Vitamin B12 comes almost entirely from animal sources, so if you eat little or no animal food, this is the one to watch. Vegetarians and vegans are the most likely to run low, along with older adults, whose bodies absorb it less efficiently with age.
Your body uses B12 to make red blood cells, keep your nervous system healthy, and turn food into energy. When intake or absorption stays low over time, that's associated with tiredness, weakness, and a kind of mental fog, and over longer periods it can contribute to nerve-related symptoms like tingling in the hands and feet. Because your liver stores B12 well, a shortfall can build for a long time before anything shows.
US national survey data (NHANES) suggests around 12.5% of people have insufficient levels, a meaningful gap even where animal foods are common. Adults need around 2.4 mcg a day, from meat, fish, eggs, and dairy, plus fortified foods like some plant milks and cereals, which matter most if you avoid animal products. There's no established upper limit, since your body clears what it doesn't use, though that's not a reason to take more than you need.