Circadian Fasting: Heart & Metabolic Health Effects

As part of cultural or religious traditions, humans have practiced periodic fasting for thousands of years. Only in the past century has much of the population had easy access to daily excess calories throughout their lifespans. This relatively recent eating pattern correlates with the onset of many chronic illnesses, including cardiovascular disease (CVD) and metabolic disorders. On the other hand, intermittent fasting strategies, including time-restricted eating^* interventions,have a range of health benefits, including the improvement of precursors to cardiovascular disease such as obesity, high blood pressure, dyslipidemia, and diabetes.^1,2

Intermittent Fasting, Time-Restricted Eating, & Cardiometabolic Health

Studies suggest that therapeutic fasting treatments may improve health through multiple pathways, potentially reducing oxidative stress,¹ enhancing mitochondrial health and DNA repair,^3,4 and triggering autophagy,⁵ a cellular recycling system that removes damaged cells and pathogens, among other actions. Intermittent fasting is a broad term used to describe food consumption cycles that alternate between periods of restricting calories and periods of not restricting calories. Different forms of intermittent fasting may be beneficial in a range of cardiometabolic conditions:

• During a prospective study (n=697 patients with and without type 2 diabetes), an intermittent fasting intervention that included periodic daily caloric restriction improved fatty liver index, a proxy for metabolic dysfunction–associated steatotic liver disease (MASLD), formerly known as non-alcoholic fatty liver disease (NAFLD), significantly and rapidly.⁶
• A systematic review (n=27 clinical trials) concluded that more research is needed but that intermittent fasting approaches (i.e., 5:2 diet, time-restricted eating, alternate-day fasting) show promise for helping to treat patients with obesity, and no serious adverse effects were documented.⁷ 
• A randomized clinical trial (n=39 adult patients with metabolic syndrome) found that those patients in the intermittent fasting group following an alternate-day fasting treatment significantly reduced fat mass, oxidative stress, inflammatory biomarkers, and improved vasodilatory parameters compared to the control group.⁸ Study results also indicated that the fasting intervention increased the production of short-chain fatty acids.⁸

Specific timing patterns of eating and fasting windows may also lead to greater improvements of cardiometabolic markers.^9,10 Time-restricted eating (TRE) is an intermittent fasting approach that is a form of circadian fasting—a dietary pattern that optimizes circadian elements such as daily rhythms for insulin peaks and glucose tolerance by consuming food and beverages within a shortened window of time during the day, extending a person’s nightly fast to 12 hours or more. Research studies suggest that in addition to reducing cardiovascular disease risk factors in healthy populations,^11,12 circadian fasting may also hold promise as a beneficial adjunctive therapy, improving cardiometabolic health for some patients with conditions such as metabolic syndrome^13,14 and MASLD.^15,16

As an example, in a 12-week randomized controlled trial (RCT), 45 middle-aged patients with MASLD were randomized to either the intervention group (followed a daily circadian fasting/TRE pattern of 16-hour fasting and 8-hour feeding in addition to a low-sugar diet) or the control group (followed an isocaloric control diet based on traditional meal distribution and eating patterns).¹⁶ In addition to the reduction of body fat, body weight, waist circumference, body mass index, inflammatory markers, and total cholesterol, the intervention group also showed a clinically significant reduction in liver stiffness measurements.¹⁶ 

Circadian Rhythms & Early Time-Restricted Eating

The circadian rhythms of the body influence sleep-wake cycles, eating and digestion patterns, and biochemical and metabolic processes. Studies suggest that interruptions to these biological rhythms may lead to an elevated risk for chronic conditions, including CVDs and metabolic disorders.^17,18 Meal timing affects circadian rhythms, and late night eating may contribute to CVD precursors such as obesity more than food consumed at other times of the day.¹⁹ This may in part be due to the diurnal rhythm of glucose tolerance, which peaks in the mornings; some evidence shows that peak is lowered by early-phase TRE.²⁰

Clinical trials have investigated the cardiometabolic and other biological impacts of early TRE.^20-23 Two of those studies included a six-hour feeding window of 8 am to 2 pm. Compared to the 12-hour control eating schedules, results indicated that the early time-restricted eating treatments:

• Decreased fasting glucose levels.^20,21
• Altered lipid metabolism and increased fat oxidation.²²
• Increased metabolic flexibility, defined as the ability to switch between the oxidation of different substrates.²²
• Increased a sense of fullness, decreased the desire to eat, and had no impact on 24-hour energy expenditure.²²

Recent meta-analyses that have assessed early TRE interventions also have suggested that these approaches may have beneficial effects on health parameters such as body weight, fasting blood glucose, HbA1c levels, and insulin resistance measurements.^24-26

Personalization & Incorporating Fasting Into Therapeutic Food Plans

It is important to note that not everyone is a suitable candidate for fasting therapies. Some preexisting conditions rule out this intervention strategy for patients such as those who are pregnant, who have type 1 diabetes, or who have a history of or are at risk of developing an eating disorder. If, however, fasting is appropriate for a patient, different forms of fasting may be blended with therapeutic food plans to create an individualized nutrition intervention. Examples of fasting approaches include the following:

• Fasting: The willful abstinence from any caloric intake for an extended period that is greater than 24 hours. This practice usually includes water only, but some methods allow tea, coffee, and minerals. Of note, some experts only define fasting in this way and do not consider intermittent approaches as forms of fasting.
• Intermittent fasting: A broad term used to describe food consumption cycles that alternate between periods of restricting calories and periods of not restricting calories.
• Time-restricted eating/prolonged nightly fasting: Calories from food and beverages are consumed only during a shortened window of time daily, ranging from four to 12 hours. This is also called “prolonged nightly fasting,” which eliminates or reduces a person’s caloric intake at night, with an extended overnight fast that is greater than 10 hours.
• Alternate-day fasting: A cycle of a complete fast on one day and eating freely on the next day.
• Intermittent energy restriction: Consecutive or non-consecutive days of alternating very low-calorie intake (as low as 400 to 500 kcal) with days of normal calorie intake.
• 5:2 diet: Two consecutive or non-consecutive days of low-calorie intake (no more than 25% of an individual’s daily caloric requirement) coupled with five days of unrestricted eating.
• Calorie restriction: Daily caloric intake is reduced for an extended period without causing malnutrition, and meal frequency is maintained.
• Fasting-mimicking diet: A periodic, multiple-day (typically followed for five days), very low-calorie, low carbohydrate food plan designed to mimic a fasting state.

Conclusion: A Functional Medicine Approach

Time-restricted eating interventions that optimize circadian elements have the potential to improve patient outcomes through addressing the precursors of cardiovascular diseases and improving cardiometabolic health. Specific eating windows and fasting lengths that yield the maximum health benefit continue to be studied; however, through the personalized approach of functional medicine care, flexible fasting strategies can be infused with personalized therapeutic nutritional interventions for each patient to create the most effective health treatment. 

A collaborative patient-practitioner relationship is an essential component of the functional medicine framework that not only helps clinicians understand a patient’s health history and their current health disturbances but also supports effective and sustainable interventions for the individual patient. Learn more about personalized lifestyle-based approaches that support and enhance cardiometabolic health at IFM’s Cardiometabolic Advanced Practice Module (APM)^TM.

*Of Note: Semantically, some experts define fasting as lasting 24 hours or more and therefore technically do not consider time-restricted eating a form of fasting.