Why Intermittent Fasting Isn't Working: 5 Evidence-Based Reasons

Obesity is one of the most prevalent and tenacious problems facing humanity today. Obesity wreaks havoc on the body's systems, disrupting hormone production, hurting insulin reception, and causing cardiovascular problems, among many other things. It's a distressing trend that shows no sign of slowing despite dozens of different diet programmes, weight loss options, and social movements aimed at addressing the issue.

Many people around the world subscribe to intermittent fasting as a weight loss approach, and for some it works very well. For others, it seems entirely ineffective. What's going on, why might intermittent fasting not work for some people, and what other options exist?

This article examines the science of weight loss and the most common reasons intermittent fasting fails to deliver results.

What is Intermittent Fasting and How Does it Work?

The theory behind intermittent fasting is relatively simple. Where many diets focus on controlling what you eat, or how much you eat, intermittent fasting instead focuses on when you eat whatever it is you're eating.

The rationale behind intermittent fasting comes from an examination of human societal norms over past decades and centuries, as well as the rates of obesity around the world. Obesity wasn't nearly as prevalent as it is today, even just 30–50 years ago, let alone centuries ago.

What has changed since then? Well, a lot of things, including the widespread use of processed ingredients and preservatives, artificial sugars, and more. Sociologists also note there has been a significant shift in culture.

In the past, when the sun went down, there often wasn't much left to do for the day. People would work on handicrafts or simple household chores in the early evening, but they soon went to bed to rest for the following day. Even with the advent of electricity and modern conveniences like radio and television, these services often shut off before midnight, and people, left with little to do, went to bed.

In the last few decades, however, several things have changed. TV and the internet, as well as games and apps, can now provide entertainment at all hours of the day. A more globally interconnected world means people have friends around the world to communicate with at any hour of the night. There is a lot more available to keep people awake and alert far later than was once culturally normal.

When staying up late, energy levels can drop. Some people take that opportunity to go to bed; others grab a quick snack to refresh themselves.

This is where the origin of intermittent fasting comes in. People are more likely to be snacking outside of the relatively narrow hours humans used to eat, which both increases the amount of time the body is processing and storing new energy rather than expending stored energy, and provides more opportunity to take in additional calories.

Since the core of virtually all weight loss comes down to the calculation of calories in versus calories out, a broader window for snacking means more calories in and, especially with more sedentary modern lives, fewer calories out.

How Does Intermittent Fasting Work Specifically?

On a more specific level, intermittent fasting defines a set period of each day during which eating is permitted. There are a variety of specific protocols to choose from, such as:

• 12:12, where eating is allowed within 12 of the day's 24 hours, such as from 7am to 7pm.
• 8:16, where eating is restricted to just 8 hours during the day, such as from 9am to 5pm.
• 5:2, where two days per week — such as Monday and Thursday — involve a deep caloric restriction (no more than 600 calories on those days); the other five days are normal.

There are other variations as well, such as alternate day fasting, a weekly complete 24-hour fast, or the extreme 4:20 Warrior Diet, wherein eating is only permitted during four hours of a day.

Various studies have shown that intermittent fasting works, at least in the short term. For example, a randomized controlled trial of 5:2 showed it was effective for weight loss, especially in the first six weeks, but that the effect dropped off over the course of a year.

Intermittent fasting works within the established understanding of weight loss in two ways. The first is that, by restricting eating windows, it eliminates a large portion of additional calories from casual snacking outside of those main hours. Since main meals are eaten within the allowed period, hunger is managed, and if the restrictions are followed, additional snacking outside those hours is avoided. It functions as a caloric restriction similar to traditional caloric restriction, but based on time rather than the amount of food eaten.

The second mechanism is that, after a certain amount of time without eating, the body switches from expecting new energy sources from food and starts to consume energy already stored as body fat. This, combined with an improvement to sleep quality, can help reduce body fat and improve overall health markers.

What Are the Main Reasons Intermittent Fasting Isn't Working?

While many people try out intermittent fasting and see results, others do not. The question then becomes: why does it work for some people and not others? The evidence points to five common reasons why intermittent fasting may not be delivering results.

1: Eating More to Compensate

Intermittent fasting relies on implicit caloric restriction, but one of the primary stumbling blocks is that some people simply condense the same amount of food they would have eaten throughout the day into those restricted hours, rather than trimming off the excess calories that would normally have come from outside-of-window snacking.

Since weight loss typically comes down to calories in versus calories out at its heart, this means that overall caloric intake isn't actually reduced, which eliminates the primary benefit of intermittent fasting. This is more of a problem with the less restrictive forms of the plan, whereas the more restrictive and full-day fasting options make it much harder to compensate.

2: Eating Too Late at Night

One of the main reasons intermittent fasting works is that the extended period without eating allows the body to enter ketosis, where it consumes stored body fat for energy. Normally, this happens during sleep and stops when breakfast is consumed in the morning.

Some people structure their intermittent fasting schedules so they eat a relatively large meal at the end of their eating window to have as much energy as possible available for the fasting period. While this seems to make sense from an energy standpoint, it counteracts the benefit of intermittent fasting and makes it harder for the body to enter ketosis.

