Kefir Versus Milk on Time-Trial Performance in Masters Athletes

Inclusion Criteria: * Provide voluntary signed and dated informed consent. * Be in good health as determined by medical history and routine blood chemistries. * Normotensive (seated, resting systolic blood pressure \< 140 mm Hg and diastolic blood pressure \< 90 mm Hg. If the first measurement is slightly elevated above these limits, the subject will be given a brief (5 minute) rest period, and two more measurements will be taken. The average of all three measurements will be used to determine eligibility. * Normal seated, resting heart rate (\< 90 beats per minute). * Age between 40 and 75 years of age * Body Mass Index \<30 kg/m2 or Body Fat \<30% for females and \<25% for males * Performing aerobic exercise \> 150 mins/week and training for at least 3 years * Well-trained will be defined as athletes that meet two of the four following criteria: training frequency is 3-7 sessions/week; duration of training sessions is 60-240 min; training background is at least 3 y; race frequency is at least once a year. Exclusion Criteria: * Individuals currently diagnosed or being treated for any cardiac, respiratory, endocrine, psychiatric, musculoskeletal, renal, hepatic, neuromuscular or metabolic disease or disorder that in the judgement of the study participant's personal physician or research nurse that will preclude their safe participation or will contraindicate quality control over the collected data * Individuals currently diagnosed with or are being treated for celiac disease, lactose intolerance, digestive insufficiencies or other gastrointestinal complications such as irritable bowel syndrome, ulcerative colitis, etc. Alcohol consumption (more than 2 standard alcoholic drinks per day or more than 10 drinks per week) or drug abuse or dependence. * Smokers or individuals who have quit smoking within the last 6 months * Participants that are unwilling to refrain from caffeine, nicotine, and alcohol intake for 12 hours prior to each study visit. * Known sensitivity to any ingredient in the test formulations as listed in the Nutrition Facts label. * Individuals who are determined to be on a ketogenic diet. * Individuals who are cognitively impaired and/or who are unable to give informed consent. * Participants that are unwilling to refrain from taking biotin at a minimum of 72 hours before visit 3 through the end of the study due to potential interference with laboratory analyses. * Individuals taking a medication known to impact digestive function or hormonal functions that may impact how our body digests, absorbs or metabolizes nutrients * Individuals that are currently taking a probiotic or a dietary supplement that may impact digestive function or hormonal function that may impact how our body digests, absorbs or metabolizes nutrients * Participants that are unwilling to refrain from fermented foods and yogurt, as well as maintain normal consumption levels of prebiotics throughout the study. * Pregnant women, women trying to become pregnant, or nursing women. * Individuals who indicate they are actively restricting calories or attempting to lose weight. * Participants that are unwilling to keep their self-directed training protocols the same throughout the course of the study * Participants that are unwilling to wear the same shoes and similar clothing for all testing visits. * Individuals who participate in aerobic exercise \< 2 days/week or have been aerobically training \< 3 years * Individuals that do not meet the criteria to be defined as well-trained * Unwillingness to intake a supplement or follow the study protocol * Any other condition which in the investigator's opinion may adversely affect the subject's ability to complete the study or its measures or which may pose a significant risk to the subject.