Different intermittent fasting strategies, and how eating outside of “normal” eating hours can adversely affect health
In this two-part series, Dr. Michael Brown, ND, takes a comprehensive look at this popular and trending approach to a healthier lifestyle. In Part I, a look at the different approaches to intermittent fasting and how nighttime eating can adversely affect health. In Part 2, Dr. Brown takes a closer look at the clinical studies surrounding intermittent fasting, as well as those comparing this dietary strategy with caloric-restriction diets.
Traditionally, fasting has predominantly been associated with religious and spiritual practices,[1],[2],leading many to assume fasting inherently entails very restricted eating or no food whatsoever for days or weeks at a time. What most don’t realize is that fasting is actually something that the majority of us partake in every night when going to bed. “Breakfast”, being the first meal of the day, actually refers to breaking the fasting period of the previous night. Fasting during this time, which usually ranges from about six to 12 hours, falls into line with our natural circadian rhythms and the rising and setting sun, which we know very much impacts our physiology and metabolism.[3]
Intermittent fasting: a modern version of an age-old tradition
Although there are a variety of ways one can fast, a newer phenomenon trending in the health and fitness world is referred to as intermittent fasting (IF), piquing the interest of not only health enthusiasts, but the scientific community as well. A quick Google search using the term “intermittent fasting” produces an astounding 55,200,000 results! This form of fasting is achieved by ingesting minimal or zero amounts of food and caloric beverages for periods of time typically ranging from 12 hours to several days.[2] IF is distinct from caloric restriction diets in which the daily caloric intake is reduced chronically by 20 to 40%, but meal frequency is maintained.
Although there are a variety of ways one can fast, a newer phenomenon trending in the health and fitness world is referred to as intermittent fasting (IF).
Of the various forms of IF, three of the more popular and studied variations are as follows:
- Alternate-day fasting: This form of fasting involves modified fasting every other day. For example, limiting your calories on fasting days from zero up to 500 calories (or a maximum caloric consumption of approximately 25% of your normal intake). On non-fasting days, you would resume a regular, healthy diet.[1],[4]
- Time-restricted feeding: In this option, you have set fasting and eating time periods. A very popular approach is the 16/8 method where you only eat between 11 am and 7 pm or noon and 8 pm. This method can be repeated several times per week and is popular due to easier compliance given a majority of your fasting occurs overnight.[1],[2],[3]
- Twice-a-week method: This is also called the 5:2 diet because five days of the week are normal eating days, while the other two restrict calories to 500 to 600 calories per day. For five days per week, you eat a regular, healthy diet. Then, on the other two days, you reduce your calorie intake to a quarter of your daily needs. You can choose whichever two days of the week you prefer, as long as there is at least one non-fasting day in between them.[5],[6],[7]
Circadian rhythms, eating patterns, and their impact on health
A circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and many other physiological processes.[3],[8] It repeats roughly every 24 hours, being sensitive not only to light and darkness but also other cues, known as zeitgebers, such as temperature and social interactions. Most living organisms, including humans, have evolved internal metabolic mechanisms that allow them to anticipate and prepare for activity, sleep, and food intake at a specific time of day.
Both food ingestion and fasting can alter our metabolic processes. Consuming calories outside the normal eating hours (i.e., late night eating in humans) may negatively impact circadian rhythms and associated cardiometabolic parameters.[3],[8]It has been demonstrated that shift-work is associated with nighttime eating and increased risks of obesity,[9] diabetes,[10] cardiovascular disease[11], and cancer (particularly breast cancer).[12],[13],[14],[15],[16] Data from trials and prospective cohort studies also support the idea that consuming the majority of calories earlier in the day, thus prolonging the night-time fasting state (with little or no food), is associated with lower weight and improved health.[17],[18],[19],[20],[21]
Data from trials and prospective cohort studies also support the idea that consuming the majority of calories earlier in the day, thus prolonging the night-time fasting state (with little or no food), is associated with lower weight and improved health.
This data is in line with animal studies demonstrating that restricting caloric intake to an eight to 12 hour window helps support natural circadian rhythm and is associated with reduced adiposity, elevated lean muscle mass, longer sleep duration, increased endurance, reduced systemic inflammation, gut homeostasis, and improvement in other clinical biomarkers.[22],[23],[24],[25],[26]
Clearly, these studies suggest that the time during a 24-hour period in which we are eating has an impact on our health. Stay tuned for Part 2, in which we take a look at the clinical studies investigating the impact that IF has on health, as well as studies comparing it to caloric-restriction diets.
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