Which vitamin is specifically noted as lacking in vegetarian diets?

Vitamin D is noted as commonly lacking in vegetarian diets due to limited dietary sources. While some vegetarian foods contain vitamin D, such as fortified plant-based milks and certain mushrooms treated with UV light, these sources may not always provide adequate amounts to meet daily requirements, especially for those who do not have sufficient sun exposure. Sunlight is a significant natural source of vitamin D, and individuals who primarily follow a vegetarian lifestyle might spend less time outdoors or may live in regions with limited sunlight, contributing to a deficiency.

In contrast, vitamin A is found in various plant foods, particularly in the form of beta-carotene, which is abundant in colorful fruits and vegetables. Vitamin C is widely present in fruits and vegetables consumed in vegetarian diets. Vitamin K can be readily obtained from leafy greens and other vegetables, making its deficiency less common among vegetarians. Thus, vitamin D stands out as the nutrient that is particularly challenging to obtain in adequate amounts in vegetarian diets.