Intermittent Fasting: Effective For Weight Loss But What Are The Risks?

Our continual search for wellness and weight loss has given rise to various diets. The Atkins diet, paleo diet, keto diet, carnivore diet, plant-based diet, and Mediterranean diet are just a few that have gained popularity in recent years.

However, one of the most common dietary eating patterns in the U.S. has nothing to do with what you eat. Intermittent fasting (IF) is focused exclusively on when you consume food.

While IF is often positively associated with weight loss and reduced body fat, other benefits, including reduced inflammation and improved heart health, appear to be linked too. But for the touted benefits, there are potential downsides. The long-term effects of this diet are still being determined, and some studies show mixed results.

Is IF a credible and helpful diet for well-being? Let’s dive into the pros and cons.

What is intermittent fasting?

IF is premised on the idea of restricting the time periods in which a person can eat. That may involve reducing the hours in which food is consumed daily or reducing calorie intake on some days of the week.

There are three main methods of intermittent fasting. These include time-restricted eating, alternate-day fasting, and whole-day fasting.

Time-restricted eating

Time-restricted eating (TRE) limits the hours in which food is consumed during the day. Typically, followers of this type of diet increase the usual overnight fasting period from 8–12 hours to 16 or even 20 hours of fasting — leaving minimal hours in the day left for eating.

TRE differs from other IF approaches. That’s because there’s no requirement for caloric restriction within eating periods and the diet is the same every day of the week.

A person, for example, may restrict their eating window to 8 hours every day of the week with no restriction on the amount they eat during those hours. 

Whether or not TRE is beneficial is currently unknown. One review, for example, found that conclusions couldn’t be drawn, given that some studies have found positive results while others haven’t.¹

Alternate-day fasting

The alternate-day fasting approach involves alternating between typical feeding days — where normal or even increased amounts of calories are consumed — and reduced feeding days, where only 25% of calories are consumed.¹

Alternate-day fasting is associated with some promising health benefits. A 2015 review, for example, found that alternate fasting trials of three to 12 weeks effectively reduced participants’ body weight, body fat, cholesterol, and triglycerides.¹

Whole-day fasting

Also known as 5:2 fasting, this method involves eating typical calories five or six days a week, then reducing caloric intake to nil for one or two days. 

This kind of fasting shows promising benefits. The same 2015 review found that whole-day fasts effectively reduced body weight and fat and improved blood lipids.

How does intermittent fasting help?

People swear by intermittent fasting for several reasons. Here are some of them.

Fasting may boost fat-burning by 14%

Proponents of IF claim that being in a fasted state can help improve the body’s metabolic processes, which can help a person lose fat. This appears to occur as fasting reduces the body’s supply of glucose.

Typically, excess glucose in the body is stored in our body as glycogen and eventually fat.²

When a person fasts, energy is maintained, not by glucose through food intake but by access to stored glycogen. This means that fasting can break down stored body fatty acids into ketones and ultimately boost our body’s ability to burn fat.³

One study even found that men on a three-day fast increased their metabolism by 14%. However, it’s important to note that the study was small and other studies have found the reverse — there were reductions in metabolism after fasting.⁴

Many researchers have concluded that IF is strongly linked with a decrease in weight.

Fasting may help our hormones

It’s been found that hormones associated with weight loss are boosted during intermittent fasting, which may help further explain why IF can help reduce weight.


Insulin plays a crucial role in influencing almost all organs in the body. It also plays a key role in the body’s process of breaking down and storing fat.

High insulin levels are associated with various ill health effects, including an inability to lose weight.

Intermittent fasting can help reduce insulin in the body. One study, which put insulin-resistant individuals on an alternate-day fasting regime, found that fasting resulted in reduced insulin in the bloodstream.⁵

Another study that required participants to alternate between fasting and feasting also showed to decrease insulin levels, thus helping the participants to lose weight.⁶

Growth hormone

While increased insulin is associated with obesity, reduced growth hormone levels are also linked to obesity and pre-obesity.⁷

It appears that fasting can help increase growth hormone secretion — something that’s associated with fat burning and weight loss.⁸

Calories may matter more at night

The time of day we eat might matter more than we think. Many of us like to snack in the evening; however, that could be why we can’t shed extra pounds.

A link has been found between the time of day we eat and how much that food will impact our weight. The reason appears to be down to our circadian rhythm.

The circadian rhythm is a natural cycle related to sunlight, which impacts our sleep-wake cycle, blood pressure, moods, and many other bodily processes. It might also affect how many calories are consumed during eating.

