Table of Contents
In the first 10 lessons we discussed quantitative and qualitative aspects of nutrition. In this lesson we will discuss considerations of “chrononutrition”, the timing of when we eat.
There are several different aspects regarding the timing of nutrition. With respect to dieting to lose weight, one consideration is to eat at a constant caloric deficit every day (“CER”: continuous energy restriction) vs breaking up dieting in daily, weekly, or longer periods (“IER”: intermittent energy restriction). With respect to nutrition in any one day, considerations include how many meals to consume each day, the timing of the meals, the size of the meals, and the total window of time within which to consume all calories (ie, a 4 hour vs 8 hour vs all day eating period). There is some overlap between these topics.
Note: Terminology in the research literature is not completely congruent with terminology used by the general public. For example, a popular eating strategy in the general public is intermittent fasting (“IF”), which is frequently done by consuming all of one’s calories in an 8 hour window any given day. In the research literature this strategy is referred to as time restricted feeding (“TRF”), while IF is typically used similarly to intermittent energy restriction (“IER”) and can describe a diet such as the 5:2 diet where there are 2 days of intense energy restriction in any given week. I will mainly be using terms here that match the research literature. When there may be ambiguity I will attempt to make this clear.
Intermittent energy restriction (“IER”) and intermittent fasting (“IF”)
Note: In this section I am using IER and IF to refer to several different types of dieting protocols, including:
- significant caloric restriction on some days of the week interspersed with days with less or no caloric restriction
- complete fasting 1+ days of the week interspersed with days with less or no caloric restriction
- caloric restriction every day for 1+ weeks interspersed with 1+ weeks with less or no caloric restriction
I am not referring to TRF studies, where caloric consumption is restricted to a specific time period any given day.
When fasting for extended periods of time metabolism shifts in the body to primarily utilize ketone bodies, which can begin to rise after fasting 8-12 hours and continue rising further as fasting extends to 24+ hours. Ketone bodies and fasting in general have been shown to help regulate several genes with health implications. This upregulation of various metabolic and genetic pathways while fasting has been associated with many potential health benefits regarding glucose homeostasis, blood pressure, resting heart rate, antioxidant utilization, autophagy, neuronal stress resistance, downregulation of inflammation, and may even impair energy metabolism in cancer cells.(de Cabo, 2020) Most of the literature supporting these benefits comes from animal studies and small human trials. While promising, it remains to be seen how much these benefits of fasting fully carry over to humans.
Additionally, as a general rule of thumb fasting protocols induce weight loss, and thus it is important to consider if the benefits are due to fasting or due to weight loss itself. A recent systematic review and meta-analysis (SR/MA) examining IER & TRF trials and their impact on BMI & glucose metabolism found that compared to CER there was a small but significant benefit for BMI (-0.75 kg/m2), fasting glucose level (-4.1 mg/dL), insulin resistance, adiponectin, and leptin.(Cho, 2019) An even more recent SR/MA including both IER and TRF trials found no significant impact on weight, but in 5 studies exclusively enrolling patients with type 2 diabetes favorable effects were seen on measures of blood glucose control.(Welton, 2020)
Note: Regarding IF or IER protocols in patients with type 2 diabetes care must be taken to prevent hypoglycemic episodes. Please discuss with your physician regarding any medication adjustments or other considerations if this applies to you.
