As a single meal, breakfast has perhaps received more emphasis and focus in the literature than any other. It has taken strong hold in public health advice for years, supported by numerous national and international health regulatory bodies.
Breakfast is certainly unique: it is the meal eaten after waking from the previous night’s sleep and fasting period, making it the first meal after the longest period without food most people undergo in any 24-hour period.
So, what can we say about breakfast as a specific meal at this point?
The Definitional Issue of “Breakfast”
One of the major limitations in meal pattern research is the lack of a standardised definition for “breakfast”, a culturally-laden term. Cross-cultural differences in the definition of “breakfast” are evident in the timing, energy content, food choices, and cultural importance of the meal across populations.
Certain research groups have attempted to place more standardised operational definitions, for example, a meal consumed within 2 hours of waking, and/or a minimum energy criteria [often ≥ 50kcal]. However, to date, meals – including “breakfast”, “lunch”, and “dinner” – have remained defined largely by cultural and social constructs, which differ between populations.
In epidemiology, breakfast consumption is generally associated with improved health outcomes, while breakfast skipping is associated with adverse cardiometabolic outcomes. However, what we are beginning to add to the picture is that both time-of-day preference and behavioural traits have important influences on breakfast consumption.
It may be, therefore, that the positive observations for breakfast reflect these factors, such as being a “morning type”.
The Relationship Between Breakfast and Chronotype
The term “chronotype” refers to an individual’s time-of-day preference, generally dichotomised into “morning larks” [morning-types] and “night owls” [evening-types]. Although this is generally referred to as a behavioural preference, chronotype has strong biological underpinnings and genetic factors leading to the expression of the behaviour.
A 2019 recent analysis investigated the influence of genetic predisposition for breakfast consumption in monozygotic and dizygotic twins. Preference for breakfast was found to have a strong genetic heritability [56%], which also corresponded to other morning-related behaviours and timing preferences.
In contrast, dinner was not influenced by genetic heritability; this suggests that dinner is influenced more by cultural, social, and behavioural factors. By implication, it means the timing of dinner may be a more modifiable dietary factor than the timing of breakfast.
Muele et al. found that evening-types have low morning hunger levels, delayed initiation of eating and distribute a majority of their energy later in the day. In addition to delaying meal timing and distributing more energy to later in the day, evening types have also been shown to have lower diet quality, compared to morning-types.
While it may seem intuitive that advice for an evening-type would be to make an effort to consume breakfast, this does not appear to be supported by the evidence. For the role of breakfast and chronotype, the most interesting analysis to date was published in 2019 by Xiao et al., which examined breakfast consumption and adiposity, stratified by chronotype. Consumption of breakfast significantly attenuated the risk of obesity, but only in morning types. In evening types, consumption of breakfast did not attenuate the risk of obesity.
The Relationship Between Breakfast Consumption and Behavioural Traits
The emerging literature on chronotype and genetic heritability for breakfast suggests that breakfast is a meal with strong behavioural underpinnings. This is supported by research which has specifically investigated behavioural outcomes. Breakfast consumption has been shown to correlate with positive attitudes toward health-promoting behaviours – beliefs about the nutritional value of the meal, weight management, and physical activity.
In addition, breakfast consumption correlates with personality traits of conscientiousness and dietary restraint. Specifically, the traits conscientiousness and agreeableness positively predicted breakfast attitudes and behaviours, while neuroticism negatively predicted breakfast attitudes and behaviours.
Thus, personality differences in breakfast attitudes appear to be mediated by behavioural preference for time-of-day, i.e., chronotype.
However, a crucial point is that it is unclear from this research whether time of day preferences are a mediating factor because of biological factors, social factors, or both. Evening types may sleep in and not have time [social], or rhythms in appetite may not align with the early part of the day [biological]. It appears likely a mix of social and biological factors.
Breakfast Consumption, Energy Balance and Weight Regulation
Most controlled interventions do not indicate that there is any advantage to breakfast vs. no breakfast in the context of weight loss. The hypothesis that breakfast consumption would influence energy balance through impacts on energy expenditure was investigated thoroughly in the Bath Breakfast Project. Participants were randomised to consume either >700kcal before 11 am [and 50% of that within 2 hours of waking], or fast until noon.
In both lean participants and participants with obesity, breakfast consumption led to significant increases in physical activity thermogenesis [PAT] compared to the breakfast skipping group. However, the increased energy expenditure matched the increased energy intake in the breakfast group, such that there was no difference in energy balance between groups.
Whole-day energy expenditure did not differ between groups, and consistent with other research, skipping breakfast did not result in any compensatory energy intake or adverse effects on energy regulatory hormones.
However, as will be discussed further below, metabolic control was improved in the breakfast group, indicating that while breakfast may not influence energy balance, it may be an important meal for metabolic health.
This lack of effect of timed eating on energy expenditure and weight loss has been further confirmed by the related findings from the Big Breakfast Study, which is the most rigorously controlled study in this area to date. The trial controlled the energy intake of participants and measured energy expenditure using the doubly labelled water technique, the gold standard for free-living energy expenditure measurements.
The intervention compared a sequence of 45%, 35%, and 20% energy at breakfast [~8 am], lunch [~1 pm], and dinner [~6.30 pm], respectively, compared to the inverse of 20%, 35%, and 45% energy, for four weeks. Total daily energy expenditure was unaffected by the distribution of energy and weight loss was practically identical at –3.33 kg and –3.38 kg in the high morning energy and high evening energy diets, respectively.
Thus, the weight of evidence to date does not support a causal role for breakfast per se in influencing energy balance, which implicates other factors.
The Relationship Between Breakfast Consumption and Metabolic Health
It is arguable that the most important role for breakfast itself, as a standalone meal, may be in relation to metabolic control, particularly for diabetes management. The most well-established characteristic of circadian effects on metabolism is the variance in glucose tolerance that occurs over the day, peaking in the morning and diminishing in the evening.
Factors which enhance the secretion of insulin, including the incretin hormones GLP-1 and GIP, are amplified during the morning, resulting in more rapid insulin responses in the morning. The net effect of this amplified insulin action is enhanced glucose disposal, resulting in lower glucose levels in response to meals, an effect which persists across the day.
When breakfast is consumed, the glucose responses to lunch and dinner are lower compared to breakfast skipping, and this effect is most pronounced in adults with type-2 diabetes [see here and here].
What Can We Conclude?
At this point, the most likely summation of the role of breakfast in health is as follows:
- Breakfast consumption may be a proxy for wider health-promoting behaviours, mediated by chronotype, genetic heritability, and personality traits;
- Adverse associations with breakfast skipping may reflect evening chronotypes, increased social jetlag, and the mediating effect of personality traits with these two factors;
- There is no clear evidence of benefit to breakfast intake from the perspective of weight management;
- There is clear evidence of a benefit to glycemic control and metabolic health, which becomes more evident with increasing glucose intolerance, i.e., the effect in diabetics is significantly more pronounced than in otherwise healthy participants.
A strong argument can be made for a specific emphasis on breakfast intake, and specifically higher energy, higher protein breakfasts, in diabetes management.
For the chronotype factor, the key aspect here appears to be counselling later chronotypes away from significant proportions of energy intake later in the biological night.
It may be useful as a heuristic to think in terms of the etymology of “break the fast”, i.e., while late chronotypes may naturally delay eating, distributing more energy to those first and second meals may be a beneficial strategy.
For the majority of healthy people, however, breakfast may not be the most important meal of the day, and may be a more preference-driven decision.
Yours in Science,
Alan
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