Research suggests eating throughout the day for 12 or more hours may influence the circadian rhythm, leading to health risks. Those health risks include cancer, heart disease, and type 2 diabetes.

A systematic review of TRE found that adhering to the diet resulted in an average 3% weight loss and a reduction in fat mass. Further, researchers found beneficial metabolic effects beyond just weight loss.⁹

Interestingly, the results were found even though participants had no caloric restrictions.

Early TRE might be even better. It’s been said that eating earlier in the day and stopping eating in the evening aligns with our natural circadian rhythms. Researchers in a 2013 study noted: “[G]reater weight loss has been observed in participants with overweight or obesity when energy intake was biased towards early, rather than late in the day, despite no differences in self-reported energy intake.”¹⁰

The adage, “eat breakfast like a king, lunch like a prince, and dinner like a pauper,” might have some truth to it after all.

The pros and cons of intermittent fasting

Like any other form of eating pattern, intermittent fasting has its pros and cons. It’s essential to know these before doing IF.

The pros: Heart health, reduced inflammation, and longevity

While IF’s potential to reduce weight is well-documented, other potential benefits have also been reported. Fasting may reduce inflammation, increase heart health, and even slow aging. It may also help protect against disease.¹¹

A 2015 review of a survey involving 2,650 women, for example, found that fasting for extended periods at night while reducing calorie intake in the evenings was associated with a reduced risk of inflammatory conditions, including breast cancer. Researchers did note, though, that further randomized trials are needed to confirm these findings.

IF may also be associated with cardiovascular health. Research including 26,092 men found that reducing eating later in the evenings through time-restricted eating may reduce the risk of experiencing heart disease. And researchers also noted that the regime is typically a safe method to adopt.¹²

A 2022 review of IF further found that this eating pattern can improve blood pressure, LDL cholesterol levels, and insulin resistance.¹³

Fasting may also improve oxidative stress — something that’s negatively associated with longevity. For example, researchers in a 2007 study found that alternate-day fasting reduced both oxidative stress and inflammation in participants.¹⁴

The cons: Fatigue, reduced exercise, and disordered eating

While IF is typically considered safe by experts, like any eating pattern, there are still risks. Many of these risks will depend on the individual.

Known for boosting weight loss, it’s important to consider that fasting doesn’t appear to be any more effective than calorie counting. When compared to a study with 139 participants, TRE was no better than simple calorie counting.¹⁵

A 2017 review also found that, while generally safe, fasting does have a range of side effects that people should be aware of.

These side effects can include:¹⁶

  • Hunger

  • Worsened mood

  • Challenges with concentrating

  • Irritability

  • Fatigue

  • Controlling feelings about food

  • Binge eating during feeding times  

There are other risks. IF doesn’t just reduce caloric intake, it also tends to impact the amount people exercise — a trend that occurs in all types of IF. A reduction in exercise is not only relevant for weight management; there are other impacts on health and well-being too.¹⁷

Exercise is well-known for improving a range of bodily processes and is critical for both heart and brain health. A 2021 study looking at alternate-day fasting found that the reduced exercise associated with fasting led to the loss of muscle mass at a rate greater than a simple calorie-restricted diet.¹⁸

The study authors also noted that alternate-day fasting could be less effective than simple calorie restriction:

“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.”

When to consult a doctor

Some individuals should avoid IF without first consulting a physician. These include the following:

Individuals with disordered eating and nutrient deficiencies

Those who experience disordered eating, or challenges around appetite, should avoid IF. Disordered eating and IF have been linked, as well as feelings of losing control around food.¹⁹

Those with nutrient deficiencies or those at risk of deficiencies may also need to avoid IF. Studies show that individuals on IF may not receive all the necessary levels of nutrients.²⁰

Individuals with diabetes

People with diabetes should also be careful. One study found fasting for individuals with type 2 diabetes increased the rate of hypoglycemia.²¹

Pregnant or breastfeeding women

Women who are pregnant or breastfeeding should not do IF. That’s because the caloric requirements and vitamin needs are higher during these times and IF can increase the risk of deficiency.

Other eating patterns may be just as useful

Research on intermittent fasting is both relatively new with some mixed findings. While some studies show benefits, others show no significant impacts. And although it’s rare, IF can occasionally have health adverse impacts.

One significant downside of IF is that the long-term impacts are currently unknown. The diet can also be challenging to follow long-term.