Two systematic reviews & meta-analyses (“SR/MAs”) that only considered IER vs CER found no benefit for weight loss, glucose, cholesterol, or blood pressure levels.(Cioffi, 2018; Schwingshackl 2020) Another SR/MA of studies lasting a minimum of 6 months found no benefit for weight loss of IER vs CER.(Headland, 2016) A separate SR/MA of studies evaluating people with overweight or obesity similarly found no benefit to IER.(Roman, 2019) A MA of studies in adults age 18-69 with overweight or obesity also found no relative benefit of IER or CER.(Enríquez Guerrero, 2020) Additionally, a SR/MA examining lipids specifically also found no benefit of IER vs CER.(Meng, 2020) A review article discussing IF in humans and going over the various health benefits seen finds that the vast majority of health effects are comparable to CER.(Mattson, 2017)
A SR/MA considering IER of fairly long duration (energy restriction periods ranging from 2-12+ weeks at different levels of caloric deficits) found no significant benefit for weight loss compared to CER.(Harris, 2018) In contrast to this, the MATADOR study compared 2 week energy restriction blocks (at 67% maintenance calories) interspersed with 2 week energy maintenance blocks (8 dieting blocks total) to 16 weeks of CER (also at 67% maintenance calories) and found better results in the intermittent dieting group.(Byrne, 2018)
Note: In the MATADOR study the intermittent dieting group had 8×2 week blocks of energy restriction and 7×2 week blocks of energy maintenance (yielding 30 total weeks) compared to 16 weeks total for the CER group. All main meals were provided. The intermittent group that completed the protocol lost 14.1 kg compared to 9.1 kg for the CER group. It is unclear how much additional weight the CER group would have lost had they continued to 30 weeks.
The more interesting aspect of this study is that both groups had 16 weeks at 67% maintenance calories but the intermittent group lost significantly more weight (and also maintained significantly greater weight loss at 6 month follow-up). This may be due in part to amelioration of adaptive thermogenesis (discussed in Lesson 1) by eating at maintenance in the 2 week blocks or due to better dietary adherence (it is likely easier to adhere to a diet in 2 week blocks than for 16 weeks consecutively).
Following the MATADOR study a trial comparing intermittent dieting in 1 week blocks to a 5:2 diet and also CER for a 1 year period was published, showing no differences in results. Maximum weight loss was at 6-9 months in all groups with regain occurring after that. After an additional year follow-up beyond the end of the trial there were still no differences between the groups.(Headland, 2019; Headland, 2020)
Overall there is very little evidence indicating superior outcomes of IF or IER protocols compared to continuous dieting schemes, and in fact the majority of the evidence suggests similar outcomes. Of note, though, is that several of the above reviews mention that the attrition rate in studies of IER vs CER are typically similar. Thus, if anyone believes following an IER protocol will lead to greater adherence this should definitely be considered. It is possible with more research we will find other tangible benefits of fasting protocols similar to those seen in mice studies, but at this point in time the evidence base is not strong enough to support recommending prolonged fasting to support better health. Additionally, as alluded to in prior lessons, fasting for prolonged periods will likely lead to loss of lean body mass, which is generally undesirable.
Tip: If a person is dieting and intends to go on vacation, this can be an excellent time to take a break from caloric restriction and attempt to eat at maintenance or a slight caloric surplus. This should allow the vacation to be more enjoyable and then dieting can be resumed after the vacation with renewed vigor. Similarly, if somebody feels run down from the dieting process this can be a good time to practice eating at maintenance to allow one to recharge and to reinforce practices to keep the weight off once dieting is complete.
A recent review indicates there is mixed evidence from epidemiologic studies indicating that higher meal frequency aids cardiovascular disease and diabetes markers, while in trials there seems to be mild benefit of increasing meal frequency on cholesterol levels, with no significant benefit on triglycerides, blood pressure, fasting glucose, or insulin.(St-Onge, 2017) Of note, many of the included trials in this review are very small.
A separate review suggests there is not much difference in weight loss with varying meal frequency though higher frequency may lead to less hunger (but also less satiety after eating), decreased protein oxidation, and more stable blood glucose & insulin levels.(Oke, 2019) More stable blood glucose and insulin is logical given caloric consumption would be spread out more with higher meal frequency; this does not contradict the prior review stating there is no impact on fasting glucose and insulin as those measurements would be obtained from a single time point and not significantly related to a prior meal.
A separate review finds increased meal frequency to be potentially beneficial for lean mass preservation in a caloric deficit (without otherwise impacting body weight or appetite).(Hutchison, 2016) Within this review they note concern from one cited study indicating increased meal frequency may be metabolically harmful in a caloric surplus.
