Dietary sugar and carbohydrate intakes will be manipulated for 3 days in a randomised crossover design. All food will be provided. Free-living physical activity energy expenditure will be measured using combined heart rate and accelerometry. Metabolic and appetite/mood responses to 3 days on each diet will be measured.

Each participant will undergo 3 days of each diet:

1. Moderate sugar - reflecting the composition of a typical European diet
2. Low sugar - similar macronutrient composition of a typical European diet but with <5% energy intake from free sugars (as per government guidelines)
3. Low carbohydrate - low carbohydrate diet with <5% energy intake from sugar and <8% energy intake from carbohydrate, replacing carbohydrate energy with fat

Guidelines calling for dietary free sugars to be reduced have been formulated from evidence that diets high in sugar are associated with high energy intake. In theory, by reducing the intake of sugars, calorie surplus will be reduced and energy balance will be maintained. However, when referring to interventions/behaviours that influence energy balance, both sides of the energy balance equation need to be considered. Alterations in nutrients may influence aspects of energy expenditure, not solely energy intake, and this may have implications for the efficacy of guidelines seeking to influence energy intake.

It is important to consider energy expenditure in the context of health. The most variable component of energy expenditure between individuals is physical activity energy expenditure (PAEE), which varies from ~600-2100 kcal per day in men of a similar demographic. Current guidelines do not regard the effect that changing dietary sugar might have on PAEE and therefore total energy expenditure.

Furthermore, there is increasing interest in restricting overall carbohydrate intake to treat metabolic disease or achieve ketogenesis. Very little is known about nutrition-physical activity interactions with this type of intervention.

This study will prescribe 3 diets to healthy males for 3 days in a randomised crossover design, where each participant will consume each of the diets. All food will be prepared and provided by the study team. The diets are as follows:

1. Moderate sugar - reflecting the composition of a typical European diet
2. Low sugar - similar macronutrient composition of a typical European diet but with <5% energy intake from free sugars (as per government guidelines)
3. Low carbohydrate - low carbohydrate diet with <5% energy intake from sugar and <8% energy intake from carbohydrate, replacing carbohydrate energy with fat

Inclusion Criteria:

• Body mass index 18.5-29.9 kg∙m-2
• Age 18-65 years
• Male
• Able and willing to provide informed consent and safely comply with study procedures
• No anticipated changes in physical activity during the study (e.g. holidays)

Exclusion Criteria:

• Any reported condition or behaviour deemed either to pose undue personal risk to the participant or introduce bias
• Any diagnosed metabolic disease (e.g. type 1 or type 2 diabetes)•Any reported use of substances which may pose undue personal risk to the participants or introduce bias into the experiment
• Lifestyle not conforming to standard sleep-wake cycle (e.g. shift worker)
• Any reported recent (<6 months) change in body mass (±3%)