I read a variety of published information and studies about potential health benefits of boron supplementation and safety of dosage. I found these to be useful summaries:
Excerpt from Walter Last. The Borax Conspiracy — How the Arthritis Cure has been stopped. 2011.
“Boron is distributed throughout the body with the highest concentration in the parathyroid glands, followed by bones and dental enamel. It is essential for healthy bone and joint function, regulating the absorption and metabolism of calcium, magnesium and phosphorus through its influence on the parathyroid glands. With this boron is for the parathyroids what iodine is for the thyroid.
Boron deficiency causes the parathyroids to become overactive, releasing too much parathyroid hormone which raises the blood level of calcium by releasing calcium from bones and teeth. This then leads to osteoarthritis and other forms of arthritis, osteoporosis and tooth decay. With advancing age high blood levels of calcium lead to calcification of soft tissues causing muscle contractions and stiffness; calcification of endocrine glands, especially the pineal gland and the ovaries; arteriosclerosis, kidney stones, and calcification of the kidneys ultimately leading to kidney failure. Boron deficiency combined with magnesium deficiency is especially damaging to the bones and teeth.
Boron affects the metabolism of steroid hormones, and especially of sex hormones. It increases low testosterone levels in men and oestrogen levels in menopausal women. It also has a role in converting vitamin D to its active form, thus increasing calcium uptake and deposition into bone and teeth rather than causing soft tissue to calcify. Also other beneficial effects have been reported such as improvement of heart problems, vision, psoriasis, balance, memory and cognition.”
Excerpt from Lara Pizzorno, MDiv, MA, LMT. Nothing Boring About Boron. IMCJ Integrative Medicine: A Clinician's Journal. 2015 Aug; 14(4): 35–48.
“Boron has been proven to be an important trace mineral because it (1) is essential for the growth and maintenance of bone; (2) greatly improves wound healing; (3) beneficially impacts the body's use of estrogen, testosterone, and vitamin D; (4) boosts magnesium absorption; (5) reduces levels of inflammatory biomarkers, such as hs-CRP and TNF-α; (6) raises levels of antioxidant enzymes, such as SOD, catalase, and glutathione peroxidase; (7) protects against pesticide-induced oxidative stress and heavy-metal toxicity; (8) improves brain electrical activity, cognitive performance, and short-term memory in elders; (9) influences the formation and activity of key biomolecules, such as SAM-e and NAD+; (10) has demonstrated preventive and therapeutic effects in a number of cancers, such as prostate, cervical, and lung cancers and multiple and non-Hodgkin's lymphoma; and (11) may help ameliorate the adverse effects of traditional chemotherapeutic agents.”
Borax is an compound of boron. Borax (sodium borate) is a natural mineral mined from the earth like salt (sodium chloride), both of which have a long history of being used to preserve food. I read that borax and salt are generally considered safe in small amounts and of similar toxicity (LD50) in large amounts.
California is home to one of the world's largest borax deposits from which 20 Mule Team Borax has been manufactured since 1891. The product is packaged in its pure form, with no chemical additives. The only processing it receives after its removal from the ground is washing, drying and packaging. The product is 99.5 percent pure, with the remaining 1/2 of 1 percent containing naturally occurring trace minerals, with no risk of heavy metals. I read that pharmaceutical grade borax is not noticeably purer or better.
I wash my hands after handling borax powder and am careful not to get it in eyes. To make a borax solution, I add one US teaspoon (leveled-off, average 4620 mg on my Weighmax CT20 milligram scale) of borax powder and 16 US fluid ounces of purified water into a cooking pot. I heat on the stove until the solution boils for one minute, and stir until the borax is fully dissolved. I pour the solution into a labeled glass jar with lid and store in a dark location at room temperature.
4620 mg borax powder * 0.113 (11.3% boron by weight) = 522.1 mg boron, divided by 64 (number of 1/2 US tablespoons in 16 US fluid ounces water) = 8 mg boron
A 1/2 US tablespoon of this solution with 8 mg boron is comparable to the estimated 5–8 mg/day boron intake for people in parts of the world such as Israel and Western Australia which have high boron levels in the soil and water (and very low rates of arthritis at 1% or less). This amount is well under the Upper Tolerable Limit (UL) of 20 mg/day boron, determined unlikely to cause any adverse effects for healthy people in long term use.