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Intermittent Fasting

  • Writer: Jitka Burger
    Jitka Burger
  • Jun 7
  • 7 min read

Intermittent Fasting text beside a clock plate with salmon, avocado, greens and nuts, plus lemon water and cutlery on a table.

Intermittent Fasting (IF) has gained significant attention in recent years as a lifestyle approach involving alternating periods of eating and fasting



Infographic titled Types of Intermittent Fasting shows three fasting schedules with icons and a clock diagram on white background.


IF has frequently been discussed in relation to weight loss, metabolic health, diabetes management, longevity, and even as a possible supportive strategy during cancer treatment. However, many of these proposed benefits remain uncertain, and the overall quality and consistency of the evidence vary considerably. Although some studies have reported potentially favorable findings, much of the research is preliminary, short-term, or based on animal models, making it difficult to draw firm conclusions about long-term health effects in humans.


Let’s take a closer look at the most common intermittent fasting protocols, the proposed mechanisms behind fasting, and what current evidence may — and may not — suggest.



What is Intermittent Fasting?


Intermittent Fasting is a form of calorie restriction that involves abstaining from food, or significantly reducing food intake, for specific periods of time.


Rather than focusing primarily on what to eat, IF emphasizes when to eat.



Common Types of Intermittent Fasting


Alternate Day Fasting (ADF)


Alternate Day Fasting involves alternating between fasting days and non-fasting days.

On fasting days, little or no food is consumed, while on non-fasting days individuals typically eat without calorie restriction.


Alternate Day Modified Fasting (5:2 Diet)


In the 5:2 approach, individuals eat normally five days per week and reduce calorie intake to approximately 500–600 calories on two non-consecutive days.


Time-Restricted Feeding (Time-Restricted Fasting)


Time-Restricted Feeding (TRF) limits food intake to a specific eating window each day, commonly between 8 and 12 hours.


For example, if all meals are consumed between 10 a.m. and 6 p.m., the eating window is 8 hours and the fasting window is 16 hours.



What Is the Theory Behind Intermittent Fasting?


Ketosis and Fat Burning


One proposed mechanism of fasting is that reduced calorie intake may eventually shift the body from using glucose as its primary fuel source to using stored fat and ketones for energy.


However, significant ketosis generally develops after approximately 24–72 hours of fasting, depending on factors such as glycogen stores, carbohydrate intake, physical activity, and metabolic health. Since many intermittent fasting protocols involve shorter fasting periods, ketosis may be limited or minimal in some individuals. In addition, the extent to which mild or intermittent ketosis translates into meaningful long-term clinical benefits in humans remains unclear.


Autophagy and Cellular Cleanup


Intermittent fasting is also commonly associated with autophagy, a natural cellular process in which the body breaks down and recycles damaged cellular components.


Autophagy may increase during prolonged fasting when glucose and insulin levels decline substantially, although the exact timing, magnitude, and clinical relevance in humans remain uncertain. Much of the current evidence comes from laboratory and animal studies, and it is not yet clear whether these findings translate directly to meaningful health outcomes in people.


Importantly, fasting is not the only factor associated with autophagy. Exercise, quality sleep, heat exposure, coffee, green tea, and certain plant compounds such as sulforaphane found in broccoli have also been linked to increased autophagic activity, although evidence for many of these associations is still evolving.


Intermittent Fasting and Cancer Treatment


One emerging area of intermittent fasting research involves its potential role during chemotherapy.


Some early studies suggest that fasting during chemotherapy may help reduce certain treatment-related side effects and improve tolerability in select patients. Researchers have proposed that fasting may help protect healthy cells while making cancer cells more vulnerable to treatment — a concept known as differential stress resistance.


However, human research remains limited, and fasting during cancer treatment should only be undertaken under medical supervision.



What are the effects of Intermittent Fasting on Health, Aging, and Disease?


Some research suggests that intermittent fasting may help certain individuals reduce calorie intake, lose weight, improve insulin sensitivity, or improve some cardiovascular risk markers.


