The causal role of breakfast in energy balance and health: a randomized controlled trial in lean adults


Background: Popular beliefs that breakfast is the most important meal of the day are grounded in cross-sectional observations that link breakfast to health, the causal nature of which remains to be explored under real-life conditions.

Objective: The aim was to conduct a randomized controlled trial examining causal links between breakfast habits and all components of energy balance in free-living humans.

Design: The Bath Breakfast Project is a randomized controlled trial with repeated-measures at baseline and follow-up in a cohort in southwest England aged 21–60 y with dual-energy X-ray absorptiometry–derived fat mass indexes ≤11 kg/m2 in women (n = 21) and ≤7.5 kg/m2 in men (n = 12). Components of energy balance (resting metabolic rate, physical activity thermogenesis, energy intake) and 24-h glycemic responses were measured under free-living conditions with random allocation to daily breakfast (≥700 kcal before 1100 h) or extended fasting (0 kcal until 1200 h) for 6 wk, with baseline and follow-up measures of health markers (eg, hematology/biopsies).

Results: Contrary to popular belief, there was no metabolic adaptation to breakfast (eg, resting metabolic rate stable within 11 kcal/d), with limited subsequent suppression of appetite (energy intake remained 539 kcal/d greater than after fasting; 95% CI: 157, 920 kcal/d). Rather, physical activity thermogenesis was markedly higher with breakfast than with fasting (442 kcal/d; 95% CI: 34, 851 kcal/d). Body mass and adiposity did not differ between treatments at baseline or follow-up and neither did adipose tissue glucose uptake or systemic indexes of cardiovascular health. Continuously measured glycemia was more variable during the afternoon and evening with fasting than with breakfast by the final week of the intervention (CV: 3.9%; 95% CI: 0.1%, 7.8%).

Conclusions: Daily breakfast is causally linked to higher physical activity thermogenesis in lean adults, with greater overall dietary energy intake but no change in resting metabolism. Cardiovascular health indexes were unaffected by either of the treatments, but breakfast maintained more stable afternoon and evening glycemia than did fasting.

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Alex’s Notes: Breakfast is always recommended as part of a healthy diet by mainstream professionals, but the belief that breakfast is beneficial for health is based purely on observational evidence. Sure, skipping breakfast is associated with an increased risk for adverse health outcomes, but habitual breakfast eaters also tend to drink less alcohol and be more physically active. So what effect does eating breakfast have on lean and healthy physically active individuals? Let’s find out.

First off, the protocol used was very extensive.The participants had assessments of their acute metabolic responses under fasted and post-prandial conditions measured, including resting metabolic rate, substrate oxidation, dietary-induced thermogenesis and systemic concentrations of key metabolites/hormones. They also had their physiological and psychological indices of appetite measured, DEXA scans for body composition taken, glucose tolerance measured, and samples of their fat taken to be assessed for insulin sensitivity. Oh, and when the participants weren’t in the laboratory, they had to keep food diaries to measure energy intake, wear combined heart-rate/accelerometer monitor to assess energy expenditure, all while wearing continuous glucose monitors to document chronic glycemic responses to the intervention and to verify compliance.

The participants were divided into two experimental groups: one that ate a 700 kcal breakfast within 2 hours of waking, and the other fasted until noon. Unfortunately, because dinner was part of the free-living conditions we do not know when this last meal was consumed and thus the fasting durations are unknown. However, assuming no one eats dinner past 9pm, the breakfast consumers probably fasted for 10-12 hours, while the breakfast skippers fasted for 15 hours.

So what happened? Well, there were no metabolic adaptations in either group and no adverse cardiovascular effects. That is, thyroid hormone was unaffected, and a range of hormones implicated in satiety and appetite did not differ between groups. The breakfast but not the fasting group showed higher fat cell insulin sensitivity (about 10% higher), which means that they are better equipped to store away the ingested nutrients in fat cells. However, the fasting group had more variable and less regulated blood glucose during the afternoon and evening, which could prove potentially problematic in overweight, less healthy, or more sedentary persons. Sleep was unaffected in either group.

Actually, the most notable difference between the groups was that the breakfast group was consistently engaged in more “light-intensity” physical activity (1.5-3 METs) during the morning. This resulted in an average of 442 kcal greater energy expenditure in the breakfast group. Of course, the breakfast group also ate more calories – an average of 539 kcal more, which completely offsets the increase in physical activity. Moreover, most of this was in the form of carbohydrates (65% of additional energy intake; 88g) and most of these carbohydrates were sugar (53g of the 88g)! This isn’t really surprising when you consider the average Western breakfast of cereal and doughnuts, and it raises the possibility that the increased morning activity was simply a result of a sugar rush that affected spontaneous behavior rather than conscious decisions to be more active. This raises even more questions about how the nutrient composition of breakfast could affect these parameters. It has been previously shown that meal mass and energy content regulate subsequent appetite and feeding, and protein is more satiating than other macronutrients.

As an important final note, overall daily eating patterns were maintained in both groups. In other words, both breakfast and fasting groups ate in a similar manner from lunch onward, demonstrating that breakfast doesn’t reduce food intake later in the day and fasting doesn’t result in overcompensation. Thus, it would be naïve to eat breakfast when attempting to lose weight with the hopes of controlling appetite later on, whereas fasting will automatically cut food intake without a subsequent increase. So contrary to all the observational evidence associating breakfast with weight-loss, fasting clearly did not result in weight gain and eating breakfast actually resulted in greater caloric intake that could not be completely compensated for by increased physical activity.


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