Meal Timing Patterns
Obesity is a growing public health problem and an ever-increasing global pandemic with severe health consequences. It has been predicted that the prevalence of individuals with obesity will approach 50% of the worldwide population by 2030. Therefore, the scientific community has endeavored to find tools to tackle this menace more effectively. Since calorie restriction, the main weapon against weight gain, often fails to bring fruitful outcomes, especially in terms of weight maintenance, new approaches are studied, including meal timing patterns and meal frequency. Especially meal timing has come to the forefront of research with increasing public interest. There is a significant amount of evidence linking meal timing to energy balance, body weight regulation, metabolic health, and other chronic diseases, all being aspects that will be addressed in the current article.
To begin with, meal timing refers to the practice of arranging and consuming meals throughout the day strategically so as to eat food only in specific hours or in particular time windows and fast for the rest of the day. In general, fasting has been shown to exert many positive effects on health, including protection against diabetes, cancer, heart disease, neurodegenerative diseases, obesity, and hypertension. Therefore, It appears that meal-time-based strategies can be employed to prevent obesity and associated metabolic disorders. Is that the case, indeed?
Meal timing is widely known as intermittent fasting, the umbrella term referring to various dietary interventions involving a complete or partial calorie restriction within defined temporal time windows on a recurrent basis. An increasing number of dieters have adopted meal-timing dietary approaches because several studies have shown that the shortening of the eating period might have beneficial effects on weight loss, cardiovascular health, and metabolic parameters, such as insulin, glucose, and hunger hormones.
The most popular approaches to meal timing patterns are the following:
Alternate-day fasting (ADF)
ADF involves eating every other day. Particularly, one may eat as much as they want (ad libitum), ideally through a healthy diet one day, and then completely fast or have one small meal of up to 500 calories the next day.
The 5:2 diet
This form of intermittent fasting involves complete fasting twice weekly, with ad libitum food consumption, ideally through a healthy diet, on the remaining five days. When the fast is undertaken on consecutive days, which is not mandatory, there may be better weight loss and metabolic health results.
Time-restricted feeding (TRF)
TRF is the most popular method of intermittent fasting. It involves food intake restriction (fasting) for different periods of at least fourteen hours daily, thereby reducing feeding opportunities to typically less than ten hours daily. As a result, meal frequency is inevitably reduced to three meals a day at the most.
The most popular TRF variation is the ‘’16:8 diet’’, where the eating window is eight hours a day and the fast sixteen hours. There are also the 14:10 and the 20:4 TRF patterns, where the eating time windows are ten or four hours and the fasting time windows are fourteen and twenty hours, respectively. During the eating windows, the subject consumes ad libitum energy intake, and during the fast, they can only drink water, black coffee, and unsweetened tea. TRF can result in reduced caloric intake and is associated with decreased body weight and fat mass.
The defining characteristic of these different meal-timing dietary patterns is the defined periods of fasting, lasting from 14 and up to 48 hours (in cases of consecutive fasting days in the 5:2 diet), as well as the decreased meal frequency, compared to regular calorie restriction dietary patterns. There is evidence that meal timing protocols may be effective for weight loss, but no data indicate that weight loss can be sustained long-term. Regarding cardiometabolic health, meal timing protocols have no effect on fasting glucose but produce fairly consistent reductions in fasting insulin, improving insulin resistance. In addition, they are useful for lowering triglyceride concentrations, inflammatory markers, and blood pressure but have little or no effect on total, LDL, or HDL cholesterol. A possible explanation of this favorable weight loss and metabolic effect might be the fasting period which leads to the depletion of glycogen stores and a metabolic switch from lipid synthesis and fat storage to fat mobilization through fatty acid oxidation. It has actually been stated that the longer the fasting window, the more effective the metabolic switch between glucose and fat oxidation, and the higher the fat-burning efficiency, thus the easier the weight loss effort.
The disheartening remark is that in the majority of meal timing studies, weight loss coexists, meaning the beneficial effects of meal timing may be mainly explained by weight loss. On top of that, studies have shown there was no greater weight loss with meal timing compared with standard daily calorie restriction methods as well as no significant differences in fat mass, fasting insulin, glucose, HbA1c, or blood lipids. As a result, whether meal-timing dietary protocols are superior to regular calorie restriction dietary methods or whether they can provide any cardiometabolic benefits outside of the energy balance context is still inconclusive.
Although meal-timing dietary approaches only dictate the duration of the eating window and not when it starts (in the morning or later in the day), this seems to play a significant role in its advocated health-promoting effects. Notably, available data suggest that if meal timing is worthwhile indeed, there may be differential effects of skipping breakfast versus dinner (i.e., evening fasting before an overnight fast vs. overnight fasting followed by continued morning fasting). It has generally been suggested that late-night eating or consuming a larger percentage of energy later in the day is related to an increased risk of weight gain, metabolic dysfunction (metabolic syndrome), and cardiovascular disease, irrespective of energy balance. One study showed that late lunch eaters lost less weight on the same hypocaloric diet than early eaters. In another study, subjects assigned to an isocaloric dietary intake with higher calories during breakfast showed greater weight loss, lower insulin resistance, and reduced subjective appetite feeling than those assigned higher calories during dinner. Taken together, studies reveal a greater weight loss in individuals who consume their main meal or the greatest percentage of their daily calories earlier in the day compared to those who do so in the evening/night.
