The short version
Manganese helps run a short list of essential enzymes, the kind involved in building bone, metabolizing carbohydrates and cholesterol, and clearing reactive oxygen species.
Primary source 01 / 06
Manganese is a cofactor for many enzymes, including manganese superoxide dismutase, arginase, and pyruvate carboxylase
It also plays a supporting role in blood clotting alongside vitamin K, though the daily requirement is tiny, measured in single milligrams rather than the gram-scale needs of a macronutrient.
Almost entirely a plant-food story
NIH's own food table draws a sharp line. Whole grains, nuts, legumes, rice, leafy vegetables, coffee, tea and spices carry manganese; ordinary servings of chicken, beef, eggs and milk register at zero on that same table.
Primary source 02 / 06
The top sources of manganese in the diets of U.S. adults are grain products, tea, and vegetables.
The table also lists shellfish, mussels and oysters especially, as unusually concentrated sources, but for anyone not eating those, grain products and tea already carry the load. One caveat tempers the picture a little.
Primary source 03 / 06
Humans absorb only about 1% to 5% of dietary manganese
Only a thin slice of what's on the plate is actually absorbed, and the rate rises when intake runs low and falls when intake runs high, which is the body's own way of holding levels steady no matter what a given meal contains.
Deficiency has essentially never been documented
This is one of the cleanest non-issues in nutrition.
Primary source 04 / 06
Manganese deficiency is very rare in humans, and signs and symptoms of deficiency have not been firmly established
NIH's own survey of at-risk groups came up with nobody worth naming.
Primary source 05 / 06
No known groups of people are likely to have inadequate manganese intakes.
The real caution runs the other way, and it isn't dietary
Manganese's genuine safety story is about exposure, not intake.
Primary source 06 / 06
No evidence shows manganese toxicity from high dietary manganese intakes
The toxicity cases on record trace to welding fumes, mining dust and drinking water with unusually high manganese levels, none of which a shift toward plant foods changes one way or the other.