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The craze of intermittent fasting has gripped the world of fitness and nutrition — but
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is this a temporary fad that will pass, or the paradigm shifting dietary intervention
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we're led to believe? Let's find out.
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Dr. Jubbal, MedSchoolInsiders.com.
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Welcome to the next episode in our Research Explained series, where we do a deep dive
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into a specific topic, spend hours scouring through the primary literature, and summarize
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the key findings so that you don't have to. This is episode 8 — you can find the
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full list on our Research Explained playlist. Link in the description.
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Nutrition is a touchy subject, on the same level of politics or religion. Just look at
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the comments on my video where I spoke about stopping a plant based diet after 5 years.
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I'm interested in talking about nutrition from a purely health focused perspective — if
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your dietary decisions are based on religion or ethics, I respect that, but we'll be
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approaching the science with a focus on health optimization. One of my favorite sources for
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nutritional, health, and longevity based information is Dr. Peter Attia.
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When discussing nutritional interventions with his patients, he describes three factors
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to his nutritional framework: dietary restriction, caloric restriction, and time restriction.
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Each factor can be manipulated independently of others to optimize for the desired outcomes.
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Dietary restriction means limiting what you do or don't eat. For example, maybe you
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avoid red meat or dairy. Caloric restriction means limiting how much you eat, which is
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the most common description of “dieting” that we think of when it comes to losing weight.
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Time restriction limits when you do and don't eat, which brings us to intermittent fasting.
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Speaking about intermittent fasting and similar interventions is confusing because most people,
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and often even researchers, aren't careful about what they're exactly talking about.
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Fasting is refraining from eating. When we refer to fasting, we normally are referring
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to water-only fasts, where consuming water is ok, or even teas and minerals, but not
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foods or drinks with any calories, sweeteners, or the like.
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When you hear the term “Intermittent Fasting” being used by the public, they're usually
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referring to time restricted feeding (TRF) instead. Time restricted feeding is where
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you limit the hours during which you eat each day to a feeding window. Most people without
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any effort normally stick to roughly a 14/10 window, meaning they'll eat for 14 hours,
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and fast for 10 hours while they sleep and a little bit before and after. The window
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can be expanded to anything you want. When starting out, you may do 12/12, but eventually
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transition to the popular 16/8 or even 18/6.
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Alternate day fasting (ADF) is somewhat of a misnomer, since you don't normally go
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a full day without eating anything. Rather, every other day you switch to doing something
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other than eating ad libitum, meaning without restriction. Usually, these days are hypocaloric
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with 1,000 or 500 calories allowed.
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Intermittent fasting (IF) in its most strict form is undergoing a fast of at least 24 hours
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at some frequency, or intermittently. For example, that could mean doing a 72 hour fast
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once every quarter, or a 5 day fast once per year. A true prolonged fast like this would
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only include water, tea, and minerals with zero calories.
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As time restricted feeding is what most people are referring to when they speak about intermittent
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fasting, that's what we'll be covering here.
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Weight loss boils down to a simple measurement of calories in and calories out. If you're
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eating more calories than are burned, you'll gain weight, and if you're taking in fewer
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calories than you burn, you'll lose weight. For this reason, dietary interventions for
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weight loss are primarily focused on continuous energy restriction (CER), or in other words,
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eating as you would normally, but just eating fewer calories. The issue with CER is that
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adherence is poor, typically declining within 1-4 months.
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A recent review by Rynders and colleagues from 2019 concluded that, based on current
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literature, “intermittent fasting does not seem to produce greater weight loss than continuous
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energy restriction.” please do a highlight effect on the relevant line on page 18 of
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the PDF, like how Jeff Nippard does so in his videos that reference scientific papers
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However, “adherence to weight loss interventions is the greatest predictor of weight loss success”
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bottom of page 14, and some patients may respond better to caloric restriction, while others
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will respond more favorably to intermittent fasting or time restricted feeding. Few studies
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were carried out long enough to properly assess adherence of the two different protocols.
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Interestingly, time restricted feeding seems better at preserving lean mass per a 2016
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study by Moro and colleagues compared to continuous caloric restriction. Count that as a win for
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time restricted feeding.
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While IF is moderately successful for weight loss, there are a variety of interesting metabolic
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effects that occur. Kahleova et al compared time restricted feeding to caloric restriction,
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and found greater improvements in fasting glucose, increased oral glucose insulin sensitivity,
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decreased C-peptide levels, and decreased glucagon levels in the TRF group.
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Sutton and colleagues in 2018 found that men with prediabetes who performed time restricted
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feeding experienced improvements in multiple cardio metabolic health markers, beyond what
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would occur from simple weight loss.
