Intermittent fasting is another trending topic that has grown in popularity recently. The most common form of intermittent fasting, involving a prolonged overnight fast and a relatively shorter window of time to eat during the day, reflects our own circadian rhythm. While fasting may be a useful tool for the general population, it also contradicts several other theories like the importance of eating breakfast, the need for several meals throughout the day, and the benefits of eating a snack before bedtime.
So is it really better to "graze all day" or to "give your gut a break"?
Let's first take a look at the most common types of intermittent fasting:
Alternate Day Fasting: alternating between days of fasting (no calories coming in) and feeding days (eating unrestrained to comfort).
Mixed results in short-term human studies with potential improvements in glucoregulatory markers, blood lipids, and inflammation, but with increased feelings of hunger. (Patterson & Sears, 2017)
Modified Fasting: 'fasting' days consist of eating 20-25% of your daily calorie needs, often used as a 5:2 where two non-consecutive days of the week have this very low calorie intake and the other five days of the week are unrestricted eating to comfort.
Human studies of 2-6 months found improved fasting glucose, insulin, blood lipids, inflammation, and significant weight loss compared to control [but NO DIFFERENT than continuous energy restriction]. (Patterson & Sears, 2017) *Maintaining a DAILY calorie deficit showed the same effects as the modified fasting of 2 very low calorie days.
Some additional findings suggest moderate improvements in mood, and feelings of hunger and satiety. (Patterson & Sears, 2017) *Increased sensitivity or awareness of hunger and satiety, which may be useful for some populations.
Time Restricted Fasting: typically extending the natural overnight fast to at least 11 hours or more. This practice is most commonly utilized in a 16:8 method where the overnight fast lasts for 16 hours and the other 8 hours of the day are the window of time available to eat.
Human studies involving at least 11 hours of overnight fasting found significantly greater weight loss compared to control. (Patterson & Sears 2017)
Eating only one afternoon meal per day (vs. three meals per day, meaning you are otherwise fasting for the rest of the day/night) was associated with significantly greater weight loss, reduced blood glucose, and improved blood lipids, but with reported increases in morning hunger. (Patterson & Sears, 2017) **It was noted that prolonged morning fasting affected appetite and satiety regulating hormones, but NOT subsequent calorie intake; i.e. greater feelings of hunger, but NOT overcompensating at the first meal. So it's a matter of how well can you handle those hunger pangs?
So what is it about fasting that can have these effects?
This is based on the theory that our biologically programmed circadian rhythm dictates appropriate activity level and metabolic influence based on the time of day. (Patterson & Sears, 2017)
It is suggested that our circadian rhythm occurs over a 24 hour period where there is a central 'clock' in the brain, and peripheral 'clocks' in the liver, fat, and muscles. (Patterson & Sears, 2017)
Now these central and peripheral clocks are already synchronized so that physiological processes like hormonal secretions, physical coordination, and sleep will be performed at the optimal time like in response to light and dark stimuli from time of day. (Patterson & Sears, 2017)
Observations suggest that nighttime shift work is associated with desynchronizing these circadian clocks, with further impaired metabolic health effects. (Patteron & Sears, 2017) There are observed changes in hormonal secretion and sensitivity based on the time of day where for instance insulin sensitivity is notably highest in the morning and decreases throughout the day into the night, whereas growth hormone secretion pulses increase at night. (Patterson & Sears, 2017) Based on this concept, it is suggested that an increase in insulin secretion and blood glucose following a meal, particularly later in the evening or night when insulin sensitivity is comparatively LOWER, could be associated long-term with insulin resistance and increased HbA1c. (Patterson & Sears, 2017) *This is often highlighted in reviews of the negative health impacts of shift work. **This would suggest that NOT eating at night could aid in synchronizing those circadian clocks to match the biologically optimal timing of eating during the daytime hours.
This circadian rhythm is noted to apply to the gut as well where gastric emptying, blood flow, and metabolic responses to glucose may be related to time of day; notably slower or decreased responses at night. (Patterson & Sears, 2017) *Most of the studies on gut microbiota responses to fasting have been limited to animal studies.
Modified lifestyle behaviors
Unsurprisingly, the use of an extended overnight fast is associated with lower total calorie intake during the day (due to LESS total time actually spent eating, and possibly due to improved hunger and satiety sensitivity) as well as improved satisfaction with sleep and increased daytime energy levels. (Patterson & Sears, 2017)
A major takeaway point for intermittent fasting is this...
