Vitamin B3 has recently been found to be a potent modifier of energy metabolism, especially the function of mitochondria. Mitochondria power up all cells in our bodies, by generating fuel, ATP, for cellular functions. In previous studies, it has been discovered that mitochondrial biogenesis and oxidative metabolism in adipose tissue is severely impaired in obesity, already at a young adult age. Here the investigators describe a proposal where they use nicotinamide riboside (NR), a form of vitamin B3 naturally found in milk, to activate dysfunctional mitochondria, in particular the SIRT/NAD+ pathway, and to rescue signs of obesity-related diseases. The investigators use a unique human study design: monozygotic twins either discordant or concordant for obesity, to examine the effects of NR on mitochondrial function in muscle, adipose tissue and the metabolism of the whole body. The upcoming upcoming results are important for understanding the links between mitochondrial dysfunction and chronic metabolic diseases in humans, as well as for clarifying mechanisms of the novel nutritional therapeutic approaches.

Inclusion Criteria:

1. BMI >18.5 kg/m2 in both members of the twin pair
2. Agreed to maintain current level of physical activity throughout the study
3. Agreed to avoid vitamin supplementation or nutritional products with vitamin B3 14 days prior to the enrolment and during the study
4. Written, informed consent to participate in the study

Exclusion Criteria:

1. Unstable medical conditions as determined by the principal investigator
2. Clinically significant abnormal lab results at screening (e.g. AST and/or ALT > 2 x ULN, and/or bilirubin > 2 x ULN)
3. Subjects who have a planned surgery during the course of the trial
4. History of or a current diagnosis of any cancer (except for successfully treated basal cell carcinoma diagnosed less than 5 years prior to screening). Subjects with cancer in full remission more than 5 years after diagnosis are acceptable.
5. History of blood/bleeding disorders
6. Immunocompromised individuals such as subjects that had undergone organ transplantation or subjects diagnosed with human immunodeficiency virus (HIV)
7. Hepatitis
8. Blood donation in the previous 2 months
9. Anemia (hemoglobin <120)
10. Participation in a clinical research trial within 30 days prior to randomization
11. Allergy or sensitivity to study supplement ingredients
12. Individuals who are cognitively impaired and/or who are unable to give informed consent.
13. Any other condition, which in the principal investigator's opinion may adversely affect the subject's ability to complete the study or its measures or which may have posed significant risk to the subject.