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But what if you're healthy without any chronic medical conditions? A meta-analysis and systematic
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review by Cho et al. in 2019 concluded that “intermittent fasting improves glycemic
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control and insulin resistance with a reduction in BMI, a decrease in leptin level, and an
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increase in adiponectin concentration,” which are generally considered favorable changes
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for most individuals. top of page 9 Cabo in 2020 also highlighted the benefits in blood
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pressure, heart rate, endurance training effects, and abdominal fat loss.
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Cognitive effects have been primarily studied with regards to caloric restriction, not time
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restricted feeding, although it's not unreasonable to assume similar findings. After all, time
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restricted feeding and other intermittent fasting protocols often lead to decreased
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caloric intake compared to feeding ad libitum.
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Studies in animals show enhances in multiple domains of cognition, including spatial memory,
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associative memory, and working memory. Caloric restriction has been associated with improved
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verbal memory in older adults, and in a separate study involving overweight adults with mild
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cognitive impairment, 12 months of caloric restriction led to improvements in verbal
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memory, executive function, and global cognition. More recently, Leclerc et al in 2020 performed
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a large multi center randomized trial demonstrating that 2 years of daily caloric restriction
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led to significant improvements in working memory.
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Stress isn't always a bad thing. With intermittent fasting and time restricted feeding, cells
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are forced to cope with the lack of a relatively continuous glucose source and engage in a
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“coordinated adaptive stress response that lead to increased expression of antioxidant
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defenses, DNA repair, protein quality control, mitochondrial biogenesis, autophagy, and down-regulation
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of inflammation.” bottom left page 2544 from Cabo 2020
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The effects on aging are likely present but not yet well established. While several studies
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have demonstrated benefits to the aging process in animals, the evidence is less clear cut
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when it comes to human longevity and healthy lifespan. After all, we haven't been able
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to research IF or TRF for decades among clinical subjects.
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In terms of cardiovascular disease, intermittent fasting has been associated with improvements
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in multiple cardiovascular markers, including blood pressure, resting heart rate, HDL and
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LDL cholesterol, triglycerides, and insulin resistance. Additionally, IF has been associated
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with reductions in markers of systemic inflammation and oxidative stress that are associated with
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atherosclerosis, which is the clogging of your arteries.
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Next, does the time of day during which your feeding window is scheduled matter? Jamshed
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and colleagues in 2019 studied early time restricted feeding (eTRF) by shifting the
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entire feeding window earlier in the day. While far from a perfect study, it demonstrated
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several interesting findings: first, early time restricted feeding resulted in lower
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average glucose and insulin levels. Second, avoiding food for a prolonged period before
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bed was also associated with lower nighttime cortisol and overnight glucose. And third,
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there was increased expression of LC3A, an autophagy gene, suggesting that more autophagy
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occurred with early time restricted feeding.
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Evidence-based medicine focuses on making treatment decisions based on the best evidence
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available in the scientific literature. It's fantastic at addressing things like infectious
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diseases, because infections are acute and binary in outcome and interventions are generally
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straightforward, like an antibiotic or antiviral, thus making measuring inputs and outputs relatively
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straightforward.
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The problem, however, is that most things related to health are much more complicated.
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Those opposed to intermittent fasting are quick to say “there isn't sufficient evidence
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to support a fasting protocol for disease X or Y, therefore you should not do it.”
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But to most accurately assess the situation, you need to compare the risk of the intervention,
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which is what we normally think of, with the risk of not doing the intervention, which
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people are less likely to consider.
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I am not saying that you should begin time restricted feeding or intermittent fasting,
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particularly if you have another pre-existing medical condition, and you should speak with
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your physician. I am saying, however, that if there is a favorable asymmetric risk profile
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for you personally, meaning the potential upside is relatively large and the potential
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downside is relatively small for any lifestyle intervention, experimentation may not be a
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bad idea.
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After consulting the literature and considering the potential upsides and downsides in my
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own personal life, I decided to give it a shot. I first started in 2017 when I was in
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plastic surgery residency, but I quickly realized that I was losing too much weight since I
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couldn't eat while in the operating room. I then started again about a year ago, back
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in 2019.
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I've been doing time restricted feeding for close to a year now, and I'm enjoying
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it enough to stick with it. It's also been a gateway intervention of sorts, paving the
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way for me to perform 36 and 48 hour fasts a few times, and has even been a practice
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in building discipline. I just released a video on my personal channel going over my
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experience, including the benefits and drawbacks I've experienced over the last year. Link
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in the description.
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Thank you all so much for watching. If you've tried intermittent fasting or time restricted
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feeding, what was your experience like? Let us know with a comment down below. Much love,
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and I will see you guys in that next one.