Intermittent fasting has become one of the most popular nutrition strategies for improving metabolic health. It is often presented as a simple way to lose weight, improve blood sugar, and reduce cardiovascular risk.
In many cases, it can do all of those things. But there is an important distinction that often gets overlooked.
Intermittent fasting can be beneficial when it is structured correctly. Most people, however, are not practicing structured fasting. They are skipping breakfast, eating later in the day, and assuming they are getting the same benefits.
From a cardiovascular health perspective, these are not the same thing.
What Intermittent Fasting Actually Improves
Across a large body of research, intermittent fasting produces modest improvements in key cardiometabolic risk factors (de Cabo & Mattson, 2019; Varady et al., 2021; Semnani-Azad et al., 2025). These can include:
- Body weight
- Blood pressure
- Insulin resistance
- In some cases, lipid levels
These benefits are meaningful, but they are not unique to fasting. In most studies, intermittent fasting performs similarly to continuous calorie restriction.
In other words, many of the benefits of intermittent fasting come from eating less overall—not necessarily from fasting itself.
Types of Intermittent Fasting and Their Effects
Alternate-Day Fasting
Alternate-day fasting typically involves alternating between a fasting day and a feeding day, with intake on fasting days often limited to around 500 kcal. Among intermittent fasting strategies, this approach tends to produce the most consistent changes in body weight and lipid markers.
A 2025 meta-analysis found an additional 1.29 kg greater weight loss compared to calorie restriction (Semnani-Azad et al., 2025).
Weight loss typically ranges from 3–8% of body weight, with improvements in blood pressure and inflammatory markers (Varady et al., 2021).
5:2 Intermittent Fasting
The 5:2 model follows a different pattern, with five days of normal eating and two nonconsecutive days of significant calorie restriction. This approach appears particularly effective for improving blood pressure and insulin sensitivity.
A 2025 study showed (Abdollahpour et al., 2025):
- Decrease in systolic BP: 123.78 vs 127.62 mmHg vs calorie restriction
- Improvements in pulse pressure and cardiovascular risk scores
16:8 Time-Restricted Eating
Time-restricted eating, particularly the 16:8 model, is the most commonly adopted approach in real-world settings. This typically involves consuming all calories within an eight-hour window, often between 12 PM and 8 PM.
A 2025 meta-analysis found (Wong et al., 2025):
- ↓ Fasting glucose
- ↓ Insulin
- ↓ HOMA-IR
- Slight ↑ HDL
These improvements are meaningful, but modest. More importantly, this is where real-world behavior often diverges from controlled studies.
The Breakfast Skipping Paradox
Randomized trials show benefits from intermittent fasting.
But large population studies show something very different (Rong et al., 2019; Cahill et al., 2013; Sun et al., 2023):
- 87% higher cardiovascular mortality (NHANES)
- 27% higher CHD risk (Health Professionals Follow-Up Study)
- 40% higher cardiovascular mortality in newer NHANES data
A 2024 meta-analysis confirmed (Wang et al., 2024):
- 28% higher cardiovascular mortality
- 27% higher all-cause mortality
Evidence at the Level of the Artery
The PESA study adds an important layer. Using vascular imaging in over 4,000 adults breakfast skippers had >2x odds of generalized atherosclerosis (Uzhova et al., 2017). Importantly, these findings remained significant even after adjusting for traditional risk factors and overall diet quality.
This suggests that breakfast skipping is not simply a marker of unhealthy behavior. It may directly influence the development of atherosclerosis.
Why Skipping Breakfast May Work Against You
Several physiological mechanisms help explain these findings.
From a circadian perspective, breakfast acts as a key metabolic signal. Eating earlier in the day helps regulate insulin secretion, glucose uptake, and overall metabolic activity. When this signal is absent, metabolic processes become less synchronized, which can impair insulin sensitivity and lipid metabolism (Knutson et al., 2025).
There is also evidence that prolonged fasting into the morning increases activity of the hypothalamic-pituitary-adrenal axis, which may elevate morning blood pressure independent of psychological stress (Rong et al., 2019). In addition, skipping breakfast disrupts the “second meal effect,” leading to higher glucose and insulin responses later in the day.
Taken together, these changes suggest that delaying the first meal may shift metabolism in a direction that is less favorable for long-term cardiovascular health.
Why This Does Not Mean Intermittent Fasting Is Harmful
At this point, it would be easy to conclude that intermittent fasting itself is harmful. That is not what the evidence shows.
The key difference is structure.
In controlled studies, intermittent fasting is often implemented in a way that aligns with circadian biology. In practice, many individuals follow a delayed eating pattern that shifts calorie intake later in the day.
Early Time-Restricted Eating as a Better Model
Early time-restricted eating represents a different approach. Instead of delaying food intake, it shifts the eating window earlier in the day, often between 7 AM and 3 PM.
