The Pros and Cons of Intermittent Fasting

Intermittent fasting is now one of the most widely practised eating patterns globally. The phenomenon has been driven by Silicon Valley entrepreneurs, fitness influencers, bestselling books, TV programmes, and early scientific studies. They all suggest that fasting can provide a host of benefits, from weight loss and enhanced performance to increased longevity.

Despite its recent popularity, intermittent fasting is not a new concept. It dates back thousands of years and has been practised for cultural, religious, and medical reasons. Long before that, early humans naturally experienced periods of fasting due to the scarcity of food. You could say that fasting is woven into the very fabric of our DNA, as our physiology has adapted to cycles of feast and famine.

So, is intermittent fasting the most natural and healthiest way to eat or yet another fitness fad?

In this article, we’ll look at the different forms of intermittent fasting and the benefits and potential risks of this approach to eating.

What is intermittent fasting?

The term ‘intermittent fasting’ refers to a broad range of eating patterns. These typically involve extended periods of little or no calorie intake, interspersed with periods of normal eating. The most common approaches include:

• Time-restricted feeding (TRF) — All daily calories are consumed within a set window, such as eight hours, while fasting for the remainder of the day.
• Alternate day fasting (ADF) — Fasting days are alternated with days of normal eating.
• Intermittent energy restriction (IER) — Periods of normal eating alternate with periods of significantly reduced calorie intake rather than restricting calories every day. A popular example is the 5:2 diet. This requires individuals to eat normally for five days per week and then limit their energy intake to about 500–600 calories on the remaining two days.

The benefits of intermittent fasting

Intermittent fasting has been associated with a diverse range of benefits, including:

• Improved brain function and cognitive health
• Enhanced weight loss and fat burning
• Improved control of blood sugar
• Improved cardiovascular health
• Reduced inflammation
• Enhanced cellular repair
• Improved physical performance
• Improved metabolic health

Which approach is most effective?

Arguably, the most common reason for adopting intermittent fasting is for weight loss and/or improvements in cardiometabolic health. Consequently, these areas have been studied most extensively.

For example, one large meta-analysis (1) of 99 randomised trials, which included all of the major fasting approaches—time restricted eating, the 5:2 diet, and alternate day fasting—found that:

• Intermittent fasting produces weight loss similar to continuous calorie restriction.
• No fasting method produced dramatically superior results.
• Modest reductions in body weight and small improvements in LDL and total cholesterol were observed.
• Alternate day fasting showed a slight advantage, but the difference was small.
• Effects on glucose control and blood pressure were minimal and inconsistent.

These findings are not unique and have been replicated across multiple reviews (2,3).

So, overall, the evidence suggests that intermittent fasting is a reasonable alternative to traditional calorie restriction—but not a superior one.

Different approaches for different goals

Although the research on weight loss and cardiometabolic health suggests that the different approaches to intermittent fasting achieve similar results, this is only part of the picture. It’s important to understand that each approach has slightly different effects and benefits beyond weight loss and cardiometabolic health.

This becomes clearer when we compare the effects of intermittent energy restriction and time-restricted feeding.

Intermittent Energy Restriction

Intermittent energy restriction can:

• Shift the body from using glucose for fuel to using fat and ketones, a process known as metabolic switching. This occurs as insulin falls and lipolysis and ketogenesis rise during fasting (4).
• Boost AMPK activity, a pathway which supports energy balance and cellular repair (7).
• Lower mTOR activity, a growth-driving pathway that, when reduced periodically, promotes cellular maintenance (8).
• Enhance autophagy, the body’s natural process for clearing and recycling damaged cells (7,8).

This makes intermittent energy restriction appealing for clients interested in cellular health or metabolic flexibility.

Time restricted feeding

Time restricted feeding aligns feeding with circadian rhythms. Eating at consistent times helps our body’s internal clock to work more effectively and supports:

• Improved insulin sensitivity (9)
• Better glucose tolerance (10)
• Reduced late night eating (11)
• More consistent metabolic patterns (12)

Another benefit of time restricted feeding is that it tends to be easier to adhere to than intermittent energy restriction (13).

