Current evidence suggests that the amount of ingested carbohydrate that we can use for energy during exercise is limited by absorption levels in the gut. Thus, strategies to increase the amount of ingested carbohydrate our body can use are of great interest.

Recent evidence has shown we may be able to use more of the ingested carbohydrate during exercise when supplementing with probiotics ("good bacteria for our gut). It is currently unknown whether the food for these "good" bacteria (known as prebiotics) can induce a similar effect. Therefore, the purpose of this study is to understand if supplementation with the prebiotic Galactooligosaccharide (onwards referred to as BIMUNO®) can increase the use of ingested carbohydrate during exercise. We also aim to understand if BIMUNO® can enhance the availability of other nutrients, such as vitamins and minerals.

Participant will attend 8 laboratory sessions at the University of Bath. The first session is a screening visit lasting no more than 30 minutes. This can take place online, over the phone, or in-person, and the remaining 7 sessions will take place in a University of Bath laboratory. The next session is preliminary testing and familiarisation (approx. 2-2.5 hours). The remaining 6 visits (4 exercise trials & 2 baseline sample collection visits) will be split across 3 main blocks of the study, each lasting 28 days. Following preliminary testing, you will be randomised into supplementing with either BIMUNO®, or placebo. Block 1 will then begin with you attending the laboratory for a baseline sample collection visit, after which you will begin supplementation for the next 28 days. After this 28-day period, you will complete two exercise trials (details below). The ambient temperature in these trials (separated by >3 days) will differ (20 & 35˚C) and will occur in a random order. The second block (termed the 'washout' period) will be a 28-day period where you will take neither BIMUNO® nor the placebo. The third block will then commence and will be identical to block 1 - the only difference being you will take the alternative supplement.

Inclusion Criteria:

• Regular exerciser - defined as participating in at least 30 mins of structured exercise at least 3 times per week
• Able to complete two hours of moderate intensity cycling
• Aged 18-60 years

Exclusion Criteria:

• Use of products marketed as prebiotics, probiotics or synbiotics within 4 weeks prior to study entry (e.g. Yakult, Actimel, Activia, VSL#3, Kefir). Regular cheese or yogurt containing lactic acid bacteria are not an exclusion criterion.
• Systemic antibiotic or antimycotic treatment within 4 weeks prior to study entry.
• Following diets likely to affect study outcomes: e.g. low FODMAP, KETO/high-fat, gluten free/coeliac, paleo, weight loss, caloric restriction, low-carb, 5:2/whole day energy restriction, Atkins/high-protein, sugar-free, single-food, juicing/any day of juicing, any other restriction diet (e.g. very low calory), or vegan diets (GOS is derived from cow's milk).
• Changes in diet, supplement use or medication likely to affect study outcomes within 2 months prior to study entry or planned during the study.

Double-blind