This study will investigate the effect of high-carbohydrate vs. high-fat overfeeding (130% of energy requirements) on whole body insulin sensitivity.

Following habitual diet, participants will be randomly allocated to either a high-carbohydrate or a high-fat diet intervention for 14-days. On days 0, 7 and 14 participants will undergo anthropometric and metabolic testing (primarily an oral glucose tolerance test [OGTT]).

Inclusion Criteria:

• 18-40 years
• BMI = 18.5-29.9 kg/m2
• Male or female
• Physically active (> 3 x 30 min moderate exercise per week)
• Non-smoker (including the use of vaporisers and e-cigarettes)
• Healthy (determined by the School's standard health questionnaire)
• No cardiometabolic or inflammatory illness

Exclusion Criteria:

• Outside of age-range
• Outside BMI range
• Inactive (< 3 x 30 min moderate exercise per week)
• Smoker
• Taking medication
• History of cardiometabolic or inflammatory illness
• Vegetarian/ vegan (as the high-fat diet is based on animal fats)
• Other dietary restrictions which prevent consumption of the experimental diet
• Consumption of probiotics within last 3 months (these influence the gut microbiota)
• Participation in another clinical trial
• Those with known or suspected food intolerances, allergies or hypersensitivity
• Women who are known to be pregnant or who are intending to become pregnant over the course of the study