Vitamin D is a fat-soluble vitamin with unique hormone-like properties that is indispensable for a wide range of physiological processes essential to human health and well-being. Unlike most vitamins, which must be obtained entirely from dietary sources, vitamin D can be synthesized endogenously in the skin through the action of ultraviolet B (UVB) radiation on 7-dehydrocholesterol, a process that is influenced by factors such as geographical latitude, season, time of day, skin pigmentation, age, and sunscreen use. It exists primarily in two forms: vitamin D2 (ergocalciferol), which is derived from plant-based sources and certain supplements, and vitamin D3 (cholecalciferol), which is produced in the skin and found in animal-derived foods including fatty fish, fish liver oils, egg yolks, and certain fortified products. Both forms undergo hydroxylation in the liver to 25-hydroxyvitamin D [25(OH)D], the main circulating form and clinical marker of vitamin D status, and then further hydroxylation in the kidney to form 1,25-dihydroxyvitamin D, the biologically active hormone. Vitamin D plays a central role in regulating calcium and phosphorus homeostasis by promoting intestinal absorption, maintaining optimal mineralization of the skeleton, and ensuring proper neuromuscular function. Adequate vitamin D status is necessary to prevent rickets in children, osteomalacia in adults, and to mitigate the risk of osteoporosis and fragility fractures in older populations. Beyond its skeletal effects, vitamin D exerts multiple extra-skeletal actions: it modulates innate and adaptive immune responses, influences muscle strength and coordination, regulates cell proliferation and differentiation, and has been implicated in cardiovascular function, glucose metabolism, and even cancer prevention. Recent studies have suggested that vitamin D supplementation may also improve plasma lipid profiles, thereby reducing risk factors associated with cardiovascular disease. The European Food Safety Authority (EFSA) has officially recognized and authorized several health claims for vitamin D, including its contribution to normal absorption and utilization of calcium and phosphorus, maintenance of normal blood calcium levels, preservation of normal bones, teeth, and muscle function, support for normal immune system activity, and reduction of the risk of falls in people aged 60 and over-a benefit particularly relevant for preventing fractures in the elderly. Despite these recognized benefits, vitamin D deficiency remains a major public health challenge across Europe, affecting populations regardless of climate. Overall prevalence is estimated at up to 40% in the general population, rising above 70% in high-risk groups such as elderly individuals, people with dark skin pigmentation, those who are institutionalized or housebound, and individuals with chronic illnesses or limited sun exposure. In Italy, although its Mediterranean climate might be expected to protect against hypovitaminosis D, deficiency is common, especially in northern regions during the winter months. This paradox is explained by modern lifestyle factors including urbanization, increased time spent indoors, sedentary habits, and cultural or occupational patterns that limit skin exposure to sunlight. Approximately one in two Italians has insufficient vitamin D levels, with the prevalence markedly higher in older age groups. The clinical consequences of prolonged deficiency are wide-ranging and extend well beyond skeletal fragility: children may develop rickets characterized by impaired bone growth, skeletal deformities, and muscle weakness, while adults may suffer from osteomalacia, chronic bone pain, and an increased risk of fractures due to osteoporosis. Extra-skeletal manifestations include compromised immune defense with greater susceptibility to respiratory infections, higher incidence of metabolic disorders such as type 2 diabetes, increased cardiovascular morbidity, reduced muscle strength leading to instability and falls, and mood disturbances including depression and seasonal affective disorder. Correcting vitamin D deficiency requires a combination of public health strategies including safe sun exposure campaigns, food fortification policies, and widespread access to effective supplementation. In Italy, the recommended daily allowance (RDA) for healthy adults is 600 IU, and over-the-counter supplements can contain up to 2000 IU per day for the management of mild deficiency, while severe cases require targeted pharmacological therapy at higher doses under medical supervision. The high prevalence of suboptimal vitamin D status has fueled a substantial global market for supplements, valued at USD 1.69 billion in 2022 and projected to grow to USD 3.44 billion by 2031, with a compound annual growth rate of 8.22% from 2023 to 2031. In parallel with this market expansion, there is growing interest in identifying sustainable, environmentally friendly sources of vitamin D as alternatives to the synthetic forms that dominate current production. One promising avenue is the extraction of vitamin D from food industry by-products such as fish waste, which not only offers a renewable resource but also aligns with circular economy principles and waste reduction strategies. This innovative approach is at the heart of the PRIN project "Innovative and sustainable processes for the development of Vitamin D nutraceutical from fish waste: extraction, formulation and clinical study for the evaluation of its bioavailability and clinical consequence" (VITADWASTE; 2022M9JL3%; National Coordinator: Prof. Gianni Sagratini, University of Camerino), funded by the Italian Ministry of University. The project seeks to develop a fish-derived vitamin D nutraceutical, evaluate its bioavailability in humans, and assess its clinical effects. Within this framework, the present randomized, double-blind, non-inferiority clinical trial aims to compare, in healthy subjects with mild vitamin D deficiency defined by plasma concentrations between 20 and 40 ng/mL, the effects of daily supplementation with 600 IU of fish-derived vitamin D-equivalent to 100% of the RDA for healthy adults-to those of an identical dose of standard synthetic vitamin D, with the primary endpoint being the change in plasma vitamin D levels after supplementation.