EAT DURING THE DAY + SLEEP AT NIGHT=
LESS time spent eating during the day (like eating within 8 hours instead of 12+)
Possibly improved sensitivity for hunger and satiety
Eating during the day when gastric emptying, blood flow, and glucose metabolism/insulin sensitivity are highest
Prioritizing sleep (since there is nothing else to do at night if you can't eat or drink...)
NOT eating right before bedtime (possibly associated with improved sleep quality) *SEE PREVIOUS POST ABOUT SLEEP AND NUTRITION...
**Easy enough to understand... no snacking after dinner, so instead you go to bed early because no one is going to stay up late, hungry, or go out with friends and fast the whole night, and then go to sleep. [You could argue that it may NOT even be the fasting itself, but rather the choice to optimize sleep that is driving the most health-promoting effects.]
This theory directly contradicts the highlighted practice of frequent meals throughout the day, and a bedtime snack, as being beneficial. Now, the use of frequent meals throughout the day (specifically consistent carbohydrate intake) has been clinically relied upon as a method to maintain stable blood glucose concentrations throughout the day, particularly for diabetic populations where insulin signaling is impaired. (Asif, 2014) Additionally, the use of a bedtime snack is already commonplace for populations, like type 1 diabetics, as a tool to avoid nocturnal hypoglycemia (or maintain blood glucose levels during sleep). (Asif, 2014)
However, the utility of frequent feedings and bedtime snacks is NOT limited to the diabetic populations, but relevant to active populations as well...
The OVERWHELMING consensus suggests that frequent feedings, specifically for protein intake (20-30g), evenly spaced throughout the day (every ~3 hours) is associated with the greatest increases in muscle protein synthesis, improved body composition, and performance outcomes. (Kerksick, et all 2017) Additionally, the use of a high protein pre-sleep snack (30-40g casein) is noted to improve muscle protein synthesis and metabolic rate, increasing morning satiety, and WITHOUT impacting lipogenesis (without increasing fatty acid production) OR negatively impacting insulin sensitivity. (Kerksick, et al. 2017)
Necessary to rebuild, repair, and replace lean tissues, enzymes, hormones, neurotransmitters, and more, that are used ALL DAY, EVERYDAY
Active populations especially require a higher amount to meet these needs (aiming for 1.2-2g/kg body weight, even as much as 2.3-3.1g/kg)
Higher thermic effect of feeding [uses MORE energy to metabolize= modest increase in metabolic rate]
POOR lipogenic factor [excess protein intake DOES NOT necessarily turn into stored body fat, but rather gets metabolized and urinated out]
Does NOT cause kidney damage in otherwise healthy kidneys
What's more...you need to take in so much protein EVERYDAY, with active populations requiring more, to where those extra protein snacks throughout the day and before bed are just more opportunities to get it in, and essentially without any adverse side effects.
And what does that mean for breakfast?
I will tread lightly with the phrasing here... "breakfast" as in "breaking the fast" from your overnight sleep is simply that FIRST meal you eat (meaning it is NOT defined by the time of day or the type of food). You could theoretically eat your 'breakfast' at any time of day, from any combination of foods; it is simply that first meal that 'breaks your fast'. *Everyone is "breaking their fast" at some point everyday, so in reality there are no 'breakfast skippers'...
This first meal of the day can therefore be an important opportunity to take in protein, especially to AVOID potential catabolic breakdown of lean body mass from prolonged fasting. (Grieske, et al. 2018) HOWEVER this is in stark contrast to the traditional Americanized breakfast items consisting of HIGH calorie intakes, primarily from carbohydrates and fats.
There is also NO specific designation of when to eat this first meal of the day ('breaking the fast') to where some individuals fare well to wait until midday to eat their first meal without any negative health affects. HOWEVER, it is suggested that activity level and goals would most likely dictate the optimal timing of that first meal of the day. In this sense, an active individual or athlete, especially one that is training in the morning, would do BEST to eat early (within at least an hour of training) to AVOID catabolic breakdown of lean tissues and impaired athletic performance. (Grieske, et al. 2018) Study findings consistently show that pre-exercise intake of carbohydrates with PROTEIN aids in supporting athletic performance and body composition, with protein intake pre-exercise specifically associated with increased fat oxidation and energy expenditure post-exercise compared to carbohydrate alone or remaining fasted. (Gieske, et al. 2018) *Pre-exercise protein intake can preserve lean body mass and increase post-exercise fat oxidation.