A more recent observational study did find benefit to higher meal frequency regarding lower BMI, however, those who ate more frequently also ate earlier in the day, which has also been shown to yield health benefits (see below).(Zerón-Rugerio, 2020)
A very recent narrative review finds increased eating frequency was associated with lower adiposity and metabolic syndrome risk but this may be due to increased eating frequency correlating with better diet quality.(Garcidueñas-Fimbres, 2021)
Tip: This same review also examined the role of eating speed and found a decreased eating speed was associated with better outcomes regarding adiposity and metabolic syndrome risk.(Garcidueñas-Fimbres, 2021) Thus, for individuals who are attempting to lose weight it may be helpful to eat more slowly. There are a variety of tools out there to help with decreasing portion sizes (ie, small utensils, glasses shaped in a specific way to slow the pace of eating, etc), though it seems these do not work well in isolation.(Vargas-Alvarez, 2021)
Perhaps using these strategies with a cognitive effort to eat more slowly will prove more effective. Other tricks include using your off-hand (ie, your left hand if you are naturally right-handed), chewing more with each bite, putting your utensil down between bites, or restricting yourself to a small first portion size and waiting 15 minutes prior to consuming a second portion. Eating with others and having a conversation during a meal can also work very well. Ultimately all of these strategies allow various gut hormones to release and travel to the brain where they decrease hunger and increase satiety.
Given the lack of strong findings above, the fact that there is no technical definition of what constitutes a meal(St-Onge, 2017) (and thus hard to do well-designed consistent trials), and the fact that while many people report having 3 meals daily their actual daily intake does not reflect this(Gill, 2015), there is not a strong enough literature base at this time to recommend a specific meal frequency. As stated in Lesson 4, spreading protein intake throughout the day will likely benefit muscle protein synthesis. Thus, I recommend a minimum of two distinct protein (and hence meal) feedings on a daily basis, and there may be additional benefit with three.
Note: Below in the TRF section there are several studies discussed that indicate a benefit to consuming calories in a restricted window, which would seem to necessitate fewer meals, yet some of the evidence above weakly suggests more meals may be beneficial. This may seem confusing but can can be consistent with the below TRF literature when considering:
- some of the above literature compared 1 meal daily to multiple meals while most TRF protocols still allow 2-3 meals daily
- some of the above literature evaluating 2-3 meals had them consumed far apart (ie, 8am, 1pm, 6pm, plus a snack), such that there was no prolonged fasting window, as this would not be consistent with TRF
Thus, these findings should not be considered inconsistent with the TRF findings discussed below.
Timing of meals
There has been emerging evidence over time that suggests eating at consistent daily times with similar intake throughout(Aqeel, 2020) or more intake preferentially early in the day may lead to better health outcomes.(Almoosawi, 2016; Pot, 2016; Patterson, 2017; Dashti, 2020; Zerón-Rugerio, 2020). This is thought to be related to the various circadian rhythms in the body, the prominent one being from the suprachiasmatic nucleus in the hypothalamus, with additional ones found in peripheral organs as well.(Chaix, 2019) This leads to various metabolic changes throughout the day, one example being increased insulin sensitivity in the morning.
However, there is not much strong evidence indicating a significant benefit to breakfast consumption, though it is possible that uncontrolled late night eating may mitigate benefits that would otherwise be seen in intervention trials.(Gwin, 2018) A very recent review on the topic of breakfast similarly concludes we need more trials to help determine if breakfast is beneficial, as there are associations breakfast intake and beneficial cardiovascular outcomes but when non-habitual breakfast consumers begin to consume breakfast regularly they typically gain weight.(Santos, 2020) It may ultimately come down to whether or not eating breakfast helps or hinders any given individual with maintaining an overall healthy diet.
Looking at a few specific studies:
– One study randomized women with an average BMI of 33 who were nondiabetic to two different isocaloric protocols: either 700/500/200 kcal for breakfast/lunch/dinner or 200/500/700 kcal for breakfast/lunch/dinner.(Jakubowicz, 2013) The group eating more kcals at breakfast had better results with triglycerides, cholesterol, glucose, insulin, had lower ghrelin (the “hunger” hormone) throughout the day, and had lower postprandial glucose and insulin levels after the 700 kcal meal.