In a randomized trial of adults with obesity, this approach produced greater weight loss, larger reductions in diastolic blood pressure, and greater improvements in insulin resistance compared to a calorie-restricted control diet (Jamshed et al., 2022). Participants lost more weight and experienced more favorable blood pressure changes while maintaining alignment with circadian rhythms.
This pattern preserves the metabolic benefits of fasting without skipping breakfast.
The Real-World Gap
The challenge is that most people are not following early time-restricted eating. Instead, their eating pattern often includes skipping breakfast, delaying the first meal until later in the day, and continuing to eat into the evening.
This shifts calorie intake to a later time of day, which appears to be metabolically unfavorable and may help explain why breakfast skipping is consistently associated with worse cardiovascular outcomes.
Clinical Takeaway
Intermittent fasting can improve cardiometabolic health, but the benefits depend heavily on how it is implemented.
When structured appropriately, it can reduce body weight, lower blood pressure, and improve insulin sensitivity. However, when it takes the form of habitual breakfast skipping and late-day eating, it may increase cardiovascular risk over time.
Bottom Line
Intermittent fasting is not just about eating less often. It is about when and how you eat.
Approaches that align with circadian rhythms, particularly those that include breakfast and avoid late-night eating, are more likely to support cardiovascular health. Skipping breakfast and shifting food intake later in the day does not replicate these benefits and may work against your cardiometabolic goals.
About the Author
Joseph Lehrberg, MS, RD is a registered dietitian specializing in cardiovascular and metabolic health and founder of CardioFunction Integrative Nutrition Services, a nutrition practice based in Boston. He works with patients with elevated cholesterol, high coronary artery calcium scores, high triglycerides, statin intolerance, and other cardiometabolic risk factors to develop evidence-based nutrition strategies for long-term heart health.
Learn more about working with him here.
References
Abdollahpour N, Seifi N, Nosrati M, et al. (2025). Comparative effects of intermittent fasting and calorie restriction on cardiovascular health in adults with overweight or obesity. Scientific Reports.
Allaf M, Elghazaly H, Mohamed OG, et al. (2021). Intermittent fasting for the prevention of cardiovascular disease. Cochrane Database of Systematic Reviews.
Cahill LE, Chiuve SE, Mekary RA, Jensen MK, Flint AJ, Hu FB, Rimm EB. (2013). Prospective study of breakfast eating and incident coronary heart disease in a cohort of male US health professionals. Circulation.
de Cabo R, Mattson MP. (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine.
Flanagan A, Bechtold DA, Pot GK, Johnston JD. (2021). Chrono-nutrition: From molecular and neuronal mechanisms to human epidemiology and timed feeding patterns. Journal of Neurochemistry.
Gabel K, Varady KA. (2022). Current research: Effect of time-restricted eating on weight and cardiometabolic health. Journal of Physiology.
Guo Y, Lu L, Chen L, et al. (2025). Effectiveness and safety of intermittent fasting on blood pressure in adults with overweight or obesity: A systematic review. International Journal of Obesity.
Jamshed H, Steger FL, Bryan DR, et al. (2022). Effectiveness of early time-restricted eating for weight loss, fat loss, and cardiometabolic health in adults with obesity: A randomized clinical trial. JAMA Internal Medicine.
Knutson KL, Dixon DD, Grandner MA, et al. (2025). Role of circadian health in cardiometabolic health and disease risk: A scientific statement from the American Heart Association. Circulation.
Rong S, Snetselaar LG, Xu G, et al. (2019). Association of skipping breakfast with cardiovascular and all-cause mortality. Journal of the American College of Cardiology.
Semnani-Azad Z, Khan TA, Chiavaroli L, et al. (2025). 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.
Sun Y, Rong S, Liu B, et al. (2023). Meal skipping and shorter meal intervals are associated with increased risk of all-cause and cardiovascular disease mortality among US adults. Journal of the Academy of Nutrition and Dietetics.
Uzhova I, Fuster V, Fernández-Ortiz A, et al. (2017). The importance of breakfast in atherosclerosis disease: Insights from the PESA study. Journal of the American College of Cardiology.
Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. (2021). Cardiometabolic benefits of intermittent fasting. Annual Review of Nutrition.
Wang Y, Li F, Li X, et al. (2024). Breakfast skipping and risk of all-cause, cardiovascular and cancer mortality among adults: A systematic review and meta-analysis of prospective cohort studies. Food & Function.
Wong PS, Wan K, Dai Z, et al. (2025). Effect of 8-hour time-restricted eating (16/8 TRE) on glucose metabolism and lipid profile in adults: A systematic review and meta-analysis. Nutrition Reviews.
