Participants will be randomly allocated to either Yakult ingestion or a control group. For the first 20 days, subjects will consume their normal diet (keeping a detailed food diary throughout). On days 21-28 they will switch to a high-fat/high-calorie diet.

The investigators hypothesise that consuming a high-fat, high-energy diet for 7 days will alter the composition of the gut microbiota and induce metabolic endotoxaemia / systemic inflammation as well as decreasing whole body insulin sensitivity (as we have shown previously). In contrast, the investigators hypothesise that consuming Yakult for 21 days before and 7 days throughout the high-fat diet will maintain a favourable gut microbiota and prevent metabolic endotoxaemia / systemic inflammation and thus maintain insulin action / insulin sensitivity.

Inclusion Criteria:

• Males and females

• Aged 18-30

• Physically active (exercising at least 3 times per week for more than 30 min at a time)

• Non-smoker

• Free from cardiovascular or metabolic disease

• Weight stable for at least 6 months

• Normal body mass index (BMI: 18.5-24.9 kg/m2)*

• Compliant (i.e. understands and is willing, able and likely to comply with all study requirements)

  • Note: If BMI is >24.9, but body fat % (as measured by bioelectrical impedance analysis) is below 21% in males and 31% in females, then the subject may still be recruited at the PI's discretion.

Exclusion Criteria:

• Using probiotic or prebiotic supplements within the previous 3 months
• Vegetarians and vegans
• Diagnosis of insulin resistance, pre-diabetes or full diabetes
• Underweight (determined as BMI less than 18.5 kg/m2)
• Overweight or obese (determined as BMI greater than 24.9 kg/m2)
• Those on a calorie controlled diet or other dietary restrictions that would prevent them from consuming the probiotic treatment and high fat/high calorie diet.
• Those who are unwilling to restrict their intake of fermented dairy products.
• 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.
• Participation in another clinical trial
• Those who have donated blood within 3 months of the screening visit and participants for whom participation in this study would result in having donated more than 1500 ml of blood in the previous 12 months.
• Participants who take any form of regular medication that is known to affect either the gut microbiota and/or insulin sensitivity or who have taken any antibiotics in the previous 3 months.
• Participants who know they would not be available for all the lab visits during the 4-week study period.

Participants will be given a unique identifier, which will not disclose the treatment group. All biological samples will be labelled with only the unique identifier.