– A different study took 420 people with overweight or obesity and put them on a weight loss program based on the Mediterranean diet, a moderate physical activity intervention, and cognitive behavioral therapy, among other education.(Garaulet, 2013) No advice was given with respect to the time of day they should eat or when they should consume most of their calories. Lunch was the largest meal of the day for this group and people who ate lunch earlier had better weight loss outcomes starting at week 5 of the intervention despite no differences in reported energy intake or energy expenditure.
– This rather interesting study not only considered when people eat most of their calories but also the relationship of this to when they typically wake up and go to sleep.(Xiao, 2019) Here they look at 800+ adults and consider the midpoint of time in bed as a cutoff; those whose midpoint was earlier than the median midpoint were considered as “early chronotypes” and those whose midpoint was later than the median midpoint were considered as “late chronotypes”. They find that while a higher percentage of energy intake early in the day is associated with reduced odds of overweight & obesity, this is more strongly seen in those with an early chronotype. Similarly, they find that that more energy intake at night is associated with higher odds of overweight and obesity moreso in those with a late chronotype. They find this mainly associates with carbohydrates and protein but not fat.
Note: I suspect over time with more research we will find this concept of chronotype and relation of normal wake/sleep time to eating patterns has a fairly significant role in the health implications of “chrononutrition”. At this point in time the vast majority of studies evaluating the timing of nutrition do not consider normal wake and sleep times. Future studies should consider this as chronotype may serve as a significant confounding variable otherwise. Much more research remains to be done to elucidate the full role that chronotype plays on the implication of eating times for overall health.
Thus, at this point evidence indicates a benefit to eating the majority of one’s calories early in the day, but it is not yet clear exactly how to define “early” (ie, based on the time of day or based on relation to when a person wakes up) or what proportion of one’s calories should be consumed early to yield greater benefits. It’s also not clear if this would change when consuming calories in a deficit, maintenance, or surplus. Additionally, these studies have not included any intense exercise interventions; for people who do workout in the evening it is conceivable these results may not apply to the same degree. Nonetheless, the research overall is promising that greater intake earlier in the day will yield health benefits.
Time restricted feeding (“TRF”)
TRF, or the concept of consuming all of one’s calories in a specific window of time, has become quite popular in the general public, with some people choosing to consume all of their calories in one large meal and others choosing to consume all of their calories in timed windows ranging from 4-12 hours. There is evidence collectively that this may influence the various circadian rhythms alluded to above and thus may benefit health beyond simply leading to decreased caloric consumption.(Waldman, 2020)
Reviews of TRF studies have shown small benefits regarding weight and metabolic outcomes, but these reviews have typically included Ramadan studies.(Rothschild, 2014; Pellegrini, 2020) Given that Ramadan goes directly against the circadian rhythm with respect to the time of eating it is possible this will underscore the actual benefits of a TRF approach. However, there are actually not many non-Ramadan studies that evaluate this, and most of the studies were published so recently that there are not any large reviews (that I’ve seen) that cover the majority of these studies. Therefore, I have summarized findings from several non-Ramadan studies, lasting at least 1 week duration, below.
Click each row for a paragraph describing the findings of the studies:
In 2015 a study by Gill and Panda was published where a smartphone app was used to monitor eating patterns of participants (this study was mentioned above as it shows many people do not follow a stereotypical breakfast/lunch/dinner eating pattern).(Gill 2015) They then took n=8 of the participants with BMI >25 who normally consumed kcals for >14 hour daily duration and put them in a 16 week pilot study where they were advised to self-select a 10-12 hour eating window (with no limitation of caloric consumption). On average the participants decreased caloric consumption 20% and lost 3.27 kilograms, with improved hunger, sleep, and energy levels. After 36 additional weeks all of the weight lost, improved sleep, and improved energy levels were maintained.
