The impact of breakfast on total daily energy intake is a matter of debate. In an experimental setting Cotton et al. [1, 2] have shown in normal weight subjects that an energy-rich breakfast leads to greater daily energy intake indicating that compensatory undereating during subsequent meals does not occur. This notion is supported by an analysis of food intake in young adults in Bogalusa . It must be kept in mind though that these findings are based on a cross-sectional analysis and accordingly these data represent interindividual differences.
Up to the author's knowledge the present data demonstrate for the first time this relationship in a free-living population of normal weight as well as obese subjects thereby confirming the earlier experimental data of Cotton and colleagues. When analysed intraindividually whole day energy intake increases proportionally to breakfast calories while other meals of the day remain largely unchanged. The only exception is a small reduction of the morning snack on the two days with the highest breakfast energy consumption in the obese group.
In a different approach de Castro [4, 5] has demonstrated that a high ratio of breakfast to whole day energy intake is associated with lower overall intakes. The present data also confirm these previous findings. An increased ratio of breakfast to whole day energy intake can be due to an augmented consumption during breakfast or to a reduced intake during all subsequent meals or a mixture of both. The direct comparison of both ways of analysis show that the range of breakfast energy intake is largely similar.
Thus, the significant reduction of daily energy intake on days with a high B/T ratio is mainly the result of below average intakes during the other meals. Moreover, it should be noted that a low B/T ratio is associated with significantly greater overall intake compared to the days with a low absolute intake.
For the purpose of reducing daily energy intake in obesity prevention or treatment, a reduction of breakfast calories can be helpful. High breakfast energy intake is not automatically associated with low whole day intake. On these days substantially reduced energy consumption during post-breakfast meals would be necessary.
With regard to obesity treatment the role of breakfast intake is controversial. Several cross-sectional studies have indicated that breakfast intake is inversely associated with body weight [12–16]. In contrast a large longitudinal study came to the conclusion that a high percentage of breakfast energy is associated with lower weight gain . It must be kept in mind though, that at the beginning of this study a high percentage of breakfast energy was associated not only with the highest overall intake but also with the highest body weight which is in contrast to the results of other cross-sectional studies [12–16]. Unfortunately food intake was assessed only at baseline but not at follow-up. Thus, lower weight gain could be due to a change of eating habits leading to reduced breakfast and overall energy intake over the three years of follow-up.
Moreover, data of the National Weight Control Registry (NWCR)  have shown that the majority (78%) regularly eats breakfast which has been suggested to be an important factor for successful maintenance of weight loss. It is important to note, however, that this is not necessarily a causal relationship since the other 22% are also successful weight loss maintainers, despite skipping breakfast regularly or occasionally, respectively. In the present analysis 5 out of 280 obese subjects (1.8%) never had breakfast. 180 patients never skipped breakfast and the other 95 occasionally skipped breakfast. All groups had a similar BMI and total daily energy intake was not different. In the normal weight group total daily energy intake of those with regular breakfast consumption (n = 68) was significantly higher (1826 ± 18.4 kcal/d) compared to the 32 subjects who skipped breakfast occasionally (1691 ± 31.5 kcal/d; p < 0.001) while body weight was not different between these 2 groups, which indicates that energy expenditure has to be considered as well. In this context it is noteworthy that the regular breakfast eaters in the NWCR reported more physical activity.
On the other hand, the only interventional study of the role of breakfast in the treatment of obesity came to mixed results . Baseline breakfast eaters lost more weight when assigned to the no-breakfast treatment group. On the other hand, baseline breakfast skippers lost more weight when assigned to the breakfast-eating treatment group. It should be kept in mind though, that the study population was very small and these very interesting data clearly deserve confirmation in a larger population.
In conclusion the present data demonstrate that higher energy intake at breakfast is highly associated with greater whole day energy intake in normal weight and obese subjects. Therefore low energy intake at breakfast can be helpful to lower daily intake and improve the energy balance during treatment of obesity Whether or not this approach really favours weight loss has to be examined in further interventional studies. At present prevailing data are rather equivocal.