By Alan Aragon and Ryan Zielonka


The ancient practice of fasting seems to periodically find its way into modern fitness subcultures. Despite its purported physiological and psychological benefits, scientific data is far from unanimously supporting it. Are there any physiological advantages to increasing or decreasing meal frequency? What are the downsides to intermittent fasting? What are the health advantages shared by fasting, exercise, and caloric restriction? Are there detrimental effects of resistance training in a fasted state? Anecdotal data is fraught with suggestion-driven bias so we will focus on what has been demonstrated objectively in scientific research.


Meal frequency—hot dogma and cold facts

Since the health nut culture’s infancy, the decree has been that one must eat every 2–3 hours, thus minimizing muscle breakdown while keeping the metabolic engine revving. Tupperware containers and coolers in gyms and cars were (and still are) a common sight, perceived by the public to be a sign of diligence and discipline. However, the long held belief in a higher-is-better frequency has been examined with the lens of science, and some interesting data has surfaced.

Effects on health and thermogenesis

Research indicates that a haphazard meal frequency, not necessarily a lower frequency, negatively impacts thermogenesis, blood lipids and insulin sensitivity (1, 2). Contrary to popular belief, a high frequency has no thermodynamic advantage over a low frequency under calorie controlled conditions (as opposed to ad libitium or free living conditions) using 24-hour indirect calorimetry (3, 4). So much for the magic of stoking the metabolic furnace with an extreme grazing pattern. It bears mentioning that lower 24-hour insulin levels as well as lower fasting and total LDL-cholesterol levels have been observed with higher meal frequencies (5, 6). However, in discovering this, studies have used unrealistic protocols for the higher frequency treatments, comparing three meals to nine or 17 meals per day.

Effects on body composition

With little exception (7, 8), the majority of controlled intervention trials show no improvement in body composition with a higher meal frequency, with treatments ranging from 1–9 meals per day (9–11, 15). Unfortunately, a scarcity of research examining meal frequency’s effect on body composition under conditions of exercise exists in the literature. To further confound the data, results from studies are mixed. The singular, full length published controlled trial showing a body composition benefit of higher meal frequency (six versus three) is limited by its poor study design. In the trial, boxers consumed a 1200 kcal liquid diet over a two-week period (8). The six-a-day group retained more lean mass than the three-a-day group.

In contrast to the boxer study, a recent abstract presented at the Twelfth Annual Congress of the European College of Sport Science reported the superiority of three meals a day to six meals a day for gaining lean mass during a 12-week period involving strength training (12). Too bad the three-a-day group also experienced a trend toward fat gain. This raises the possibility that the three-a-day group simply ate more calories overall. This wouldn’t be surprising, considering that comparative research shows an association of greater hunger with meal frequency reduction.

Effects on appetite

Studies indicating the disappearance or lack of hunger in dieters occur in either complete starvation or very low calorie regimes (800 kcal/day or less) (13, 14). Much of this data is irrelevant for most of you reading this. Furthermore, none of those studies systematically measured appetite throughout waking hours. Convenience issues aside, for the purposes of controlling appetite, research indicates the superiority of a higher frequency over a lower one.

In two separate studies led by Speechly, both lean and obese subjects had greater appetite control when pre-test meals were consumed at frequent intervals in contrast to the same amount of food consumed at a single meal (15, 16). In the ad libitium test meal that followed the pre-test meals, subjects given the single meal consumed on average 26.5 percent more calories.

Stote’s team compared one versus three meals per day (17). Among other results, the one-a-day group reported significantly higher levels of hunger and an increased desire to eat with the severity of both phenomena increasing throughout the length of the trial. In a recent alternate day fasting study (18), Heilbronn’s team concluded that: “Overall, these results suggest that a prolonged schedule of fasting and feasting would be marred by aversive subjective states (e.g., hunger and irritability), which likely limits the ability of most individuals to sustain this eating pattern.”

In a recent review, Johnstone suggests that a hierarchy of hunger response exists (at least in obese subjects) whereby hunger directly correlates with the severity of the caloric deficit, citing fasting as the obvious limit of acute deficit (13). The persistence of hunger was one of the primary reasons he did not recommend fasting as the optimal dieting strategy despite having been a principal investigator in fasting research. To quote the paper’s conclusion, “There is, however, the problem of elevated hunger during food restriction and this may provide too great a challenge to a ‘faster’ in not breaking compliance to the dieting regime and reaching for the biscuit barrel.”