In 2016 Tinsley et al. published an 8 week trial with 28 individuals split into a TRF or normal diet (“ND”) group with a full body workout being done 3 days weekly.(Tinsley, 2017) The TRF group employed a 4 hour feeding window between 4pm and midnight on nontraining days. N=10 completed the trial in the TRF group and n=8 completed the trial in the ND group. There were no significant differences in body weight, body composition, muscle cross-sectional area, or strength between the two groups at the completion of the trial. However, the ND group did gain 2.3 kg of lean body mass compared to a loss 0.2 kg in the TRF group. Of note, the protein intake averaged 1.4 grams/kilogram (g/kg) for the ND group but only 1.0 g/kg for the TRF group, which may have hampered the TRF group’s results.
In 2016 Moro et al. published an 8 week trial with n=34 resistance-trained males split into a TRF group and a normal diet (“ND”) group, both consuming maintenance calories.(Moro, 2016) The TRF group consumed calories at 1pm (40%), 4pm (25%), and 8pm (35%). The ND group consumed calories at 8am (25%), 1pm (40%), and 8pm (35%). Three exercise sessions were done weekly between 4-6pm. The TRF group lost more body fat (-16.4% vs -2.8%) while both groups maintained their fat-free mass. The TRF group had more substantial beneficial changes in blood glucose, insulin, triglycerides, and adiponectin. The only potential negative findings in the TRF group (from a muscle hypertrophy standpoint) that were not seen in the ND group were decreases in total testosterone and IGF-1, though these values were still within normal reference ranges.
In 2018 Sutton et al. published a 5 week crossover isocaloric trial in n=8 men with prediabetes where a 6 hour eating window (starting between 06:30-08:30) was compared to a 12 hour eating window (also starting between 06:30-08:30).(Sutton, 2018) The TRF period relative to the control period yielded benefits for insulin levels, insulin sensitivity, blood pressure, and 8-isoprostane (a measure of oxidative stress to lipids). Triglycerides were worsened but the authors speculate this may be due to the longer fasting period prior to testing them in the TRF group.
In 2018 Gabel et al. published a 12 week pilot study where they had n=17 adults (BMI 30-45, non-diabetic) employ TRF with an 8 hour eating window from 10:00-18:00.(Gabel, 2018) Relative to a historical control group by the same authors the TRF group lost 2.6% body weight (energy intake decreased ~340 kcals daily) and had an improvement in systolic blood pressure. No improvements were seen with diastolic blood pressure, cholesterol levels, glucose, or insulin resistance. Of note, the participants were metabolically healthy at baseline which would make it less likely to see changes in these markers.
In 2018 Antoni et al. published a 10 week pilot study with adults with BMI 20-39 with n=7 in the control group and n=9 in the TRF group.(Antoni, 2018) Here TRF consisted of delaying the first energy intake of the day by 1.5 hours and advancing the last energy intake of the day by 1.5 hours. On average the eating window decreased by 4.5 hours. This was found fairly difficult to stick to (rating 7/10) due to interference with social events. There was no difference in body weight changes between the two groups, but the TRF group had an average decrease in body fat percentage of 1.9%. The control group had an increase in fasting glucose levels while the TRF group did not; it’s unclear what caused this increase.
In 2019 Hutchison et al. performed a trial examining 9 hour feeding windows starting at 8am or 12pm (each participant did 1 week with each time period separated by a 2 week wash out period) in n=15 men at risk of type 2 diabetes.(Hutchison, 2019) TRF with either window improved the glycemic response to a test meal; however both windows as well as a baseline testing period (consisting of monitoring of intake when not eating in a specific window of time) yielded ~1% loss in body weight. Thus, it’s unclear if the improved glycemic response may be secondary to a decrease in body weight, and it seems the TRF window was not responsible for the decrease in body weight, rather simply monitoring may have been. There were no strongly meaningful differences between the two TRF time periods.
In 2019 Anton et al. published a 4 week pilot study where they took elderly overweight sedentary adults and advised them to follow TRF with a self-selected 8 hour window.(Anton, 2019) The mean age of the n=9 participants was 77 years old. There was a mean decrease in body weight of 2.6kg and a small increase in walking speed and a trend towards improved quality of life. Most of the participants stated they would be willing to continue this eating style with some modifications. Body composition changes were not measured (thus it is possible lean body mass was lost), and it is possible the increase in walking speed was due to a practice effect as the six-min walk test was used to assess the walking speed before and after the intervention.
