Boron is a trace element that is naturally present in many foods and available as a dietary supplement. It is not classified as an essential nutrient due to insufficient research, however, it may have beneficial effects on reproduction and development, calcium metabolism, bone formation, brain function, insulin and energy substrate metabolism, immunity, and the function of steroid hormones (including vitamin D and estrogen).
The body absorbs about 85–90% of ingested boron, but very little is known about how or where in the gastrointestinal tract absorption actually occurs. Bone, nails, and hair have higher boron levels than other body tissues. Boron is excreted mainly in the urine, and small amounts are excreted in the feces, sweat, breath, and bile.
Plant foods including fruit, tubers, and legumes contain the largest amounts of boron. Main sources in the United States are coffee, milk, apples, dried and cooked beans, and potatoes. Among toddlers, 38% of boron intake comes from fruits and fruit juices and 19% from milk and cheese. For adolescents, milk and cheese products account for 18%–20% of boron intake, whereas beverages, especially instant coffee, represent the highest dietary source of boron for adults.
Boron deficiency signs and symptoms have not been firmly established but limited data suggests that a boron deficiency might affect brain function by reducing mental alertness. In addition, a low-boron diet (0.25 mg boron/2,000 kcal) might elevate urinary calcium and magnesium excretion and lower serum concentrations of estrogen in postmenopausal women. Low boron intakes (0.23 mg boron/2,000 kcal) also appear to reduce plasma calcium and serum 25-hydroxy vitamin D levels and raise serum calcitonin and osteocalcin levels in men and women, which can affect bone mineral density.
Like any other mineral, the amount of boron in plant foods depends on the boron content of the soil and water where they were grown. Areas of the world with limited boron in the soil include Brazil, Japan, and most of the United States, mainly because of high levels of rainfall which leaches boron from the soil. In contrast, arid regions of the world including California and parts of Turkey, Argentina, Chile, Russia, China, and Peru have higher boron concentrations.
The World Health Organization estimates that an “acceptable safe range” of boron intake for adults is 1–13 mg/day, but the most common amounts of elemental boron in dietary supplements range from 0.15 to 6 mg.