Skipping breakfast = not too brilliant

Rampersaud’s team examined the results of 47 studies on various effects of breakfast consumption among children and adolescents (19). Interestingly, while breakfast eaters consumed more daily calories, they were less likely to be overweight. Children who consistently ate breakfast tended to have superior nutritional profiles. This concurs with a cohesive body of data indicating that adults who eat breakfast meet their daily micronutrient needs better than habitual breakfast skippers (20–22). While cognitive effects are inconsistent in well nourished children, breakfast skipping degrades mental performance in malnourished children. Overall, the evidence points to regular breakfast consumption improving cognitive function, test grades, school attendance, memory, and nutrient status. The latter effect pertains to macronutrients and essential vitamins and minerals. The impact of skipping breakfast on the intake of other functional nutrients hasn’t been studied.

In a controlled intervention trial on lean subjects, Farshchi’s team found that skipping breakfast decreased post-meal insulin sensitivity and increased LDL-cholesterol, despite a high (six-a-day) meal frequency (23). This data points to the possibility that the body is “metabolically primed” to eat a meal soon after an overnight fast.

Concurring with the above results, noted protein researcher, Donald Layman, asserted in a recent review that the most critical meal of the day is breakfast after an overnight fast (24). This is partially due to circadian protein synthesis rates being lowest at this time. He states that the anabolic impact of a meal lasts roughly 5–6 hours based on the rate of post-meal amino acid metabolism. Therefore, dietary protein should be provided at approximately five-hour intervals throughout the day. This recommendation can be challenged by the fact that other studies show longer durations of plasma glucose and amino acid elevations caused by casein or a mixed meal (25, 26). However, the latter research didn’t measure the effect of exercise on plasma amino acid flux. In the final analysis, Layman’s suggestions are a safe bet without any major convenience impingements.

In three separate controlled experiments, Benton and Parker examined the effect of breakfast versus fasting on cognition (27). In the first study, fasted subjects took significantly more time than the fed group to complete both the spatial memory task and the word recall. In the second study measuring information processing and short-term memory decay, the fasted group lacked the improvements shown in the breakfast group. In the final trial, memory and intelligence were measured. Although breakfast didn’t enhance abstract thought, it was superior to fasting for recalling a story read aloud. The researchers concluded that these trials were in agreement with a substantial body of previous research demonstrating that breakfast benefits memory.

On the observational research front, the National Weight Control Registry (NWCR) is the largest ongoing study of individuals who have successfully maintained substantial weight loss over the long term. To qualify, participants must maintain a weight loss of at least 13.6 kg (30 lbs) for at least one year. According to a formal analysis led by Wyatt (28), 2313 subjects (78 percent) ate breakfast every day. Only 114 subjects (4 percent) reported skipping breakfast. This obviously isn’t cause-and-effect data, but it shows the crucial commonalities in the habits of dieters with long-term success. Daily breakfast is clearly one of those habits. Thus, intermittent fasting human research is interesting but inconclusive.

Alternate day fasting

Intermittent fasting (IF) can be any number of variations of feed/fast intervals. Alternate day fasting (ADF) is simply defined by its name. As of this writing, the human literature on ADF contains three human trials conducted within the last two years (18, 29, 30). Control groups were absent in all of those studies. As such, no comparative conclusions can be drawn between ADF and linear caloric intake. Animal research has shown promise for the health effects of ADF. However, human research hasn’t quite lived up to the luck of rats. Nevertheless, the data still provides food for thought and further investigation.

Heilbronn’s team put non-obese men and women on an ADF for a total of 22 days (29). Subjects lost an average of 2.1 kg total body weight despite instructions to eat double their typical day’s intake every other day. Men maintained normal glucose metabolism and improved insulin response. Impaired glucose tolerance occurred in women by the end of the trial. Although a trend toward increases in resistance to stress occurred in the study, both men and women showed no changes in gene expression involved with fatty acid oxidation.

In another 22-day ADF study led by Heilbronn, non-obese subjects lost an average of 2.5 percent of initial body weight (18). Positive markers of ADF included a decrease in fasting insulin levels and respiratory quotient, indicating an average fat oxidation increase of roughly 15g per day. Negative markers include an increase in hunger on the first fasting day, a condition that remained elevated for the duration of the trial.

Halberg’s team examined the effect of an ADF for a total of 14 days on non-obese young men (30) and observed an increase in insulin sensitivity. In contrast to the previously discussed study, no change in body weight or body fat occurred. As a result of the ADF, insulin sensitivity and glucose uptake in muscle increased. However, there was also an increased sensitivity, or uptake readiness in the fat cells, evidenced by an inhibition of insulin-mediated adipose tissue lipolysis. The next study we’ll examine deserves its own section but not for the reasons you might expect.

Stay tuned for part 2!


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