In 2019 Tinsley et al. published an 8 week trial in resistance training females with an 8 hour TRF window from 12:00-20:00.(Tinsley, 2019) Protein intake was at least 1.4 grams/kilogram/day (g/kg/d) and they aimed for a 250 kcal/d deficit. They trained 3 times weekly with training sessions between 12:00-18:00. There were 2 separate TRF arms, one of whom was given supplemental beta-hydroxy beta-methylbutyrate (HMB) (thought to decrease muscle protein breakdown when in a catabolic state), as well as a control arm. The intention to treat analysis had n=40 while the per protocol analysis had n=24. There was no difference in results between the groups, indicating TRF did not inhibit skeletal muscle hypertrophy or strength acquisition, and HMB did not prove beneficial.
In 2020 McAllister et al. published a 4 week pilot study in n=22 physically active college-age men where they were split up into 2 groups; one group was given isocaloric intake, the other was allowed to eat ad libitum, and both followed a 16 hour daily fasting protocol.(McAllister, 2020) The average eating time was ~11:30-~19:00. Both groups had similar caloric intake and had positive changes in cholesterol and inflammatory markers as well as blood pressure and a decrease in heart rate with no change in body weight, body fat percentage, alertness, or oxidative stress markers. There were no meaningful differences in results between the groups.
In 2020 Wilkinson et al. published a 12 week TRF trial with a self-selected 10 hour window in n=19 adults with metabolic syndrome.(Wilkinson, 2020) The eating windows decreased on average 4.3 hours. Results included a decrease in body weight (3.3 kg), waist circumference (4.5 cm), body fat percentage (1.0%), visceral fat rating, blood pressure, and cholesterol. There was no change in CRP, a trend toward improved fasting glucose & insulin levels, and positive effects were seen on sleep parameters. The benefits in lipids and blood pressure were also seen in participants on lipid & blood pressure medications. The authors contacted all participants 16 months after completion and 5 were still following TRF while 7 were doing it part time. Improvements in markers were not correlated with the amount of weight lost. There was no control group.
In 2020 Zeb et al. published a 25 day trial of young healthy males in China where the TRF group (n=56) had an 8 hour eating window from 19:30-03:30 and compared this to a control group (n=24).(Zeb, 2020) There were improvements in cholesterol, triglycerides, and liver enzymes, with no significant change in inflammatory markers. Additionally, the gut microbiome was significantly enriched in the TRF group. Thus, even with a night time TRF protocol there were overall positive changes.
In 2020 Stratton et al. published a 4 week trial with a TRF group (n=13, 8 hour window from 12-8pm or 1-9pm) compared to a normal diet group (n=13) combined with resistance training and a 25% caloric deficit with at least 1.8 grams/kilogram of daily protein.(Stratton, 2020) While various changes were seen in both groups over time, between the two groups there were no meaningful differences regarding body composition, strength gains, muscular hypertrophy, testosterone, leptin, or adiponectin. TRF did not negatively impact response to training (regarding metrics mentioned above), recovery, energy, or hunger; however, the surveys for the latter items were completed during the feeding window which likely impacted the measurements.
In 2020 Cienfuegos et al. published an 8 week trial comparing TRF windows of 4 hours (3-7pm) and 6 hours (1-7pm) to a control group (no time restriction) in sedentary adults with obesity.(Cienfuegos, 2020) Each group had n=14-19 participants complete the trial. Most of the adults had insulin resistance but none had diabetes. The two TRF groups lost ~3.2% body weight (no weight loss in the control group). There were no meaningful changes in glucose, insulin, or cholesterol levels. Blood pressure did not change in a statistically significant manner but the TRF groups had a decrease of 4-5 mmHg in systolic blood pressure while the control group had an increase of 3.7 mmHg. There was a decrease in 8-isoprostane (a measure of oxidative stress) in the TRF groups with no change in inflammatory markers. By self-report the TRF groups decreased caloric intake by ~500 kcal/d. There were no overly meaningful differences between the two TRF groups.
