The Effect of Dairy on Insulin Sensitivity.

This study involves recruiting participants over 18 years of age to a 10 week, randomised crossover intervention. Each participant will be exposed to two diets, lasting four weeks each; a high dairy diet, and a low dairy diet. These diets are separated by a two week period where the participants will resume their habitual diets. Screening: Participants are weighed, height is measured, and an oral glucose tolerance test is given to determine their glucose tolerance. Participants are also instructed to provide a 3 day weighed food record to this visit. Arm 1 The high dairy diet consists of consuming 4-6 serves of dairy per day, with at least two of these serves being yoghurt. There are no restrictions on the fat content of the dairy, and dairy products are not supplied by the investigators. The participants are compensated for their grocery expenses. Arm 2 The low dairy diet requires participants to eat just 0.5 serves per day. After each four week period, participants will undergo two insulin sensitivity tests, and a pulse wave velocity test. The first insulin sensitivity test is the Low Dose Insulin and Glucose Infusion Test, or LDIGIT. It involves infusing a small amount of insulin (25mU/kg body weight/hr), and glucose (4mg/kg/min) into a fasting patient, and taking blood samples at regular time points to assess serum glucose, insulin and c-peptide levels. The second test is the hyperglycaemic clamp, involving the infusion of glucose into a participant, where the serum glucose level will be held at 10mmol/L, and blood samples will be taken at regular time points to assess serum glucose and insulin levels, and c-peptide response. Finally, pulse wave velocity will be assessed in the participant to determine an individual's risk of developing cardiovascular disease after consuming high and low dairy diets. A blood pressure cuff is placed on the upper thigh while the participant lies in the supine position, and a pulse probe is placed on the carotid pulse site. The cuff automatically inflates, and a computer records and calculates the resulting pulse wave velocity. To ensure compliance to the intervention, participants meet with the investigator halfway through each diet, and weighed food diaries are prepared. The investigator then assesses the diaries, and makes suggestions to aid the participants with compliance. Participants are required to remain weight stable during this intervention, and so it is necessary for them to remove foods that have similar energy content to the extra dairy that they are adding into Arm 1 of the study. In Arm 2 of the study, participants are required to substitute in foods with similar energy content to account for the dairy removed from the diet. A guide is provided for both arms of the study for participants to refer to, and also includes suggestions for including more dairy into the diet. A serve of dairy is defined as * 1 cup or 250ml of milk (fresh, UHT, reconstituted powdered milk ) * 40g, equivalent to 2 slices, or 4x3x2cm cube of hard cheese (eg cheddar) * 1/2cup or 120g ricotta cheese * 3/4 cup or 200g of yoghurt