The adverse cardiometabolic risk profile (increased risk of type-II diabetes and coronary heart disease) of late-night eaters, such as night workers, may be attributed to the ‘’wrong’’ coupling of food intake to the body’s circadian rhythms. Circadian rhythms are 24-hour cycles running to carry out essential physiologic functions, including hormones, metabolism, eating behavior, sleep patterns, etc. The body’s circadian rhythms are controlled by a central clock in the brain and peripheral clocks in the organs, such as the liver, skeletal muscle, and fat tissue. As a result of circadian regulation, better meal timing has been shown to associate with more effective body weight regulation and better cardiometabolic health.
Several possible biological mechanisms have been described in this relationship, including better insulin sensitivity and glucose tolerance, favorable alterations in the gut microbiome, lower cortisol concentrations as well as increased diet-induced thermogenesis in the morning than in the afternoon/evening. Increased diet-induced thermogenesis in people eating more calories earlier in the day can enhance overall energy expenditure. These data suggest that early eating aligns with circadian rhythms, and weight loss is favored when greater caloric intakes occur in the morning until early afternoon vs. evening until late night. Therefore, restricting the eating window to the earlier time of the day, for example, TRF, where dinner is before 3 pm, may be more beneficial for weight loss and cardiometabolic health. Additionally, maintaining a diurnal (daytime) eating phase could positively affect the gut microbiome and result in better sleep quality and longer sleep duration.
Conversely, consuming food outside humans’ normal feeding phase (during late night in humans) may disrupt circadian rhythms. This circadian misalignment could induce higher caloric intake, thus weight gain, due to increased levels of the hunger hormone ghrelin and decreased levels of the satiety hormone leptin. It can also increase the risk of type-II diabetes, cardiovascular disease, and mental disorders. Hence, not only the duration of the fasting window but, most importantly, aligning the eating window with the day and the fasting window with the night by consuming a greater proportion of calories earlier in the day might contribute to better body weight management and cardiometabolic health through synchronizing central and peripheral circadian clocks.
Evaluating the data presented above collectively, one could conclude that a meal-timing dietary protocol which would encompass early eating windows until early afternoon, where most of the calories (bigger meal portions) would be consumed during morning hours, would be the most effective in terms of weight loss and cardiometabolic health, even outside the strict confines of standard calorie restriction. However, no recommendation can still be made since the data is controversial and heterogeneous.
Although much research in recent years suggests a positive health effect of limiting daily food intake to 4-10 hours compared to the regular daily meal distribution (i.e., 3-5 meals, spread from breakfast to late dinner), there has been a debate regarding low vs. high meal frequency. As stated above, meal timing is unavoidably associated with skipping meals and a low meal frequency due to the time-restricted eating periods. Often infrequent meal patterns are associated with an irregular eating approach that could cause weight gain, increase hunger-related hormones, and ultimately lead to metabolic disturbances, such as metabolic syndrome, that may increase cardiovascular risk. For example, some studies have shown that those who consistently skip breakfast have a higher risk of weight gain and coronary heart disease compared to those who eat breakfast. Furthermore, greater eating frequency has been associated with a lower risk of obesity. One possible explanation could be that diet-induced thermogenesis (DIT) decreases from morning to night, thus, increases in the body weight of people who skip breakfast would make sense. However, the relationship between eating frequency and obesity is mixed and inconclusive since there are also studies showing that higher meal frequency may lead to higher calorie intake.
While meal timing has gained a lot of popularity and may indeed have some positive aspects in terms of weight loss, metabolic and cardiovascular health, specific population groups are not safe to practice it. Therefore, pregnant women, people at risk for hypoglycemia, such as type-1 diabetic subjects, and people at risk of an eating disorder or who have a history of eating disorders should not engage in meal-timing patterns. Regarding patients with type I diabetes, several eating occasions may need to take place to treat hypoglycemic episodes, which does not coincide with the principles of fasting and eating only during specific timespans. Regarding eating disorders, research suggests that failing to consume regular meals and skipping meals significantly correlates to the risk of eating disorders. Moreover, food intake restriction may lead to binge eating episodes and contribute to eating disorders relapse. Lastly, individuals at risk for sarcopenia (age-related progressive loss of muscle mass and strength) are not recommended to engage in meal-timing dietary patterns because such a diet could exacerbate muscle loss. However, if they do, they need to ensure adequate protein intake.
Overall, meal timing and the strategic distribution of caloric intake throughout the day may be important considerations in terms of weight management and cardiometabolic health. Whether these can function independently of an energy balance scope is a concept that needs further investigation since research is still fairly limited and the data too diverse and heterogenous to allow for definitive conclusions. However, dietary approaches based on meal timing represent a promising strategy and may be an attractive and easy-to-adapt tool for individuals who wish to lower their caloric intake through such a diet. Therefore, focusing on meal timing and frequency as a starting point to accomplish a negative energy balance and improve dietary quality can be beneficial. This is because having a meal structure on which to superimpose calorie restriction may improve the individual’s ability to adhere to the weight loss plan and help them better control their hunger.
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