Is intermittent fasting overrated? New evidence questions its weight-loss edge

Why intermittent fasting is in the spotlight

Intermittent fasting has become a mainstream wellness trend. Over the past decade, it has been widely discussed in the media, promoted on social platforms, and endorsed by celebrities such as Jennifer Aniston , fitness personalities like Joe Rogan , and podcast hosts such as Gin Stephens.

Its appeal lies partly in its simplicity: rather than asking people to count calories or eliminate certain foods, it sets clear time boundaries around eating.

In an environment where many people feel overwhelmed by conflicting nutrition advice, intermittent fasting can seem refreshingly straightforward. The idea that simply adjusting when you eat might improve metabolism, burn fat more efficiently, or even extend lifespan is compelling.

However, popularity and persuasive messaging do not always reflect the strength of the scientific evidence. As enthusiasm for intermittent fasting has grown, so too has the need to carefully examine whether the claims surrounding it are supported by high-quality research.

What is intermittent fasting? A simple explanation

Intermittent fasting is an eating pattern which cycles between periods of eating and not eating. Instead of focusing on what foods you eat, it focuses on when you eat them. 

During fasting periods, you avoid food and drinks that contain calories. During eating periods, you can eat (ideally balanced and nutritious) foods within a set time.

Common examples include the 16:8 method, where an individual fasts for 16 hours each day and eats within an 8-hour window, and the 5:2 diet, where normal eating occurs five days per week and calorie intake is reduced to approximately 500–600 calories on two non-consecutive days.

The theory behind intermittent fasting is that fasting triggers certain metabolic changes. Insulin levels decrease, which may encourage the body to use stored fat for energy. Levels of human growth hormone may increase, and cellular repair processes such as autophagy are thought to be stimulated (source, source). These responses are often interpreted as survival‑oriented adaptations to periods of low food availability.

Supporters suggest that these changes could support weight management, and improve metabolic health. However, research in this area is limited.

It is also important to note that many people experience side effects, particularly when first starting intermittent fasting. Commonly reported side effects can include fatigue, irritability, headaches, dizziness, difficulty concentrating, and digestive issues such as constipation or bloating (source). 

Intermittent fasting is not suitable for everyone. It is generally not recommended for individuals with a current or past history of eating disorders, those under 18 years of age, or individuals who are pregnant or breastfeeding. People with underlying medical conditions or those taking medications that require regular food intake should seek medical advice before considering this approach.

What does the latest research say about intermittent fasting and weight loss?

This systematic review of 22 studies, involving nearly 2,000 adults found that intermittent fasting does not appear to outperform traditional dieting for weight loss. 

This research included adults who were over the age of 18 and who were overweight or obese. In these studies, participants were randomly assigned to follow an intermittent fasting plan, receive standard dietary advice, or receive treatment, which included being placed on a waiting list.  The waiting list meant that participants were scheduled to start a programme later, allowing researchers to compare them with people who began treatment straight away.

When compared with standard dietary advice, intermittent fasting may make little or no difference in weight loss. The evidence suggests that any difference in weight loss is very small, and it is also unclear if intermittent fasting helps individuals achieve a meaningful 5% reduction in body weight. 

Additionally, intermittent fasting may have little or no effect on quality of life. Overall, the certainty of this evidence ranged from low to very low, meaning the true effect could be different from what the studies suggest.

When compared with doing nothing or being placed on a waiting list, intermittent fasting likely makes little to no difference in weight loss. Evidence about its effects on quality of life in these comparisons was also very uncertain.

Importantly, none of the studies reported whether participants were satisfied with intermittent fasting, nor did they assess diabetes outcomes or broader measures of other health conditions.

How reliable is the evidence on intermittent fasting?

One key finding is that this review gives moderate confidence that when intermittent fasting is compared with no intervention, it is unlikely to lead to a meaningful difference in weight loss. In other words, the evidence here is considered reasonably reliable.

However, confidence was low to very low for most of the other outcomes. These included whether people achieved a 5% reduction in body weight, changes in quality of life, and the risk of adverse events. Important outcomes such as overall health measures and participant satisfaction were not reported at all. Much of this uncertainty is because many of the studies were small, did not use the strongest research methods, or produced unclear results. 

This means the true effects of intermittent fasting could be different from what these studies suggest.

There were also gaps in the participants included. Most of the participants were from high-income countries and were predominately from white populations. This means there is uncertainty about how the findings apply to those in low- and middle-income countries or to more diverse populations. 

Additionally, there are uncertainties around gender differences. Only one study focused on men only, while several included only women. This makes it difficult to understand whether intermittent fasting affects genders differently.

Finally, most of the studies included followed participants for at least six months, this is about the point where weight loss often starts to plateau or level off. This means that there is limited evidence on whether intermittent fasting is effective or sustainable over the long-term. 

What do these findings mean for people trying to lose weight?

As appealing as a one-size-fits-all solution to weight loss might be, intermittent fasting isn’t it. Our bodies are complex, and they respond to dietary changes in different ways. Based on current evidence, intermittent fasting doesn’t appear to be significantly better than other forms of energy restriction when weight loss is the goal. 

For many people, the choice may come down to what feels realistic, sustainable, and appropriate for their individual circumstances. Some prefer time-restricted eating because it simplifies decisions about food. Others find long fasting periods difficult to maintain and may do better with steady, moderate calorie reduction.

However, it is also important to note that prolonged or overly restrictive fasting may increase the risk of nutrient deficiencies, and loss of lean muscle mass (source, source). 

Additionally, U.K clinical practice guidelines for managing overweight and obesity do not specifically recommend intermittent fasting, and instead prioritise lifestyle interventions that combine dietary change, physical activity, and behavioural support (source). 

It’s also worth noting that the research doesn’t tell us much about long-term sustainability, satisfaction, or broader health outcomes like diabetes risk. That means decisions shouldn’t be based on weight loss alone. 

In practical terms, the most effective approach is likely the one you can maintain consistently. Whether that’s traditional dietary changes, or another structured eating plan ideally with support from a relevant healthcare professional, especially if you have any underlying health conditions.

The bottom line: what we know, and what we don’t

The evidence does suggest that intermittent fasting may lead to weight loss, but not in a way that is meaningfully different from other forms of calorie restriction.

What remains unclear is whether intermittent fasting offers advantages in long-term sustainability, satisfaction, diabetes prevention, cardiovascular outcomes, or other broader health measures. Many studies are small, short in duration, or limited in participant diversity. 

Importantly, these findings challenge many popular claims made on social media. Intermittent fasting is often promoted as a way to “boost metabolism”, “switch the body into fat-burning mode”, or produce superior weight loss compared with traditional dieting. This research does not support the idea that intermittent fasting has a unique advantage. This highlights why credible, evidence-based nutrition advice matters. While intermittent fasting can be a useful tool for some people, it is not a magic solution.