Timing Matters when Intermittent Fasting

In the 15th century, the word breakfast became used to describe “breaking the fast.” However, when many people start intermittent fasting they tend to skip breakfast and begin eating at lunchtime. Unfortunately they are undermining their weight loss goals, as studies show that the benefits of fasting tend to be diminished when calories are shifted to the latter part of the day. 

These results are due in large part to our body’s circadian rhythm, an internal process that allows us to anticipate and respond to environmental changes such as light and dark. This perpetual rhythm leads to the timely expression of clock-controlled genes, especially those encompassing digestion/enzymes and regulatory molecules that mediate metabolism. Thus the strong relationship between the circadian clock and metabolism, as they share some common regulators. 

Studies show that the benefits of fasting tend to be diminished when calories are shifted to the latter part of the day. 

Recent studies on fasting showed that when most of the calories were consumed later in the day, there was no change – or even worsening – in glucose levels, cell responsiveness, blood pressure and lipid levels. When eating was restricted to the morning, however, we saw an improvement in metabolic markers and greater health benefits, such as a lower risk of weight gain, obesity and cardiovascular disease. 

Photo by Toa Heftiba on Unsplash

Additional findings from recent fasting-related studies: 

  • Skipping breakfast and repeated late-night eating is associated with higher risks of metabolic syndrome. Each 10% increase in proportion of total caloric intake consumed in the evening is associated with a 3% increase in inflammatory markers. 
  • Those who skip breakfast are less likely to meet the recommended daily requirements for vitamins and minerals, compared to those who eat breakfast. 
  • When studying intermittent fasting and breast cancer in mice, there was a lower recurrence rate when the majority of calories were consumed earlier in the day. 
  • Young adults who skip breakfast tend to eat more added sugars and score lower on healthy index scores. 

So, while fasting and time-restrictive eating can improve metabolic function, there may not be a benefit when the majority of food intake is shifted to the afternoon and evening. As the human metabolism has evolved to be in sync with day and night cycles – eating during the day and sleeping during the night – we must consider that both the amount of time spent eating and the time of day at which food is consumed are relative to our circadian rhythm and are critically important to the effects of diet on health and longevity.  

Worth Reading

Ard, Jamy, Baskin, Monica L., Chiuve, Stephanie E., et al., Meal Timing and Frequency: Implications for Cardiovascular Disease Prevention: A Scientific Statement From the American Heart Association, Circulation/2017

Baumann, Thalke, Herzog, Nina, Kistenmacher, Alina, et al., Twice as High Diet-Induced Thermogenesis After Breakfast vs Dinner On High-Calorie as Well as Low-Calorie Meals, The Journal of Clinical Endocrinology & Metabolism/2020

Bernier, Michel, de Cabo, Rafael, Di Francesco, Andrea, et al.,  A time to fast, Science/2018

Beyl, Robbie, Cefalu, William T., Kate S., Early, et al.,  Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes, PubMed.gov/2018

University of California, San Diego, When — not what — obese mice ate reduced breast cancer risk: Intermittent fasting aligned with circadian rhythms improved insulin levels and reduced tumor growth, ScienceDaily/2021