Much scientific research been directed towards an improved understanding of how breakfast may be related to population health. It has long been established that individuals who do not frequently consume breakfast are more likely to be overweight and suffer from a range of associated negative health outcomes such as hypercholesterolaemia, coronary heart disease and diabetes. Surprisingly however, little longitudinal research or evidence from randomised controlled trials is available to confirm the existence of any direct causal relationship (or mechanisms of effect) between breakfast and health. Given that people who usually skip breakfast also tend to adopt other less healthy lifestyle habits (e.g. higher fat/alcohol intakes, lower fibre/micronutrient intakes and a tendency to be less active), further research is therefore clearly warranted not only to establish whether there is a cause-effect relationship between breakfast consumption and health but also to determine the mechanism for this effect, whether it be via improved diet or increased physical activity. While many findings arising from this research certainly remain equivocal, the general pattern which is emerging is that the relative inclusion or omission of breakfast from the diet does not have a substantial impact on energy intake or aspects of resting metabolism. The only remaining possibilities are therefore either that breakfast increases physical activity or that any underlying metabolic effects may just take a period of weeks to occur. It is therefore interesting that no existing study has objectively and reliably measured physical activity habits in response to breakfast relative to extended daily fasting. To address these gaps in current understanding, the proposed study will therefore directly compare the effects of having or omitting breakfast from the diet, firstly in relation to the acute metabolic and behavioural (i.e. appetite) effects of a standardised breakfast relative to an extended fast on a given day and secondly in relation to the chronic effects of habitual consumption/omission of breakfast. The first phase of testing will reveal the extent to which resting metabolic rate and energy intake at lunch are affected under controlled laboratory conditions, while the second phase of testing will make use of recent technological advances to accurately gauge responses to each treatment under free-living conditions. For this second phase, half the study cohort will ingest a prescribed amount of energy before 11 am every day for six weeks, while the other half will extend their overnight fast until 12 pm every day for six weeks. Energy intake and energy expenditure will be monitored over this period using weighed food records and a novel physical activity monitoring system, respectively, while glucose concentrations will also be monitored continuously using a portable device. Following the 6 week intervention, an advanced form of X-ray analysis (DEXA) will be used to measure how each participant may change throughout the experiment in terms of their total body mass and fat/lean tissue composition. Small samples of sub-cutaneous adipose tissue will also be acquired before and after this intervention both to directly assess the sensitivity of this tissue to insulin with each feeding regimen and also to determine the expression of genes relevant to regulation of appetite/physical activity behaviours, insulin resistance and chronic low-grade inflammation. In relation to the latter, various markers of inflammation will also be measured in the blood given that these measures have been implicated in and therefore provide and indication of cardiovascular-disease risk. Finally, participants will repeat their acute breakfast trial exactly as was performed under the first phase of testing to determine whether the previously observed effects in terms of metabolic rate and energy intake at lunch may have been modified by prolonged exposure to each intervention.