The term intermittent fasting is defined as; intermittent energy restriction or intermittent calorie restriction, diet plans that cycle between a period of fasting and non-fasting over a defined period.
In 2013, Mosley & Spencer published a best-selling book titled “The FastDiet” which describes the benefits of severely restricting energy intake for 2 days a week but eating normally during the rest of the week. This was a big push for this popular concept.
In 2016 an internet search using the terms “intermittent/ diet- fasting” had more than 210,000 hits. In contrast, there is a shortage of evidence-based support for intermittent fasting that can be used to generate recommendations for public health practices.
The bulk of scientific evidence for the health benefits of intermittent fasting primarily comes from studies of male rodent models. Human studies have largely been limited to observational studies of religious fasting (e.g., during Ramadan) and experimental studies with modest sample sizes.
In randomized, intervention trials studied thus far the efficacy of intermittent fasting on human health supports weight loss. Data is lacking regarding the impacts of intermittent fasting on other health behaviors, such as diet, sleep, and physical activity.
Alternate Day Fasting
It’s typically assumed that pre-agricultural humans and our hominid ancestors experienced long periods of famine, or, at the very least, weren’t grazing and snacking all day. It’s more likely that they feasted and fasted, going some length of time between successful hunts. Today, this would be called alternate day fasting.
It has alternating fasting days, during which no calories are consumed, and feeding days, during which foods and beverages are consumed freely. This fasting regimen was as effective as simple daily, caloric restriction in reducing obesity-associated body weight and fasting insulin and glucose concentrations. Alternate-day fasting in rodent models of obesity has also been shown to reduce total plasma cholesterol and triglyceride concentrations, reduce liver steatosis and inflammatory gene expression and have beneficial effects on cancer risk factors, such as cell proliferation.
Other considerations for these study purposes and the practice in general is hunger and mental status, as well as post-fast energy intake/calories which can offset the positive effects and are important outcomes to consider with extended fasting during waking hours.
In a recently reported study 16 women did a 2-day fast resulting in distraction, but not hunger, and was associated with a low mood and perceived work performance compared with 2 days prior to and following the fasting period.
The sparse data on alternate-day fasting suggest that this regimen can result in modest weight loss and lead to improvements in some metabolic parameters. However, reports of extreme hunger while fasting indicate that this may not be a feasible public health intervention.
Modified Fasting Regimens
Modified fasting regimens may be tolerated a bit better. They generally specify that energy consumption is limited to 20–25% of energy needs on regularly scheduled fasting days. The term fasting is used to describe periods of severely limited energy intake rather than no energy intake. This type of regimen, also called intermittent energy restriction, is the basis for the popular 5:2 diet which involves energy restriction for 2 nonconsecutive days per week and unrestricted eating during the other 5 days of the week.
The outcome here was decreased visceral fat, leptin, and resistin, and increases in adiponectin. Similar studies conducted by this research group also found that in mice these fasting regimens appear to reduce adipocyte size, cell proliferation, and levels of insulin-like growth factor 1.
Time restricted feeding that had daily fasting intervals ranging from 12 to 21 hours in numerous rodent models were associated with reductions in body weight, total cholesterol, triglycerides, glucose, insulin, interleukin 6 (IL-6), and TNF-α, as well as with improvements in insulin sensitivity. Interestingly, positive health outcomes occurred despite the variable effects of on weight loss.
Prolonged Morning Fasting
The long-term metabolic benefits associated with eating or not eating breakfast that is, extending the nighttime fast until the lunch meal—are of great research and public interest.
Satiety and appetite-regulating hormones and peptides were affected by prolonged morning fasting, but these alterations did not significantly affect calorie intake. In their 6-week controlled trial, they observed no benefit with respect to weight change, glycemic control, lipids, or inflammatory markers for the group omitting the breakfast meal compared with the control group.
Night time eating or not eating is another tool to consider
Data from the National Health and Nutrition Examination Surveys (known as NHANES) have shown that each 3-hour increase in nighttime fasting duration was associated with significantly reduced odds of elevated HbA1c and significantly lower CRP concentrations in women who ate less than 30% of their daily calories after 5:00 pm.
A published analysis of the nightly fasting interval in 2,337 breast cancer survivors in the Women’s Healthy Eating and Living (known as WHEL) Study indicated that cancer survivors who fasted <13 hours per night during 7 years of follow up had an increased risk of recurrence.
The same concept extends into prolonged, into the night, or night eating that we see in shift work. This is, in part, due to the circadian rhythm of insulin secretion and the insulin-impeding action of growth hormone, the pulsatile concentrations of which increase at night. Postprandial insulin and glucose responses to meals increase across the day and into the night. Thus, meals consumed at night are associated with greater postprandial glucose and insulin exposure than content-matched meals consumed during the day, leading to increased HbA1c levels and risk of type 2 diabetes over time. Short-term intervention studies designed to mimic circadian rhythms in human participants have metabolic consequences. Inducing circadian misalignment in humans by extending the day from a 24-hour to a 28-hour cycle causes insulin resistance after only 3 cycles.
So, fasting regimens also have the potential to foster modifiable health behaviors. A study in 8 overweight young adults found that increasing the nightly fasting duration to ≥14 hours resulted in statistically significant decreases in energy intake and weight, as well as improvements in self-reported sleep satisfaction, satiety at bedtime, and energy levels.
Modified fasting regimens appear to promote weight loss and may improve metabolic health. However, there are insufficient data to determine the optimal fasting regimen, including the length of the fasting interval, the number of fasting days per week, the degree of energy restriction needed on fasting days and recommendations for dietary behavior on non-fasting days. Which could potentially be successful with individual metabolic analysis for sustainable levels of deprivation.
Fasting appears to have numerous beneficial effects, but that doesn’t mean it’s appropriate for everyone. Individuals with type 2 diabetes can fast safely but under medical supervision is advised. Individuals taking certain medications may be advised to avoid fasting or to be sure to work with their doctor to make any necessary adjustments to medications that are typically taken with food, or whose potency or pharmacology may be affected by fasting.
Fasting is not recommended for women who are pregnant or breastfeeding, nor for adolescents in an active growth stage. It’s also not advised that anyone with an eating disorder or a history of eating disorder adopt a fasting practice. Another group of people for whom fasting is not recommended is the elderly, except under medical supervision. Many older people are already are under-consuming protein. It wouldn’t be wise for an individual already experiencing sarcopenia or dynapenia to become even more protein deficient or malnourished overall.
As we know, some form of calorie restriction especially in relation to sugar and starch type foods is a positive step for most, for a metabolic upgrade. Because it is so impactful it is prudent to tender with a clinical approach and some individual evaluation.