Sunlight exposure is probably the most effective natural source of vitamin D. You just go outside and find the sun; that’s all you need to do. But, there are many dietary sources for taking vitamin D. Vitamin D is doing holistic vital functions: bone, immunity, heart and, etc. Here are the top 3 categories for vitamin D.

Fish
Salmon, sardines, mackerel, and herring are a good sources of vitamin D. 100 g serving produces about 600 International Units of vitamin D. This approach is the daily recommended intake for adults. Cod also produces vitamin D, especially cod liver oil. Tuna in can with oil has plenty of vitamin D; 1 can of canned tuna generally contains over 200 IU of vitamin D. 

Farm Food
Foods from pharm have much vitamin D. For example, egg yolks have some vitamin D, and 1 large egg yolk contains 41 IU of vitamin D. And beef liver is also a good vitamin D supplement, and 100 g of it is 60 IU. Milk and yogurt have some, and in the countries with not enough sunlight exposure, fortify these foods with vitamin D. 

Fungi(Mushroom)
Mushrooms provide vitamin D a lot. Especially shiitake and maitake mushrooms when they were exposed to sunlight or UV while they were growing.

Vitamin D-rich foods are so simple and easy to get. And Vitamin D is fat-soluble, which means it stays in the body for a long time. With proper sunlight exposure, diet is so effective in increasing the level of vitamin D in the body. Even there are many supplements and medicines for uptakes. 

Mushrooms
What I want to say is, let’s go deep inside of mushrooms because it has many other good nutrients for us, especially for the elderly. Vitamin D is useful for bone health, helping calcium to be absorbed in the bones and making them dense. Vitamin D is involved in muscle to get strength, which is important for the aged. Mushrooms, always useful for medical uses, are great vegetables for diabetes. For those who with diabetes, mushroom us super great food. Vitamin D regulates insulin and glucose metabolism. Mushrooms have a low glycemic burden, lots of fiber, and pharmaceutical functioning components. It has been proven that the potential glycemic response of foods was significantly lowered by including mushrooms (4). For dinner, try mushrooms such as chestnut mushrooms and oyster mushrooms.
As medicinal plants, mushrooms not only have fibers but are rich in polysaccharides, phenolics, and alkaloids that provide antidiabetic, antioxidant, and antihyperlipidemic effects (5). Mushroom polysaccharides act as prebiotics and help regulate microflora in the gut. This helps reduce insulin resistance. Also, in edible mushrooms, beta-glucan shows a great effect on diabetes and obesity by modulating either cellular function or biochemical pathways (6). Beta-glucan inhibits insulin secretion by delaying sugar absorption in the body. it is also effective in preventing cardiovascular disease, one of the commonly expected complications of diabetes. It also prevents blood vessels from narrowing or blocking.

Back to the starting point, vitamin D is crucial to maintaining healthy bones, the immune system, and many other biological functions. And there are so many superfoods for it. I think mushrooms with rich vitamin D are useful for many reasons, including bone health, diabetes, and immunity, which are all required for the elderly.

References
4) Margaret A. Brennan†, Emma Derbyshire†, Brijesh K. Tiwari†, and Charles S. Brennan*†§ Enrichment of Extruded Snack Products with Coproducts from Chestnut Mushroom (Agrocybe aegerita) Production: Interactions between Dietary Fiber, Physicochemical Characteristics, and Glycemic Load. J. Agric. Food Chem. 2012, 60, 17, 4396–4401
5) Rubiya Khursheed, Sachin Kumar Singh, Sheetu Wadhwa, Monica Gulati, Ankit Awasthi. Therapeutic potential of mushrooms in diabetes mellitus: Role of polysaccharides. International Journal of Biological Macromolecules. Volume 164, 1 December 2020, Pages 1194-1205
6) Sushil K. Dubey, Vivek K. Chaturvedi, Divya Mishra, Anand Bajpeyee, Aprajita Tiwari & M. P. Singh . Role of edible mushroom as a potent therapeutics for the diabetes and obesity. 3 Biotech. 2019, 9, 450
7) Alexander Little, Kevin Murphy and Patrick Solverson. Quinoa’s Potential to Enhance Dietary Management of Obesity and Type-2 Diabetes: A Review of the Current Evidence. Diabetology 2021, 2, 77–94

3 F for vitamin d intake.

Fish
Salmon, sardines, mackerel, and herring are a good sources of vitamin D. 100 g serving produces about 600 International Units of vitamin D. This approach is the daily recommended intake for adults. Cod also produces vitamin D, especially cod liver oil. Tuna in can with oil has plenty of vitamin D.

Farm Food
Foods from pharm have much vitamin D. 1 large egg yolk contains 41 IU of vitamin D. Milk and yogurt have some. Try vitamin d fortified foods of these.

Fungi
Mushrooms provide vitamin D a lot. Especially shiitake and maitake mushrooms when they were exposed to sunlight or UV while they were growing.

And here are..