The goal of this randomised, double-blinded, placebo-controlled trial is to investigate the immune, metabolic and cognitive effects of four weeks of daily ketone supplementation in adults aged 60 to 80 with stable health. The main objectives are to assess the effects of the intervention versus placebo on markers of metabolic health, inflammation, immune function, adipose tissue, and cognitive performance.

Participants will undergo two weeks of baseline monitoring followed by a four-week supplementation period in which they will drink a ketone monoester drink or taste-matched placebo three times a day. During these periods, participants will record their diet and supplement intake and their physical activity and blood glucose will be monitored using wearable devices. At the beginning and end of the supplementation period, participants will undergo testing in the university physiology laboratories, involving blood, expired air and adipose tissue samples, as well as cognitive tests, physical tests and questionnaires.

Background: Research shows that ketones have beneficial effects on metabolism, inflammation and brain health in humans. In mice, they have also been shown to influence pathways involved in ageing. Ketones are natural molecules that are produced by the body when people fast (abstain from eating) for longer than 16-24 hours or eat a diet low in carbohydrates. It is now possible to consume ketones in the form of a drink.

Aims: This study aims to investigate if consuming a ketone drink for four weeks improves immunometabolic and cognitive health in adults aged 60 to 80 years. The main objectives are to assess the effects of the intervention versus placebo on:

1. Markers of metabolic health, including glucose control, lipid profile, blood pressure and body composition;
2. Systemic inflammation, immune cell activation and pro-inflammatory cytokine production;
3. Gene expression and secretory profile of subcutaneous adipose tissue; and
4. Cognitive performance and physical function.

Methods: The study is a randomised, double-blinded, placebo-controlled trial. Thirty participants (male and female) aged 60 to 80 years old with stable health will be recruited. Participants will undergo two weeks of baseline monitoring followed by a four-week supplementation period in which they will drink a ketone monoester drink or placebo three times a day. During these periods, participants will record their diet and supplement intake and their physical activity and blood glucose will be monitored using wearable devices. For two days in the supplementation period, participants will replicate their food consumption and physical activity so that they match two days in the baseline period (these are known as matched meal and activity days).

At the beginning and end of the supplementation period, participants will come to the physiology laboratories at the University of Bath for testing. Here, measures will be taken of their body weight, body composition, blood pressure, cognitive function and physical function, as well as samples of expired air, fat tissue and blood for analysis. Sleep and quality of life will be assessed via questionnaires.

Inclusion Criteria:

• Age 60 to 80 years
• Postmenopausal women must be >1 year since last menses
• Able to provide informed consent
• Willing and able to comply with all study procedures including randomisation into any of the experimental groups; maintenance of habitual dietary intake, exercise, medication and supplement use over the 28-day intervention period; blood draws and adipose tissue biopsies; and abstinence from alcohol (>24 h), food (>10 h) and strenuous exercise (>3 d) prior to trial days.

Exclusion Criteria:

• Living in a residential care home
• Unstable or clinically active pulmonary, cardiac, hepatic, renal, endocrine, hematologic, immunologic, neurologic, psychiatric or biliary disorders. 'Unstable' refers to complications of a condition that are not controlled by medication or lifestyle and which require frequent monitoring and testing by a health professional. Stable chronic disease is not an exclusion criterion unless specified.
• Diagnosed Type 1 or Type 2 Diabetes Mellitus
• Diagnosed gastrointestinal condition which would potentially impact ability to consume study drink (e.g. inflammatory bowel disease, history of gastrointestinal ulcers or bleeding)
• Diagnosed autoimmune condition
• Previous major cardiovascular event (e.g. heart attack, stroke)
• Past or current cancer diagnosis and treatment excluding non-melanoma skin cancers
• Severe hypertension (systolic blood pressure ≥180 mmHg and/or diastolic blood pressure ≥120 mmHg), as defined by blood pressure measured at Visit 1
• Current tobacco or recreational drug use
• Reported changes to use of thyroid, antihypertensive, antidepressant or statin medications within 30 days of Visit 1
• Taking medications that will interfere with the study outcomes
• Known negative reaction to lidocaine anaesthetic and/or taking warfarin
• Currently following a ketogenic diet or taking ketone supplements
• Not weight stable in the prior 3 months (>5% weight change)
• Unable to converse in English