Fermented Foods and Bone Health: Investigating the Gut-Bone Axis in Premenopausal Vegan Women

Plant-based diets are increasingly adopted for health and environmental reasons, but they are associated with lower bone mineral density, higher fracture risk, and elevated bone turnover markers, particularly in women. These effects are partly explained by lower intakes of calcium and zinc, and higher concentrations of phytates and oxalates, substances that inhibit mineral absorption from plant foods. Evidence-based dietary strategies to support bone health in vegan populations beyond supplementation remain limited. Fermented plant-based foods may help address this gap through two complementary mechanisms: first, by delivering live microorganisms that beneficially modulate gut microbiota and promote the production of short-chain fatty acids, which support mineral absorption and reduce bone resorption, and second, by reducing antinutritional factors such as phytates during microbial fermentation, thereby improving mineral bioavailability. This study investigates whether the daily consumption of fermented plant-based foods, specifically lacto-fermented vegetables, calcium-fortified plant-based yogurt alternatives with live cultures, and Rhizopus-fermented tempeh, reduces bone resorption and improves calcium metabolism in premenopausal women following a vegan diet. Participants will follow each dietary condition (fermented or matched non-fermented control foods) for 12 weeks in randomized order, separated by an 8-week washout period. Blood, urine, and stool samples are collected at each study visit to assess bone turnover markers, gut microbiota composition, short-chain fatty acid production, inflammatory markers, and a range of metabolic and nutritional parameters.

Inclusion Criteria: * Female, aged 28-43years. * Body mass index (BMI) between 18.5 and 24.9 kg/m². * Premenopausal, with regular menstrual cycles. \* * Adherence to a vegan diet for ≥1 year, confirmed by questionnaire. * Willingness to consume both fermented and non-fermented plant-based study foods. * Ability to comply with all study procedures, including run-in, washout, sample collection, and clinic visits. * Signed written and oral informed consent. * Regular menstrual cycles are defined as spontaneous menstrual bleeding occurring every 21-35 days, with cycle-to-cycle variability not exceeding 7-9 days over the prior three months, and bleeding duration of 3-8 days, in the absence of hormonal contraception or endocrine disorders. Exclusion Criteria: * Chronic diseases or conditions affecting bone metabolism (e.g., osteoporosis, thyroid disorders, chronic kidney and liver disease, menstrual cycles disorders affecting bone health, conditions associated with malabsorption). * History of diagnosed eating disorders. * Chronic inflammatory or gastrointestinal diseases (e.g., IBD, celiac disease). * Recent antibiotic use (within 3 months prior to screening). * Use of Vitamin D supplements exceeding 2000 IU/day. * Abnormal DXA results (T-score ≤ -2.5). * Pregnancy, intention to become pregnant, or lactation during the course of the study * Use of hormonal contraceptives. * Use of medications influencing bone metabolism (e.g., glucocorticoids, antiresorptives). * BMI \24.9 kg/m². * Known or suspected non-compliance, drug or alcohol abuse * Inability to follow the procedures of the studies, e.g. due to language problems, psychological disorders, dementia, etc. of the participant or anticipated prolonged absence (e.g. extended travel). * Participation in another clinical study within the last 3 months. * Known allergy or intolerance to study foods.