The risks of intermittent fasting

As popular and potentially beneficial as intermittent fasting may be, recent evidence suggests that it is not risk free. It may even be potentially harmful for some individuals, as it has been linked to a range of negative side effects (14), including:

• Headaches
• Dizziness
• Irritability
• Difficulty concentrating
• Intense hunger
• Cravings
• Occasional binge eating tendencies
• Fatigue
• Low energy and reduced exercise performance
• Hypoglycaemia-like symptoms
• Hormonal disturbances
• Increased stress and anxiety
• Increased risk of disordered eating
• Constipation, bloating, and nausea

Research shows that while most of these side effects are mild, they also tend to be under-reported (14). This suggests that intermittent fasting may not be well tolerated by all individuals—especially those with underlying health conditions or poor metabolic flexibility.

Intermittent fasting and heart disease: a cause for concern?

A recent study presented at the American Heart Association reported that an 8-hour eating window was associated with a 91% higher risk of cardiovascular mortality compared with a typical 12–16-hour window (15).

Although this sounds alarming, it’s important to understand that this study has several major limitations:

• It relied on only two days of dietary recall to estimate long-term eating habits.
• It was observational, meaning it cannot show cause and effect.
• Individuals with existing illness may naturally eat in shorter windows.
• The findings conflict with other research showing short-term cardiometabolic benefits.

In light of this, experts caution against drawing strong conclusions until higher-quality evidence is available (16).

Does intermittent fasting reduce muscle protein synthesis?

To optimise muscle growth and recovery, guidelines recommend spreading protein intake across the day, ideally consuming a protein-rich meal every 3–4 hours (17). This guidance appears to directly contradict the principles of intermittent fasting. Consequently, concerns have been raised regarding intermittent fasting’s effect on muscle protein synthesis, with fears that it may even cause a loss of muscle.

Current evidence suggests that these fears are unfounded, as intermittent fasting, including time-restricted feeding, has been shown not to impair muscle protein synthesis, even when daily eating windows are shortened. For example, a study by Parr and colleagues (18) found that muscle protein synthesis rates were unchanged when participants followed an 8-hour eating window compared with normal eating, provided total daily protein and calorie intake are maintained. Several other randomised trials report similar findings (19).

The anabolic bonus

Rather than reduce muscle, there is evidence that time-restricted feeding may actually support muscle protein synthesis, as it can enhance the anabolic signalling pathways that help muscle cells grow and repair. It can also improve insulin sensitivity within muscle tissue and increase the uptake of glucose and branched-chain amino acids—key building blocks for muscle protein synthesis (20).

Is intermittent fasting right for every client?

Not necessarily. It may be suitable for clients who:

• Prefer structured eating windows
• Struggle with constant calorie tracking
• Want a simple, rule-based approach
• Are interested in metabolic or cellular health benefits
• But it may be inappropriate for clients who:
• Have a history of disordered eating
• Experience strong hunger or energy crashes
• Have certain medical conditions
• Are pregnant, breastfeeding, or under high training loads
• Have difficulty maintaining performance during periods of fasting

What does this mean for nutrition coaches?

Intermittent fasting can be a useful dietary strategy, but nutrition coaches need to understand its strengths, limitations, and the differences between each approach. Although current evidence suggests that it’s no more effective than traditional calorie restriction for weight loss, it can provide a range of other benefits. Also, many clients may find intermittent fasting more convenient and easier to adhere to than other approaches to weight management.

As appealing and effective as it might be, it’s important to recognise that intermittent fasting is not appropriate for everyone and may be potentially harmful for some individuals. Because of this variability, it should always be individualised rather than applied as a universal strategy.

For nutrition coaches, the key priority is to help clients choose evidence-based approaches that align with their goals, lifestyle, and health status, while supporting long-term adherence and wellbeing. Developing a deeper understanding of nutrition science and practical application through recognised nutrition courses can help practitioners make more informed, confident, and client-centred recommendations.

References

1) Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025 Jun 18;389:e082007. doi:10.1136/bmj-2024-082007.

2) Hua Z, Yang S, Li J, Sun Y, Liao Y, Song S, Cheng S, Li Z, Li Z, Li D, Guo H, Yang H, Zheng Y, Li X. Intermittent fasting for weight management and metabolic health: an updated comprehensive umbrella review of health outcomes. Diabetes Obes Metab. 2025 Feb;27(2):920 932. doi:10.1111/dom.16092. Epub 2024 Dec 1.

3) Gu L, Fu R, Hong J, Ni H, Yu K, Lou H. Effects of intermittent fasting in human compared to a non intervention diet and caloric restriction: a meta analysis of randomized controlled trials. Front Nutr. 2022 May 2;9:871682. doi:10.3389/fnut.2022.871682. eCollection 2022.

4) Longo VD, Mattson MP. Fasting: molecular mechanisms and clinical applications. Cell Metab. 2014;19(2):181 192.