However, these findings should be interpreted cautiously. Many intermittent fasting studies have been:


  • short-term,

  • relatively small,

  • and conducted primarily in overweight young and middle-aged adults.


Long-term sustainability, safety, effectiveness, and applicability across diverse populations remain uncertain.


As noted in the New England Journal of Medicine review by de Cabo and Mattson:


“It is crucial to note that the long-term sustainability of IF and its applicability to diverse age groups remain uncertain. Clinical studies have predominantly focused on overweight young and middle-aged adults, limiting the generalization of observed benefits and safety.”


In many cases, the benefits observed with intermittent fasting may be related less to fasting itself and more to:


  • reduced calorie intake,

  • weight loss,

  • improved food choices,

  • or reduced late-night eating.


It also remains difficult to determine whether intermittent fasting offers unique physiological advantages beyond those achieved through overall calorie reduction or improved dietary habits.



Is Intermittent Fasting Superior to Other Eating Patterns?


One of the longest and most rigorously controlled human trials on intermittent fasting compared Alternate-Day Fasting with standard daily calorie restriction over one year.


Researchers found that intermittent fasting was not superior for:


  • weight loss,

  • weight maintenance,

  • or improvements in cardiovascular risk factors.


Participants in the fasting group also had higher dropout rates, suggesting that intermittent fasting may be more difficult for some individuals to sustain long term.


Similarly, while some studies on Time-Restricted Feeding have reported improvements in insulin sensitivity and blood pressure, these studies have generally been short-term, involved small numbers of participants, and often lacked long-term follow-up.


At this time, current evidence does not clearly demonstrate that intermittent fasting is superior to other healthy, sustainable dietary patterns for long-term health outcomes. Overall, the evidence remains mixed, and additional large, long-duration human trials are needed before stronger conclusions can be made.



Potential Challenges of Intermittent Fasting


While some people report finding intermittent fasting helpful or easier to follow than other eating patterns, others may experience difficulties such as:


  • excessive hunger,

  • irritability,

  • fatigue,

  • headaches,

  • overeating during eating windows,

  • social inconvenience,

  • or difficulty maintaining regular exercise performance.


For some individuals, prolonged fasting periods may also make it more difficult to consume adequate calories, protein, and nutrients consistently.


Additionally, some people become overly focused on meal timing while paying less attention to overall dietary quality, sleep, stress management, physical activity, and other important lifestyle factors.



Who Should Use Caution with Intermittent Fasting?


Intermittent fasting may not be appropriate for everyone. Individuals who are pregnant or breastfeeding, underweight, frail, elderly, prone to disordered eating, or taking glucose-lowering medications such as insulin or sulfonylureas should consult a healthcare professional before attempting prolonged fasting protocols. Medical guidance is especially important for anyone with diabetes, a history of eating disorders, cancer, kidney disease, or other chronic conditions that may be affected by changes in food intake, hydration, or medication timing. 


Children and adolescents generally should not practice prolonged fasting unless medically supervised.



In Conclusion


For many people, intermittent fasting may be difficult to maintain long term, particularly when fasting schedules interfere with hunger cues, exercise routines, family meals, or social activities such as dining out with friends.


Although intermittent fasting is sometimes associated with weight loss or improvements in certain metabolic markers, current evidence does not clearly establish that fasting itself is responsible for these effects, nor does it demonstrate that intermittent fasting is necessary to achieve them.


Many of the reported benefits associated with intermittent fasting may also be achievable through other sustainable dietary and lifestyle approaches without prolonged fasting.


A practical and sustainable strategy for many people may simply involve:


  • avoiding late-night eating,

  • maintaining a moderate overnight fasting period,

  • prioritizing adequate sleep,

  • exercising regularly,

  • managing stress,

  • and focusing primarily on a nutrient-dense dietary pattern rich in whole plant foods.


Intermittent fasting may be one dietary strategy that some individuals choose to explore, particularly if it helps them structure eating habits or reduce overall calorie intake. However, current evidence remains limited in several important areas, and there is insufficient evidence to conclude that intermittent fasting is superior to other balanced, sustainable dietary approaches for long-term health outcomes.



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