Quick overview of what you’ll learn from this blog post:
- Intermittent fasting explained
- Differences between intermittent fasting (IF) and caloric restriction (CR)
- Efficacy of CR and IF for Longevity in Animal Studies
- Theoretical Benefit of CR and IF in humans
Too often the overarching notion of caloric restriction becomes synonymous with whatever nutrition program is in vogue. Right now, it seems clear that the diet – for lack of a better term – of the times is intermittent fasting (IF). And while there is an element of inherent caloric restriction built into the fundamentals of IF, there’s slightly more science behind this novel approach.
What is Intermittent Fasting?
But first, here are the spark notes on Intermittent Fasting – namely, what specifically we mean when we talk about fasting and feeding windows, and how those states support not just weight loss, but actual wellness.
Intermittent fasting refers to a program that outlines a specific stretch of time in which one should remain fasted and a subsequent feeding window. Where many diets focus on what to eat, intermittent fasting is more concerned with when you eat.
This notion of timing as it relates to diet and nutrition can take many forms:
- Feeding within a purposefully limited window each day (i.e., between the hours of 6 and 8 pm).
- Individuals fast every other day as opposed to incorporating a fasting window into daily routines. Following this model, every other day is as close to 100% fasted as one can tolerate offsetting unrestricted eating days.
- Similar to alternate-day fasting, however, instead of every other day, individuals are unrestricted for five days, offset by two days of complete fasting.
- Short periods of near-complete fasting amid a month of unrestricted eating.
Here is one example from a 2020 research report authored by Dr. Ayse Mindikoglu, Dr. Mustafa Abdulsada, and Antrix Jain from the Baylor College of Medicine in Houston, Texas:
Subjects fasted (no eating or drinking) for more than 14 h. daily for 29 days, from May 06, 2019, until June 03, 2019. The minimum required duration of daily fasting from dawn to sunset was 14 h., 8 min for the shortest day (May 6, 2019), and 14 h., 42 min for the longest day (June 03, 2019).
For the Baylor study, the fasted window was based on Ramadan, an annual Muslim practice of fasting and prayer in commemoration of Muhammed’s First Revelation. Ramadan is unique in that fasting during this holy season is set to the lunar calendar. During Ramadan, fasting is “exclusively practiced during the human activity hours from dawn to sunset and is for both eating and drinking,” and in this way, mimics the natural circadian rhythms of early man.
Ramadan fasting is also far less lenient than popular IF programming where light eating is allowed during the fasted window. Some secular fasts also encourage ample water intake and even allow individuals to indulge in coffee for its appetite-suppressing properties as a way to augment energy levels during the fasted part of the day.
So, How is IF Different from CR?
A valid question and one not easily answered since the smaller feeding window associated with an IF diet inherently restricts daily average caloric intake. If an individual is adhering to the timestamps of their particular IF programming, there will simply not be enough time to comfortably achieve a caloric surplus.
And so, while IF and CR appear similar when viewed through the caloric lens, conceptually the two could not be more different.
Ultimately, the distinguishing factor is intention. IF deals in fasted and fed states not because that is a great way to achieve caloric deficit (though that is also true), but for the litany of health benefits unrelated to calories. IF’s ability to facilitate a deficit is merely one more happy consequence.
Efficacy of CR and IF for Longevity in Animal Studies
The results of various animal studies, as outlined by the NIA, of both caloric restriction and intermittent fasting as it relates to longevity and the occurrence of age-related disease are sort of a mixed bag.
For example, studies of rodents controlled for a 10-40 percent reduced caloric intake generally showed extended lifespans. This is mainly due to reduced rates of certain cancers rather than factors related to aging. That said, results were largely dependent on mouse strain, and for some, lifespan among the CR group was actually shorter. Results were similarly contradictory in various insect studies, which showed 40 percent lifespan increases in worms. A study of fruit flies, on the other hand, showed similar results from calorie restriction, but not intermittent fasting.
The National Institute on Aging (NIA)-supported two separate studies on the effects of a 30 percent reduction in daily caloric intake over the course of twenty years in rhesus monkeys. Although the two studies followed slightly different methodology and even featured different breeds, both showed evidence of reduced age-related diseases, such as cancer, heart disease, and diabetes. While in both studies, incidents of disease were less, the ultimate impact on longevity proved inconsistent as one study showed extended lifespans in the majority of control group monkeys, while the other did not.
Theoretical Benefit
In a review article from the NIA, John Hopkins University, and Harbor Hospital, researchers describe evidence suggesting overall health and longevity benefits of both CR and IF. As potential lifestyle interventions, IF and CR possess the theoretical potential to boost the overall health span of the nervous system by impinging upon fundamental metabolic and cellular signaling pathways that regulate lifespan.
In addition, CR and IF impact energy and oxygen metabolism, as well as the body’s cellular stress response systems. In both instances, the interventions facilitate the protection of neurons against chronic aging as a result of certain genetic and environmental factors. CR and IF benefit neurons through various pathways and molecular mechanisms including FoxO transcription factors, sirtuins, and peroxisome proliferator-activated receptors. These pathways stimulate the production of protein chaperones, neurotrophic factors, and antioxidant enzymes, all of which help cells cope with stress and resist disease.
Humans and CR
In terms of the impact of caloric restriction on humans, as of right now more research is needed before we can fully grasp how CR plays into long-term human health. What is clear is that calorie restriction leads to lower blood pressure and cholesterol levels, both of which are markers for age-related disorders like diabetes, heart disease, and stroke. That said, bone density, lean body mass, and aerobic capacity also decrease slightly as a result of the calorie-restriction strategy.
Humans and IF
Only a handful of clinical trials exist focused on intermittent fasting in humans with the majority of the studies focusing on the weight loss element of fasting, mainly in obese persons. The majority of research looks at individuals who already practice various forms of intermittent fasting, whether for weight loss and nutritional benefits or as a part of a religious belief system as in the case of Ramadan. The findings of this research are thus based on observations of IF as it exists and manifests anecdotally rather than in a controlled setting. As a result, there are innumerable external factors at play beyond the method of fasting. Still, it was generally found that people who fast on a regular basis (either as a religious practice or for other reasons) had effectively reduced their risk of coronary artery disease when compared to individuals who did not practice any form of fasted diet.
More definitive clinical research is necessary before we can speak to the net benefit of either CR or IF on these systems. The main point of contention is regarding whether or not the properties by which IF and CR may prevent neurodegenerative disorders outweigh the deleterious effects of extended calorie and nutrient deficiency that is the result of many CR or IF diets.