Multiple sclerosis (MS) is a chronic disease that causes cognitive and physical disability whose pathological hallmarks include inflammation, demyelination, and axonal loss. On magnetic resonance imaging (MRI), blood-brain barrier breakdown, inflammatory lesions, and neurodegeneration are frequently present in the brain of people with MS (Trapp et al., 1998).

Low vitamin D, a fat-soluble vitamin, is considered a risk factor for MS but there is conflicting evidence for the utility of vitamin D supplementation for treating MS progression (Doosti-Irani et al., 2019; Feige et al., 2020; Zheng et al., 2018). However, other fat-soluble vitamins and micronutrients have been investigated to a more limited extent (Besler et al., 2002; Loken-Amsrud et al., 2013b; Wikstrom et al., 1976; Wong et al., 1993). Wickstrom et al. did not find differences in vitamin E levels (Wikstrom et al., 1976), and Wong et al. found no differences in absorption of retinol or β-carotene (Wong et al., 1993) between MS patients and controls. However, Besler et al. reported 16–36% lower levels of retinol, β-carotene, α-tocopherol, and ascorbic acid levels in 24 secondary progressive MS patients relative to healthy controls. Loken-Amsrud et al. examined the effects of α-tocopherol, which was present in the corn-oil placebo and omega-3 fatty acids supplementation treatment arms of a trial of omega-3 fatty acids supplementation that randomized 88 relapsing-remitting MS (RR-MS) patients prior to, and following initiation of interferon-β treatment (Loken-Amsrud et al., 2013b). They found that α-tocopherol levels were 56% (Besler et al., 2002) (Loken-Amsrud et al., 2013b) lower in the omega-3 fatty acids supplementation treatment arm, and association trends for greater baseline α-tocopherol levels with fewer new contrast-enhancing and T2 MRI lesions were observed; no associations were found for relapses or disability progression (Loken-Amsrud et al., 2013b). In the context of the same the omega-3 fatty acids supplementation trial, each 1 µmol/L increase in retinol levels reduced the number new contrast-enhancing and T2 MRI lesions by 49% and 42%, respectively (Loken-Amsrud et al., 2013a). However, retinol levels are not associated with MS relapses (Runia et al., 2014).

This research focuses on retinol, tocopherols, and carotenoids (RTC), which are important fat-soluble vitamins and micronutrients (Tanumihardjo et al., 2016), in a cohort of MS patients and controls with 5-year follow-up. Carotenoids are categorized as carotenes if the underlying tetraterpene hydrocarbon structure lacks oxygen and as xanthophylls if the oxygen is present. Carotenes such as α-carotene and β-carotene, and xanthophyll carotenoids such as β-cryptoxanthin containing β-ionone rings have provitamin A activity because they undergo biotransformation by the enzyme β-carotene 15,15′-monoxygenase to retinol, a form of vitamin A (Burri et al., 2011). Retinol is converted to 11-cis-retinal, the light-sensing chromophore covalently linked to rhodopsin, the retinal rod cell photoreceptor that triggers visual signal transduction. Adequate dietary intake of vitamin A is therefore essential for low-light vision. retinol is also converted to retinoic acid, which is important in immune cell homeostasis (Brown et al., 2015) (Raverdeau and Mills, 2014). Lutein and zeaxanthin are isomeric xanthophyll carotenoids that are enriched in the macula pigment. In MS patients with optic neuritis, macular pigment optical density is reduced compared to healthy controls (Cerna et al., 2021).

Liver, fish liver oils, dairy products and egg yolk are dietary sources of retinol (Tanumihardjo et al., 2016) whereas carotenoids are obtained from dietary fruit and vegetable intakes (Al-Delaimy et al., 2005a,b; Pfeiffer et al., 2013). Lutein/zeaxanthin and β-cryptoxanthin are associated with the intake of vegetable and fruits, respectively. Green leafy vegetables (e.g., kale, spinach) and intensely yellow or orange-colored fruits and vegetables (e.g., carrots, corn, orange bell peppers, pink grapefruit, peach, papaya, mangoes, cantaloupe) contain pro-vitamin A carotenoids (Tanumihardjo et al., 2016). In Europe and United States, serum levels of carotenes are associated with carrots and root vegetables, and lycopene is associated with tomatoes and tomato products (Al-Delaimy et al., 2005a,b; Pfeiffer et al., 2013). Carotenoids are also scavenger antioxidants that can inactivate singlet oxygen and peroxyl radicals (Stahl and Sies, 2003) (Edge and Truscott, 2018). There is great interest among patients and caregivers in nutritional interventions for MS (Wahls et al., 2021). Nonetheless these carotenoids, which could be sentinel biomarkers in nutritional trials and precision nutrition strategies for MS, have not been investigated as dietary fruit and vegetable biomarkers in MS.

Tocopherols such as α-tocopherol, δ-tocopherol, and γ-tocopherol have vitamin E activity. Tocopherols protect cell membranes from oxidative injury because they incorporate in cell membranes and can reduce peroxyl and other free radicals. Vitamin E deficiencies can cause a range of neurological disorders including peripheral neuropathy, myopathies, and retinopathy (Monsen, 2000).

There is a significant unmet knowledge gap in our understanding of the role/s, if any, of RTC in MS. The working hypothesis was that retinol and pro-vitamin A carotenoids (α-carotene, β-carotene, β-cryptoxanthin) might be associated with low contrast vision in MS. The specific aims of this study were to systematically investigate: i) the inter-dependencies of RTC with each other, with MS disease course, and their changes between baseline and 5-year follow-up, ii) the associations of RTC with neuroaxonal injury and disability in MS, and iii) the associations of retinol, which has a central role in visual function, with low contrast letter acuity in MS patients.