5) Mandic M, Misirkic Marjanovic M, Janjetovic K, Vucicevic L, Harhaji-Trajkovic L, Trajkovic V. Multifaceted role of AMPK in autophagy: more than a simple trigger? Am J Physiol Cell Physiol. 2025;329:C1380–C1397.

6) Smiles WJ, Ovens AJ, Kemp BE, Hawley JA. New developments in AMPK and mTORC1 cross talk. Essays Biochem. 2024;68(3):321–336.

7) Saxton RA, Sabatini DM. mTOR signaling in growth, metabolism, and disease. Cell. 2017;168(6):960 976.

8) Park JM, Lee DH, Kim DH. Redefining the role of AMPK in autophagy and the energy stress response. Nat Commun. 2023;14:1234.

9) Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early time restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1212 1221.e3.

10) Jamshed H, Beyl RA, Della Manna DL, Yang ES, Ravussin E, Peterson CM. Early time restricted feeding improves 24 hour glucose levels and insulin sensitivity in men with prediabetes. Nutrients. 2019;11(6):1234.

11) Potter GD, Cade JE, Grant PJ, Hardie LJ. Circadian rhythm and sleep disruption: causes, metabolic consequences, and countermeasures. Nutr Res Rev. 2016;29(2):150 164.

12) Panda S. Circadian physiology of metabolism. Science. 2016;354(6315):1008 1015.13) Gabel K, Hoddy KK, Haggerty N, Song J, Kroeger CM, Trepanowski JF, et al. Effects of 8 hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: a pilot study. Nutr Healthy Aging. 2018;4(4):345 353.

13) Gabel K, Hoddy KK, Haggerty N, Song J, Kroeger CM, Trepanowski JF, et al. Effects of 8 hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: a pilot study. Nutr Healthy Aging. 2018;4(4):345 353.

14) Harris L, Hamilton S, Azevedo LB, Olajide J, De Brún C, Waller G, et al. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta analysis of adverse events. Nutr J. 2024;23(1):pages pending.

15) Yang Y, et al. Association of time restricted eating with mortality outcomes in U.S. adults: results from NHANES 2003–2018. Presented at: American Heart Association Scientific Sessions; 2024 Nov 16–18; Chicago, IL.

16) Harris E. Study examines intermittent fasting and cardiovascular mortality. JAMA. 2024;331(17):1440. doi:10.1001/jama.2024.5158.

17) Jäger R, Kerksick CM, Campbell BI, Cribb PJ, Wells SD, Skwiat TM, Purpura M, Ziegenfuss TN, Ferrando AA, Arent SM, Smith-Ryan AE, Stout JR, Arciero PJ, Ormsbee MJ, Taylor LW, Wilborn CD, Kalman DS, Kreider RB, Willoughby DS, Hoffman JR, Krzykowski JL, Antonio J. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr. 2017 Jun 20;14:20. doi: 10.1186/s12970-017-0177-8. PMID: 28642676; PMCID: PMC5477153

18) Parr EB, Kouw IWK, Wheeler MJ, Radford BE, Hall RC, Senden JM, Goessens JPB, van Loon LJC, Hawley JA. Eight hour time restricted eating does not lower daily myofibrillar protein synthesis rates: a randomized control trial. Obesity (Silver Spring). 2023;31(1):e23637. doi:10.1002/oby.23637

19) Kouw IWK, Parr EB, Wheeler MJ, Radford BE, Hall RC, Senden JM, et al. Short term intermittent fasting and energy restriction do not impair rates of muscle protein synthesis: a randomised, controlled dietary intervention. Clin Nutr. 2024;43(11):174 184.

20) Jones R, Pabla P, Mallinson J, Nixon A, Taylor T, Bennett A, Tsintzas K. Two weeks of early time restricted feeding (eTRF) improves skeletal muscle insulin and anabolic sensitivity in healthy men. Am J Clin Nutr. 2020;112(4):1015 1028. doi:10.1093/ajcn/nqaa192.

Author

Paul Orridge

Paul Orridge BSc (Hons)

Paul Orridge is a graduate in the field of sport, exercise and health, and has over 30 years’ experience within the fitness industry. In this time, he has performed a variety of roles including personal training, lecturing and writing. Paul now works as a freelance technical author and subject matter expert within the fitness industry. His work is based on his practical experience gained working with a diverse range of people from very unfit, overweight individuals to highly conditioned athletes, and is underpinned by the latest research.