The goal of this descriptive clinical study is to investigate daily oscillations in glycaemic control between healthy adults and adults with excess weight and who have early-stage prediabetes or T2D and are not taking medications for glycaemic control. The study also explores how these differences relate to changes in daily patterns in oral microbiome and metabolome, circadian markers, and lifestyle factors such as eating, physical activity, light exposure and appetite.

The main questions to answer are:

1. Do adults with excess weight and prediabetes/T2D exhibit a disruption of the circadian system compared to healthy individuals? If so, are these disruptions manifested in hormone levels, gene expression, microbiota composition and function, metabolite levels and appetite regulation?
2. Does chrono-disruption contribute to the dysregulation of glucose metabolism and responses to lifestyle factors in individuals with excess weight and prediabetes/T2D?

Researchers will compare two groups:

• Healthy adults with normal weight.
• Adults with excess weight (overweight or obesity) with prediabetes/T2D who are not on diabetes medications.

The study will involve both semi-controlled settings (where food intake and physical activity are controlled) and free-living conditions.

Participants will:

• Wear devices: Use a continuous glucose monitor and a circadian monitoring device for 14 days.
• Attend clinical visits: Visit the Nutritional Intervention Unit (NIU) 4 times for body composition measurements, sample collection (blood, saliva a faeces) and to answer questionnaires. .
• Participate in a 12.5-hours clinical visit day in the NIU under semi-controlled conditions, with the purpose of collecting blood samples before and after breakfast and saliva samples every 4 hours, covering a full 24-hour cycle.
• Keep track of daily habits: Maintain their usual lifestyle while keeping a food diary and recording appetite related feelings.

Compelling evidence suggests that type 2 diabetes (T2D) can often be prevented by adopting healthy lifestyle. However, current standard recommendations may not fully account for individual differences, limiting their effectiveness. Factors such as genetics and daily lifestyle patterns influence how individuals respond to these recommendations. For example, disrupted sleep patterns or irregular eating schedules can make it difficult to maintain healthy habits, ultimately affecting blood glucose control. Such disruptions are associated with disturbances in the body's internal clock, also known as circadian rhythms. In this sense, chrono-disruption (i.e., disruption of circadian rhythms) has been shown to impair glucose metabolism. Individuals at high risk of T2D are characterized by loss of diurnal rhythmicity in the insulin oscillatory pattern. Similarly, recent studies suggest that disruptions in daily fluctuations of hormone levels, gene expression, microbiota composition, and metabolite profiles are associated with poorer blood glucose control.

The present project relies on the hypothesis that individuals with excess weight and drug-naïve prediabetes or type 2 diabetes, compared to healthy individuals, exhibit a disruption of the circadian system. This is manifested by loss in daily oscillations and altered oscillatory patterns across multiple physiological processes, including hormone production, gene expression, microbiota composition and function, and metabolite composition and secretion. Chrono disruption contributes to the dysregulation of glucose metabolism, changes in appetite regulation and response to external cues and, consequently, impaired glycaemic control in this population.

The Kronodiabet study is a 2-week observational study with two parallel study groups, matched by age and gender, according to the following conditions: adults with overweight or obesity and impaired glucose metabolism (drug-naive prediabetes or type 2 diabetes); and adults with normal weight and no alterations in glucose metabolism.

Experimental design: Each participant, once recruited, will attend the NIU at the Center for Nutrition Research of the University of Navarra on 4 occasions. At visit 1 (V1), participants will come to the NIU and will be fitted with a continuous glucose monitoring sensor and a circadian monitoring device, which will be worn during the main study visit (V2) and the following 12 ± 2 full days (field period phase) and body composition will be analysed through dual energy x-ray absorptiometry (DEXA). At this visit (V1), participants will bring a stool sample collected no later than 48 hours prior to arrival at the NIU. In visit 2 (V2), lasting 12 hours and 30 minutes, participants will arrive to the NIU (where the entire visit will take place) after at least 8 hours of fasting. During this visit, participants will be provided with the meals to be consumed throughout the day and blood draws and completion of questionnaires will be carried out. In addition, 4 out of the 7 saliva samples required for the analysis of the oral microbiota, metabolome and gene and hormone determination will be collected every 4 hours. The remaining saliva samples (3/7) will be collected at home, until the 24-hour period is completed. On the following day (V3), participants will come back to the NIU to deliver the samples collected at home. In addition, they will be given the necessary material for the field study period. During the field phase (which will last 2 weeks) participants will be asked not to change their habitual lifestyle, including diet, exercise and timing. In the final visit (V4), body composition measurements will be taken and the circadian and glucose monitoring sensors and completed questionnaires will be collected, concluding the study.

