Intermittent fasting has become one of the most discussed dietary strategies of the past decade. Advocates claim benefits that range from enhanced metabolic flexibility to improved cardiovascular health and even extended lifespan.
Yet, despite its popularity, one fundamental question remains largely unanswered: are the benefits of fasting due to the fasting itself, or are they simply the result of better calorie control and improved energy balance? A recent systematic review and network meta-analysis published in Journal of American Medical Association sought to address this question by comparing the effects of various intermittent fasting strategies on cardiovascular disease risk factors (1).
The analysis, conducted by Kibret and colleagues, pooled data from 56 randomised controlled trials involving almost 4,000 participants. The findings provide valuable insight into which fasting patterns appear most effective, while also highlighting the need for a more individualised and sustainable approach to nutrition.
What the Research Found
The authors compared several forms of intermittent fasting, including alternate-day fasting (ADF), the 5:2 diet, and time-restricted eating (TRE), against traditional continuous calorie restriction (CCR). Across the studies, ADF produced the most notable improvements in body weight, waist circumference and blood pressure, with average reductions of around 5 kilograms, 4 centimetres, and 7 mmHg respectively.
Time-restricted eating showed similar benefits and additionally reduced fasting blood glucose levels by approximately 4 mg/dL. However, this approach was also linked to a reduction in lean mass, an important consideration for individuals wishing to preserve muscle tissue.
Despite these encouraging findings, the authors cautioned that many of the improvements diminished in longer trials and were less consistent when lower-quality studies, many of which were excluded from the analysis. They also noted that few trials directly compared the different fasting methods head-to-head, limiting the ability to draw definitive conclusions about which is most effective.
In short, the review concluded that both alternate-day fasting, and time-restricted eating can be beneficial for improving cardiovascular risk markers, but the evidence remains context-dependent (1).
Calorie Restriction or Fasting?
The improvements seen in body weight, blood pressure and glucose control are undoubtedly positive, yet it is important to interpret these results carefully. Most intermittent fasting approaches naturally restrict the time available to eat, which in turn tends to reduce overall calorie intake. As such, the metabolic benefits may not necessarily arise from fasting itself, but from improved energy balance and subsequent weight loss.
In essence, intermittent fasting may simply serve as a structured way to help individuals achieve caloric deficits, a concept that has long been known to support cardiometabolic health.
Reduced calorie intake can improve insulin sensitivity, lower triglycerides, decrease inflammation and reduce oxidative stress, all of which play a key role in cardiovascular disease prevention (2,3).
This does not mean fasting has no additional benefits. Some research suggests that it may help the body align better with its natural day–night rhythm, support cellular repair processes, and improve how efficiently the body uses energy. However, these effects are likely secondary to the main reason fasting works for most people, it helps manage calorie intake and maintain a healthier energy balance overall.
The Risk of Losing Lean Mass
One important finding from the meta-analysis was the loss of lean body mass observed in some time-restricted eating interventions. From a functional medicine standpoint, this is a crucial consideration. Maintaining muscle mass is vital not only for metabolic health but also for physical function, glucose regulation, hormonal balance and longevity.
When caloric intake becomes too restricted, or when protein distribution is inadequate during eating windows, muscle loss can occur. For low muscled individuals, particularly older adults, this could offset some of the cardiovascular benefits gained through weight loss.
Therefore, any fasting strategy should include sufficient protein intake and ideally be combined with resistance training to protect muscle tissue. The goal is not merely weight loss, but healthy body composition change, which is why we strongly recommend testing body composition as part of any weight loss journey.
Personalisation Over Prescription
The varied results across studies reinforce an important point: there is not a single optimal fasting approach. Some individuals thrive on structured fasting, experiencing improved energy, focus and appetite regulation. Others may find it disruptive to mood, sleep, hormonal balance or performance.
From a functional medicine perspective, nutrition should always be tailored to the individual. This means considering the person’s metabolic profile, hormonal status, stress levels, activity patterns, nutritional needs, sleep rhythm and psychological relationship with food.
For instance, someone with significant insulin resistance may benefit from a defined fasting window that improves glycaemic control and reduces visceral fat.
In contrast, an individual with thyroid dysfunction or high stress levels may respond poorly to prolonged fasting due to increased cortisol output and reduced metabolic efficiency.
Laboratory testing can help inform these decisions, allowing us to identify patterns such as impaired insulin sensitivity, elevated inflammatory markers or nutrient insufficiencies that may influence how an individual responds to different dietary structures.
A Sustainable Framework for Success
The most effective fasting pattern is the one that is sustainable, supports lean mass retention and fits the realities of daily life. For many, a moderate time-restricted eating window of 12 to 14 hours, is both achievable and beneficial. I will often use this as an initial starting point for most clients.
Others may prefer a more structure such as the 5:2 approach, which includes two reduced-calorie days per week, or even alternate-day fasting. Either way, you should remain adaptable and don’t get married to a specific approach.
Fasting should also never exist in isolation. Its benefits are amplified when paired with a wholefood diary, well-balanced macronutrients; regular movement; quality sleep; and effective stress management. These factors collectively determine the success of any nutritional intervention.
A Functional Medicine Viewpoint
This latest research supports the idea that intermittent fasting can be a useful tool for improving cardiovascular risk factors, but it should not be viewed as a standalone solution. The primary mechanism remains caloric regulation, not the fasting window itself.
In functional medicine, we look beyond protocols to understand why an individual may be struggling with weight regulation, insulin resistance or elevated blood pressure in the first place. Factors such as chronic stress, inflammation, disrupted sleep, lack of movement and exercise, poor dietary habits and micronutrient deficiencies all contribute to metabolic dysfunction. Addressing these root causes, alongside appropriate caloric and macronutrient balance provides a more durable foundation for cardiovascular health than any one dietary strategy alone.
If you would like to take a deeper look at your own metabolic and cardiovascular health and uncover the underlying factors influencing your results, you can learn more about our Functional Medicine services.
The key takeaway is that fasting can be helpful, but only when applied intelligently and within a broader context of personalised nutrition, movement, recovery and emotional wellbeing. Intermittent fasting should not be about “restriction”, it is just another method that can work for some but not others. The trick is finding the methods that work for you!
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References
- Kibret T. M., et al. (2025). Intermittent Fasting for the Prevention of Cardiovascular Disease: A Systematic Review and Network Meta-analysis. JAMA Network Open. 8(1): e250046. https://pmc.ncbi.nlm.nih.gov/articles/PMC12289860/
- Fontana L., Partridge L., Longo V. D. (2010). Extending Healthy Life Span — From Yeast to Humans. Science. 328(5976): 321–326.
- Ravussin E., Redman L. M. (2021). Caloric Restriction in Humans: Impact on Physiological, Psychological, and Behavioral Outcomes. Antioxidants & Redox Signaling. 34(2): 165–179.
