The purpose of this study is to evaluate the effects of kefir on gastrointestinal health, sleep quality, and biochemical parameters in adults who have sleep problems. Further measurements of cognitive function, arterial stiffness and blood pressure will also be assessed.

The following questions will be addressed during the study:

1. What effect does repeated ingestion of kefir have on the gut microbiome and can this positively influence sleep performance (quality and duration) in adults with sleep problems?
2. Does repeated ingestion of kefir impact LDL, triacyglycerols and non-HDLc in adults with sleep problems?
3. What effect does repeated ingestion of kefir have on cognitive function and mood?
4. What effect does repeated ingestion of kefir have on vascular function, inflammation and markers of immunity in adults with sleep problems?

In a randomised, crossover design study, participants will consume 250 ml of kefir (original flavour) or milk for 4 weeks with a 4-week washout.

The gut microbiome is recognized as a key regulator of host health, influencing various physiological processes. Dysbiosis of the gut microbiome has been implicated in a range of adverse health outcomes, including disruptions in sleep architecture and impairments in cardiometabolic function. There is growing interest in identifying dietary interventions that can modulate gut microbiota composition and functionality. In particular, the consumption of fermented foods, such as kefir, has emerged as a potential strategy for promoting microbial homeostasis and improving health outcomes

This study conforms to a randomised, double-blind 2-arm crossover design. Participants will consume either Kefir (original flavour) or milk. Each arm will involve daily consumption of the drink for 4-weeks with a 4 week washout before crossing over to the opposite condition. Drinks will be administered in a counterbalanced order to minimise potential order effects.

The primary outcome of this study is a 5% change in LDL based on a previous study (Bellikci-Koyu et al., 2022).

Assuming type I error probability α = 0.05 and power of 80%, and effect size of 0.2 G*power calculated that 32 participants would need to be recruited.

To account for potential drop out and balanced randomization, we will aim to recruit 40 participants.

Participants will be recruited through the use of posters around Sheffield Hallam University and the University of Leeds, through emails to staff and students, and contacts at surrounding universities. Potentially eligible participants will be identified through the use of a recruitment questionnaire before being invited to a screening visit for full consideration for the study. All participants will provide written informed consent before participating.

A linear mixed model analysis will be used to assess the effect of kefir on outcome parameters.

Inclusion Criteria:

• Aged 40-70 years old
• Suffer with poor sleep (measured using the PIttsburg Sleep Quality Index with a score > 5)
• BMI in range of 25-35 kg/m2
• Willingness to consume each study drink daily for 4 weeks and attend the laboratory on 4 separate occasions
• For women: Medication in the form of contraception or HRT providing these have not been modified 3 months prior to the first study day

Exclusion Criteria:

• Intolerance or allergy to milk-based products
• Pregnancy/breastfeeding
• Insufficient English language skills to complete all study questionnaires and perform the cognitive tests
• Gastrointestinal disorders
• Inability to attend the laboratory due to travel/mobility difficulties