The specific objectives are:

1. To characterize the phenotype, behavioral traits and lifestyle habit, as well as the microbiome, metabolome and transcriptome in healthy individuals and individuals with overweight/obesity and drug-naïve prediabetes/type 2 diabetes.
2. To investigate, under free-living conditions, daily oscillations in glycaemia, food behavior and circadian parameters of the main circadian synchronizers.
3. To compare daily oscillations in oral microbiota and metabolome, along with daily oscillations of circadian markers, under semi-controlled conditions, between healthy individuals and individuals with overweight/obesity and drug-naïve prediabetes/type 2 diabetes.
4. To study, from an integrated perspective, daily oscillations in glycemic control, through the integration of omics (metagenomics, metabolomics), clinical and lifestyle data, in the absence and presence of excess weight and impaired glucose metabolism.

Inclusion Criteria (T2D group):

• Women and men between the ages of 30 and 70.
• Meet any criteria diagnosis of prediabetes or type 2 diabetes according to the American Diabetes Association (ADA), without pharmacological treatment for glycaemic control.
• Excess weight, according to body mass index (BMI) value > 25 kg/m2.
• Body fat percentage, measured by bioelectrical impedance analysis (BIA), above the normal range, according to age and sex, following the classification from Gallagher D, et al. Am J Clin Nutr. 2000;72(3):694-701.

Inclusion Criteria (Healthy group):

• Women and men between the ages of 30 and 70.
• No impaired glucose metabolism according to ADA diagnostic criteria
• Normal weight, according to BMI value (18,5 - 24,9 kg/m2)
• Body fat percentage, measured by BIA, within the normal range, according to age and sex, following the classification from Gallagher D, et al. Am J Clin Nutr. 2000;72(3):694-701.

Exclusion Criteria:

• Chronotype within the categories "extreme evening" (<34) or "extreme morning" (>76), according to the Hörne-Orberg Morning-Evening Questionnaire (MEQ) classification.
• Weight changes (± 4.5 kg) during the last 3 months.
• Habitually consuming ≥ 14 units of alcohol/week in women and ≥ 21 units of alcohol/week in men.
• Self-reported use of drugs of abuse in the last 12 months.
• Be an active smoker or have quit smoking within the last 3 months.
• Night or late shift work (finishing later than 11 pm on a permanent basis).
• Recent travel across 2 or more time zones during the last month before the start of the study.
• Show poor cooperation or, in the opinion of the investigator, have difficulty following the study procedures.
• In the case of women, being pregnant or breastfeeding .
• Blood donation in the 3 months prior to the beginning of the study.

Medical conditions:

• Oral diseases, inflammation or lesions at the time of the study.
• Any clinical condition, including chronic metabolic diseases, systemic intestinal, hepatic or renal diseases, when the research team consider that it may influence the results of the study.
• Severe hyperlipidaemia, severe hypertension or hypothyroidism, untreated or treated for less than 3 months with stable dose.
• Have relevant functional or structural abnormalities of the digestive tract, such as malformations, angiodysplasias, active peptic ulcers, chronic inflammatory diseases or malabsorption.
• Subjects who have undergone surgery of the digestive tract with permanent sequelae, including surgical treatment for obesity.
• Any type of psychological/psychiatric impairment such as depressive pathology, anxiety or bipolar disorder, when the research team consider that it may influence the results of the study.
• Any type of cancer or be undergoing treatment for it, or that a period of at least 5 years has not elapsed since its eradication.
• A diagnosed eating disorder (such as bulimia or binge eating disorder)
• Presence of any type of health problem that may affect sleep
• Any infection at the time of the study, especially respiratory or intestinal infection.

Medication:

• Be under continuous pharmacological/hormonal treatment that may affect glycaemic control.
• Current or within the previous 2 months use of prescription or non-prescription medication or nutritional supplements that have the potential to affect glycaemic control, as well as biochemical analyses. For food supplements containing biotin, they are permitted provided that their consumption is omitted 48 h before blood draws.
• Current or within the previous 2 months use of nutritional supplements or prescription or non-prescription drugs that may affect the microbiota (probiotics, prebiotics, fibre, antibiotics, antifungals).
• Taking any medication that may seriously affect circadian rhythms (melatonin receptor agonist drugs (tasimelteon, ramelteon), drugs for the treatment of drowsiness (modafinil, armodafinil), or with sedative effects (benzodiazepines, zolpidem)) or nutritional supplements that contain melatonin and are likely to affect circadian rhythms, except if on a stable dose for 3 months or more.
• Cholesterol-lowering medication, except if on a stable dose for 3 months or more.
• Blood pressure lowering medication, unless on a stable dose for 3 months or more.
• Medication that may affect appetite and body weight (e.g. antidepressants, thyroid) unless on a stable dose for 3 months or more and without significant side effects.

Individuals from a region in the north of Spain. People registered in the Nutrition Research Centre's own database will be invited. In addition, other recruitment channels will be used, such as flyers, advertisements in local press, medical centres and hospitals, social networks and websites.