But that catabolic breakdown from fasting may be useful for some populations...
When in a fasted state (like during sleep), food nutrients go to work for their respective needs like glucose being metabolized and stored as glycogen, amino acids incorporated into proteins for tissue repair, etc. During this state of INACTIVITY, fatty acids become a major fuel source, which OF COURSE leads to the notion that extending this fasted state would equate to greater total time of oxidizing fatty acids (or 'tapping into your fat stores').
Interestingly, in an 8 week study with resistance-trained males, when total calories, protein, and exercise prescription were controlled for, the group that followed an extended overnight fast (16:8) showed a decrease in fat mass, as well as decreased testosterone and IGF-1. (Moro, et al. 2016)
What is interesting about these study findings is that:
BOTH groups ate 100% of their calorie needs (NO calorie deficit)
There was only a 4 hour difference in meal distribution and overnight fasting (16 hr fast vs. standard 12 hr overnight fast)
Same macronutrient ratios for both groups (~2900 kcal, ~53% carbs, ~24% fat, ~22% protein)
Both groups were experienced in strength training, and performed the exercise protocol in the afternoon (4-6pm) after eating a meal at 1pm *NOT fasted exercise
BOTH groups increased strength, and maintained lean body mass *DESPITE the 16:8 fasted group showing decreased testosterone and IGF-1
The key factor to keep in mind: this study was only 8 weeks in duration. While it is interesting that both groups maintained lean body mass and increased strength, the decrease in anabolic hormones for the 16:8 fasted groups opens up the question of what the long-term effects could be if continued at that rate. Even more surprising is the noted decrease in fat mass despite no calorie deficit for the 16:8 fasted group, which brings about the question: how accurate were the measurements for calorie intake? Energy balance (calories in vs calories out) is THE primary driver for loss of body fat, to where there is surely an unaccounted-for difference between these two groups in that the 16:8 fasted group is either taking in less calories or expending more calories than the other group. These findings do open up a world of discussion on short term weight maintenance/fat loss plans though, like for an athlete that needs to continue training while trying to cutting excess weight and preserving lean body mass.
However there are a few key details to keep in mind:
Fatty acids are oxidized for fuel primarily under certain conditions (low activity or inactivity)
Fatty acids are a metabolically 'expensive' fuel source (require MORE oxygen per gram vs glucose)*Meaning it is actually much HARDER to burn the same amount of total calories from fat than from carbohydrate
Body fat loss requires a calorie deficit, regardless of the type of diet
Long term improvements in body composition depend upon maintaining lean body mass, NOT catabolic tissue breakdown *Increased lean body mass increases basal metabolic rate, athletic performance, aesthetics, and overall health
Bottom line is this...
An extended overnight fast MAY be a useful tool for some populations for making lifestyle changes.
For body composition, calories are KING and optimizing protein is QUEEN (the type of diet does NOT matter, and fasting itself is NOT magic).
EVERYONE can benefit from prioritizing sleep.
Asif M. The prevention and control for type 2 diabetes by changing lifestyle and dietary pattern. Journal of Education and Health Promotion. 2014; 3:1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3977406/
Gieske B, Stecker R, Smith C, et al. Metabolic impact of protein feeding prior to moderate-intensity treadmill exercise in a fasted state: a pilot study. Journal of the International Society of Sports Nutrition. 2018; 15(1):56. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6267781/
Kerksick C, Arent S, Schoenfeld B, et al. International society of sports nutrition position stand: nutrient timing. The Journal of the International Society of Sports Nutrition. 2017; 14: 33. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5596471/
Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine. 2016; 14: 290. https://www-ncbi-nlm-nih-gov.proxy.lib.fsu.edu/pmc/articles/PMC5064803/
Patterson R, Sears D. Metabolic Effects of Intermittent Fasting. Annual Reviews in Nutrition. 2017; 37:371-393. https://www.annualreviews.org/doi/full/10.1146/annurev-nutr-071816-064634?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed
Stockman M, Thomas D, Burke J, et al. Intermittent Fasting: Is the Wait Worth the Weight? Current Obesity Reports. 2018; 7(2):172-185. https://link.springer.com/article/10.1007%2Fs13679-018-0308-9