This weight loss study will investigate the impact of calorie distribution across a day (large breakfast meals and smaller evening meals versus small breakfast meals and large evening meals) on body weight, and physiological and behavioral mechanisms regulating energy balance.

Participants will undergo 2 x 4 week energy restriction protocols in a randomized cross over design; big breakfast (45% of calories in the morning meal, 20% at dinner) and big dinner (45% of calories in the evening meal, 20% at breakfast). We predict that timing of eating will influence energy balance, because morning energy expenditure is amplified in comparison to the evening. This study will allow us to assess whether the increased energy expenditure in the morning is linked to natural biological circadian rhythm or behavioral adaptions.

Dietary advice for weight management is broadly based on the assumption that a 'calorie is a calorie' and it does not matter when calories are consumed across the day. Recent evidence has challenged this assumption, suggesting that we may utilize calories more efficiently when consumed in the morning relative to the evening, and this could be used as a beneficial strategy for weight loss - this is a newly developing field of investigation which merges circadian biology with nutrition (chrono-nutrition).

Timing of food consumption is a modifiable factor influencing energy balance and body weight (and thus, disease risk). Previous research has shown that calories ingested at different times of the day have different effects on energy utilization, leading to differential weight loss, even at iso-caloric amounts. This study will aim to increase our understanding of the underlying behavioral and physiological mechanisms associated with differential weight loss and energy balance when calories are consumed predominantly in the morning versus in the evening.

This study will be a cross-over study comparing large breakfast versus large evening meals (percent daily calories split between breakfast, lunch and dinner as 45-35-20 (breakfast-loaded) or 20-35-45 (evening-loaded)) during energy restriction (Fed to measured RMR) on energy balance, through differences in both physiological and behavioural changes in energy expenditure and substrate utilization.

Inclusion Criteria:

• BMI ranging from 27-42 kg/m2;
• Those habitually consuming breakfast (at least 5 times a week).

Exclusion Criteria:

• women who are pregnant, planning to be pregnant or breastfeeding
• subjects with food allergies which prevent consumption of the study diet.
• diagnosis of diabetes, hypertension, renal, hepatic, haematological disease, coronary heart disease, metabolic disease, gastrointestinal disease
• having given a pint of blood for transfusion purposes within the last month
• unsuitable veins for blood sampling
• inability to understand the participant information sheet
• inability to speak, read and understand the English language
• those on any prescription medications (other than oral contraceptives) which will adversely affect the study outcomes (i.e. medications affecting, circadian timing, sleep or metabolic function).
• those on any specific diet regimes
• those on any weight loss programmes (that may be affecting lifestyle, physical activity and diet).
• Extremes of chronotypes, sleep patterns and physical activity.