthy individuals or individuals with obesity and other metabolic disorders, such as diabetes. Moreover, most of these clinical trials have assessed only the short-term safety of intermittent fasting.
The authors noted that the long-term safety of intermittent fasting remains to be studied. The safety and efficacy of intermittent fasting schedules also need to be tested in a more diverse group of individuals.
The study’s lead author Dr. Nathorn Chaiyakunapruk, a professor at the University of Utah, told MNT that there was a lack of evidence on the long-term efficacy and safety of fasting.
“There still is a paucity of evidence demonstrating clear and sustainable clinical benefits and safety of intermittent fasting in the long-term. Studies should also investigate the effects of intermittent fasting in a broader range of participants with a broader scope of outcomes such as cancer, cardiovascular event, liver outcomes, diabetes remission, and the gut microbiome.”
– Dr. Nathorn Chaiyakunapruk
“Studies directly comparing between types of [intermittent fasting] are lacking. Therefore, it remains unknown which type is better than the others,” added Dr. Chaiyakunapruk.
Similarly, Dr. Varady said that the next steps in research should focus on addressing the effects of intermittent fasting in different populations.
“In particular, more studies are needed in people with type 2 diabetes, fatty liver disease, polycystic ovarian syndrome (PCOS), inflammatory bowel disease (IBS), and thyroid disorders. It is of great interest to see if intermittent fasting can be used as a non-pharmacological therapy to help these individuals manage their conditions,” he said.
Dr. Benjamin Horne, a cardiovascular and genetic epidemiologist at the Intermountain Medical Center Heart Institute, Utah, pointed out another limitation.
“The paper did not examine what happens to people who stop their intermittent fasting regimen and whether the weight loss can be sustained without continuing the regimen. This is a critical issue that plagues all weight-loss diets, especially intense weight-loss diets,” he told MNT.
Dr. Horne also noted that the study suggests that intense intermittent fasting schedules, such as the modified-alternate day fasting and the 5:2 diet, produced the best results. However, these schedules may be hard to sustain over long periods.
Consult a doctor before fasting
Dr. Horne stressed that these intermittent fasting regimens may be unsustainable in the long run and said, “[f]uture studies need to evaluate the ability of the average person to adhere to these regimens.”
Although there are valid concerns about the safety of intermittent fasting, some concerns may be overstated.
“It’s important to note that intermittent fasting has been shown to be a safe diet therapy for weight loss. There are many concerns about intermittent fasting – for instance, people worry that these diets slow metabolism and cause eating disorders. Evidence from recent clinical trials shows that intermittent fasting has no negative effect on metabolic rate and does not cause eating disorders in people who do not have a history of eating disorders,” said Dr. Varady.
Kai Liu, a dietitian and a doctoral student at the University of Adelaide, Australia, advised that individuals consult with a doctor before embarking on an intermittent fasting diet, especially if they have special nutritional needs.
“I think dietary interventions, including intermittent fasting, should consider the nutritional need of the target population,” she said.
“For example, [intermittent fasting] trials that recruited people with diabetes suggested close monitoring from health professionals and active reviewing and adjusting the dose of hypoglycaemic agents. Also, [older] people tend to have an increased need for some micronutrients, such as calcium. Therefore, assessing people’s needs first is always crucial before giving dietary advice.
“[A]lways consult with health professionals first,” added Liu.