Whether or not to adopt IF may come down to your personal preferences and health history. It’s always best to follow dietary recommendations and choose a way of eating that suits your needs and lifestyle. Other eating patterns, such as the ones below, can offer similar benefits.

Calorie restriction

For those who wish to lose weight, tracking calories can be a useful way of understanding how much energy you’re consuming. Research has also shown that there are no additional benefits to fasting as opposed to the traditional restriction of calories.²²

It’s important to note though, that like fasting, calorie counting is not safe for those who experience disordered eating.

The Mediterranean diet

The Mediterranean diet is well-known for good reason. In 2022, the U.S. News & World Report ranked the Mediterranean diet number one in their “Best Diets Overall” for the fifth year in a row. Those eating a Mediterranean diet tend to have a high life expectancy, low cancer rates, and low rates of cardiovascular disease.

Research, for example, shows that the Mediterranean diet reduces the chance of experiencing a heart attack or stroke by up to 30%. That’s just one of its many benefits.²³

Plant-based diet

Plant-based diets are also becoming popular for good reason. Researchers have found that plant-based diets are associated with a lower risk of cardiovascular events.²⁴

Studies have also shown that plant-based diets may be a healthy strategy for weight control.²⁵

Adopting a balanced plant-based or Mediterranean style of eating also has a major plus when compared to IF in that fasting isn’t required to receive similar — and potentially more robust — benefits.

The lowdown

While intermittent fasting does offer some potential benefits, there are pros and cons. Keep in mind that before adopting any new eating pattern, it’s important to consult your physician or a registered dietitian to discuss your personal nutritional needs and requirements.

IF shows promising benefits for weight loss and reductions in body fat. Other potential benefits also include reductions in inflammation, improved blood pressure, and lower bad cholesterol. However, research in the area of IF is relatively new, so the long-term health impacts are not yet known.

Fasting is also not suitable for everyone. Pregnant women, those who experience disordered eating, and those with pre-existing health conditions should avoid IF unless advised by a physician.

While IF might be useful for some, other healthy eating patterns, such as the Mediterranean diet, may offer the same benefits without the restriction of fasting.

The information provided is designed to support, not replace, the relationship that exists between a patient/site visitor and his/her existing health care professional(s).

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  1. Effects of intermittent fasting on body composition and clinical health markers in humans (2015)

  2. Physiology, glucose (2022)

  3. Intermittent fasting and metabolic health (2022)

  4. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine (2000)

  5. Differential effects of alternate-day fasting versus daily calorie restriction on insulin resistance (2019)

  6. Practicality of intermittent fasting in humans and its effect on oxidative stress and genes related to aging and metabolism (2015)

  7. Insulin and growth hormone balance: Implications for obesity (2020)

  8. Multiple effects of growth hormone in the body: Is it really the hormone for growth? (2016)

  9. Food timing, circadian rhythm and Chrononutrition: A systematic review of time-restricted eating’s effects on human health (2020)

  10. Optimising intermittent fasting: Evaluating the behavioural and metabolic effects of extended morning and evening fasting (2020)

  11. Flipping the metabolic switch: Understanding and applying health benefits of fasting (2018)

  12. Time-restricted eating to improve cardiovascular health (2021)

  13. Clinical application of intermittent fasting for weight loss: progress and future directions (2022)

  14. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma (2007)

  15. Calorie restriction with or without time-restricted eating in weight loss (2022 - The New England Journal of Medicine)

  16. Potential benefits and harms of intermittent energy restriction and intermittent fasting amongst obese, overweight and normal weight subjects—A narrative review of human and animal evidence (2017)

  17. Anticipation of 24 h severe energy restriction increases energy intake and reduces physical activity energy expenditure in the prior 24 h, in healthy males (2020)

  18. Templeman et al 2021 STM | Nottingham Repository (Download as File)

  19. Intermittent fasting implementation and association with eating disorder symptomatology (2022)

  20. Intermittent fasting 5:2 diet: What is the macronutrient and micronutrient intake and composition? (2020)

  21. Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial (2018)

  22. Calorie restriction with or without time-restricted eating in weight loss (2022)

  23. Randomised trial of coconut oil, olive oil or butter on blood lipids and other cardiovascular risk factors in healthy men and women (2018)

  24. Plant-based diets and cardiovascular health (2018)

  25. Plant-based diet as a strategy for weight control (2021)

Chloe Garnham is a writer exploring a broad range of topics, including healthcare, education, and technology.

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