A review of nighttime eating found that this impact could be mitigated, either by adjusting what is eaten (such as focusing on protein-rich foods only) and by incorporating exercise. A survey of nighttime eating also found that pre-bedtime meals increased the incidence of waking up during the night. Without these mitigating factors, however, the primary benefits of intermittent fasting are undermined.

3: Not Supporting the Weight Loss

Another reason intermittent fasting may not be working is if additional steps to improve health and weight through other means are not also being taken. Intermittent fasting has a relatively modest impact on its own, such that, in some cases, it can seemingly fade into the background of natural weight fluctuations.

The impact of intermittent fasting can be improved through the use of complementary weight loss tools, such as shifting to a healthier diet rich in whole foods and lean protein (such as the Mediterranean diet), and incorporating exercise into a daily routine. Absent these additional interventions, intermittent fasting alone is less impactful.

4: Diminishing Returns

One of the biggest challenges with intermittent fasting is the body's natural adaptability to changes in circumstances. Many studies have observed that intermittent fasting can be quite effective when done properly for the first several months. However, after a year, the beneficial results tend to drop off.

This isn't unique to intermittent fasting. Nearly any health plan — from caloric restriction to weight lifting — can reach a "plateau" where results level off and progress appears to stall. The body is simply very good at adapting to a "new normal" and will need further encouragement at those points to continue progressing.

5: Unrelated Health Issues

Intermittent fasting is beneficial in helping to control weight and has a demonstrated positive impact on a variety of markers, including cholesterol levels, insulin control, and some markers of cardiovascular health. However, it is less effective at certain things, like blood pressure, than a general caloric restriction diet.

There are, unfortunately, a variety of health conditions, illnesses, and diseases that can make intermittent fasting either less effective or actually dangerous:

• Type 1 diabetics. Intermittent fasting is beneficial for those with type 2 diabetes, but type 1 diabetics need stricter blood sugar control and should avoid the hypoglycaemia associated with intermittent fasting.
• People with eating disorders. A disordered relationship with food can be exacerbated by the restrictions of intermittent fasting.
• Individuals with immune deficiency illnesses, which can be impacted in a variety of ways by the imbalance in caloric intake.

Intermittent fasting is also not appropriate for pregnant women.

Are There Risks to Intermittent Fasting?

Despite the widespread interest, relatively few long-term studies have been performed on intermittent fasting, and of those that have been performed, they aren't always the most methodologically robust. However, there have been a few causes for concern, either in at-risk populations or more generally.

For example, one systematic review that looked at 27 studies found no serious adverse events in intermittent fasting populations. However, a narrative review found that participants in some trials experienced reduction in bone density and lean body mass.

One concerning observational analysis that looked at over 20,000 people found a 91% higher risk of cardiovascular death in those undergoing intermittent fasting compared to those who weren't. While this figure sounds alarming, it is important to note that this was an observational study showing only a correlation, not a causative relationship. Confounding factors — such as people with pre-existing illness adopting intermittent fasting in hopes of improving their health — may significantly influence the observed association.

Overall, considerably more research is needed to determine whether intermittent fasting carries any serious long-term risks, and for whom.

What Should You Do to Lose Weight?

Whether intermittent fasting is the right approach depends on individual circumstances. The research suggests that addressing body weight is a complex problem that cannot be solved with any single change.

Within the calories-in versus calories-out framework, the evidence consistently supports increasing physical activity and reducing overall caloric intake, regardless of the specific dietary approach used. A proper balance of macronutrients, alongside assurance that micronutrient needs are being met, is an important foundation. It matters less which diet protocol is followed, and more that the body is consistently getting what it needs to function well.

Regardless of the dietary approach taken, ensuring adequate micronutrient intake supports the body's overall health. A balanced, whole-food diet should come first; a well-formulated micronutrient supplement can help fill gaps that are difficult to address through diet alone.

At the end of the day, there is no singular medication, diet, or exercise plan that is perfect for everyone. Working with a healthcare professional to determine what is appropriate for individual health circumstances, and pursuing personalised goals in the most effective way, remains the best approach.

Sources

1. Overweight & Obesity Statistics – National Institute of Diabetes and Digestive and Kidney Diseases: https://www.niddk.nih.gov/health-information/health-statistics/overweight-obesity
2. A randomized controlled trial of the 5:2 diet: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0258853
3. Association of Eating and Sleeping Intervals With Weight Change Over Time: The Dail24 Cohort: https://www.ahajournals.org/doi/10.1161/JAHA.122.026484
4. The Health Impact of Nighttime Eating: Old and New Perspectives: https://pmc.ncbi.nlm.nih.gov/articles/PMC4425165/
5. Does the Proximity of Meals to Bedtime Influence the Sleep of Young Adults? A Cross-Sectional Survey of University Students: https://pubmed.ncbi.nlm.nih.gov/32295235/
6. Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses of randomized controlled trials: https://pubmed.ncbi.nlm.nih.gov/38500840/
7. Intermittent fasting and weight loss: Systematic review: https://pubmed.ncbi.nlm.nih.gov/32060194/
8. Beneficial effects of intermittent fasting: a narrative review: https://pmc.ncbi.nlm.nih.gov/articles/PMC9946909/
9. 8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death: https://newsroom.heart.org/news/8-hour-time-restricted-eating-linked-to-a-91-higher-risk-of-cardiovascular-death