Low-carbohydrate Diet and Low-Density Lipoprotein - Cholesterol (LDL-c) Interindividual Variability

The study will be a controlled feeding replicate randomised controled trial with 3 cycles, each involving the pair of diets [low carbohydrate (K) and control (C) diets]. There will be 6 x 1-day periods (3 days low carbohydrate and 3 days control). Twenty individuals (body mass index: 18.5-35 kg/m2) will be recruited and randomised into one of the 8 possible sequences:

• KC KC KC
• KC KC CK
• KC CK KC
• KC CK CK
• CK CK CK
• CK CK KC
• CK KC CK
• CK KC KC A week before the interventions start, participants will visit the laboratory so that the investigators can perform indirect calorimetry and collect a blood sample for genotyping. During the week before the interventions, participants will wear activity monitors to determine habitual physical activity. Before each visit, participants will be asked to come to the lab in a rested and overnight fasted state. During the first trial visit, participants will have a dual-energy X-ray absorptiometry (DEXA) body scan to determine body composition. During all 12 study visits the investigators will perform a venepuncture in an antecubital vein and will collect 20 mL of blood.

Additionally, the investigators will also collect urine samples. After odd visits (1st, 3rd, 5th, 7th, 9th and 11th), participants will be supplied with their respective meals for the day (low carbohydrate or control) and will be asked to not eat anything else during the day.

Finally there will be a wash-out period of 6 days between laboratory visits. To keep consistency between trial days, participants will be asked to replicate physical activity levels performed during the 24 hours before the first trial, and during diet day (before second visit), so that these can be replicated for the following visits.

Inclusion criteria

• Age: >18 years
• Body mass index: 18.5-35 kg·m-2
• Low-Density Lipoprotein-cholesterol <4.9 mmol/L
• Non-fasting triglycerides <2.3 mmol/L

Exclusion criteria

• weight instability (>5% change in body mass within last 3 months)
• smoker
• diagnosis of hypercholesterolaemia or any other lipid-related disease
• diagnosis of diabetes
• diagnosis of atherosclerotic cardiovascular disease
• elevated fasting glucose (>5.4 mmol/L)
• dietary intolerances or allergies or to other study procedures
• diagnosis of gastrointestinal disorders
• any other condition/medication that could introduce bias to the study
• already following a ketogenic diet
• not willing to follow a ketogenic diet
• unable to understand English language