Both in ankylosing spondylitis (ASP) and psoriatic arthritis (PsA), osteopenia is present in one-third of women and men, whereas osteoporosis mainly affects men, even in their 30 s. Subclinical gut inflammation has been described in patients with AS or PsA. Joint involvement also occurs with other gastrointestinal diseases such as celiac disease. We tested the hypothesis, whether elevated serum levels of human anti-tissue-transglutaminase-IgA (htTG) are associated with changes in disease activity, vitamin D metabolism and bone mineral density (BMD) in patients with ASP and PsA. In a cross-sectional study, we evaluated both biochemical markers of bone turnover, BMD and htTG in 76 patients with ASP and 120 patients with PsA. A reduction of BMD in lumbar spine was determined both in ASP (42.7%) and in PsA (57.3%). Furthermore, a significantly higher prevalence of htTG was detected only in ASP (14/76). ASP patients with negative htTG status have significant higher 25-vitamin D3 levels. ASP patients with positive htTG status are younger. A positive htTG status entails the risk of a bad vitamin D supply, which should be considered in young patients since this constellation favors a reduction in bone density. The coincidence of ASP along with detection of htTG and clinically asymptomatic celiac disease as an accessory source of inflammation with a negative influence on the bone metabolism is also conceivable. Clinicians need to be aware of patients younger than 45 years with ASP, which have important implications for the correct treatment and vitamin D supplementation.