In 2020 Jones et al. published a study with n=8 young healthy males undergoing TRF from 08:00-16:00 for 2 weeks, determined the caloric intake decreased ~400 kcal/day, and then recruited a matched control group to eat at the same deficit for 2 weeks with no time restriction.(Jones, 2020) Both groups lost a similar amount of weight. In response to a protein + carbohydrate drink (with no exercise in the prior 24 hours) consumed pre & post intervention, the TRF group had improved glucose & insulin post-intervention. Additionally, after the drink skeletal muscle glucose and branch chained amino acid uptake increased in the TRF group but not in the control group. Thus, TRF may potentiate skeletal muscle nutrient sensitivity.
In 2020 McAllister et al. published a study with n=16 healthy male firefighters undergoing TRF for 10 hours (time period not specified) for 6 weeks, while following a resistance training program. Decreases were seen in advanced oxidation protein products (~31%) as well as advanced glycated end products (~25%), both of which are thought to have negative health effects. No differences were seen in the hunger hormones, cholesterol or glucose/insulin levels, or other inflammatory markers. Without a control group it’s unclear if the observed improvements were due to the TRF, the resistance training program, or some other mechanism.
So what can we learn from all of these TRF studies? From a health standpoint most of these studies show that following a TRF protocol leads to a natural reduction in caloric intake and subsequent weight loss. Beyond this, several (though not all) studies show benefit for glucose and insulin levels, cholesterol, and blood pressure. These are seen despite the varying protocols in different populations of individuals. The interventions that include resistance training with sufficient protein intake do not show significant detriments to skeletal muscle hypertrophy, which is one of the big concerns that would arise from not spacing protein consumption throughout a full day.
Of note, several trials were in metabolically healthy individuals, which may preclude seeing benefits that would be seen in metabolically unhealthy individuals. Additionally, it’s important to keep in mind that all of these studies were small, and most were done for <=8 weeks. This would make it harder to find statistically significant differences and thus may underscore the overall effects. However, caution must be taken when drawing strong conclusions from a small body of research that includes small trials of relatively short duration. Thus, overall, TRF seems quite promising with no significant adverse effects seen and at this point can likely be considered a viable dietary strategy, but until larger trials are done I believe it is premature to provide strong recommendations to follow a TRF protocol. Additionally, certain medical conditions may make prolonged fasting periods dangerous; consider consulting your medical provider prior to making any drastic dietary changes.
Note: in trials compliance was an issue at times when the eating window was earlier in the day because this precluded social eating at night. As discussed earlier in this lesson there is evidence that shifting caloric intake earlier in the day leads to health benefits, and this may also extend to TRF, but there were also benefits seen in TRF trials here that started midday. Thus, if a person wants to try TRF and use a feeding window that includes evening social events to aid compliance, I do not see any particular reason to discourage this at this time given these trials still collectively showed benefit.
Tip: Anecdotally many people find TRF helps to stabilize/control hunger levels. It is likely that TRF will need to be used for several days or even 1+ weeks before physiologic adjustments in appetite/hunger stabilize. Thus, if attempting TRF I would commit to it for at least 1 week prior to giving up on it as a viable strategy.
There are several different aspects of “chrononutrition” as discussed above. TRF seems like a valid approach that can be employed with resistance training as long as protein intake is sufficiently high. IER additionally seems valid and may aid compliance with a diet. If doing this I would prioritize protein intake on the restricting days. Eating at regular times is likely beneficial for overall health and consuming most of one’s calories earlier in the day also seems beneficial, though a conclusive benefit to adding in breakfast for individuals who prefer to skip it has not been found. Research thus far does not promote extended fasting in humans, though more research is needed. Additionally, more research is warranted relating timing of nutrition to one’s normal wake & sleep times.
Most of this research does not consider people who are actively exercising; for those who exercise in the evening it may make more sense to consume calories in the evening. Anyone with medical conditions that may preclude fasting periods should discuss these dieting strategies with their provider. People who perform shift work, particularly night shifts, may minimize the metabolic risks by eating at similar times each day; more research needs to be done regarding time-restricted feeding protocols in night shift workers.
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