In plain terms: Boron has been tested in human trials aimed at the relevant outcome and did not show benefit at the doses and durations studied. The animal research may be encouraging, but it has not yet translated to humans in these trials.
Boron
The evidence as the record states it
Controlled trials, all null: Ferrando and Green (Int J Sport Nutr, 1993; PMID 8508192) and the same 19-subject trial reported again by Green and Ferrando (Environ Health Perspect, 1994; PMID 7889885), boron 2.5 mg/day for 7 weeks in male bodybuilders, no effect on testosterone, lean mass or strength (both groups improved with training); Meacham et al. (Environ Health Perspect, 1994; PMID 7889886), 3 mg/day for 10 months in 28 college women, bone density change not improved by boron; Beattie and Peace (Br J Nutr, 1993; PMID 8329361), postmenopausal women in a metabolic unit, a low-boron diet then 3 mg/day, no effect on minerals, sex steroids or bone-turnover markers. The claim behind the supplement: Nielsen et al. (FASEB J, 1987; PMID 3678698), a non-randomized metabolic-unit study of 12 postmenopausal women in which 3 mg/day after a low-boron diet raised serum estradiol and testosterone; context, not grade evidence. Rondanelli et al. (J Trace Elem Med Biol, 2020; PMID 32540741): narrative review of 11 human supplementation studies concluding 3 mg/day supports bone health; its conclusion rests largely on non-randomized metabolic studies, and two of its authors work in the research unit of a commercial botanical-ingredient company. Grade D, Route 2: the controlled trials found no effect, and no trial reports a clinical outcome.
Trials showing no benefit
Low. Small controlled trial (PubMed types it as a clinical trial, not a randomized one); null.
Low. Small controlled trial (PubMed types it as a clinical trial, not a randomized one); no boron effect.
Low. Small crossover-style metabolic study; no effect on any measure; the low-boron diet appeared to induce calcium hyperabsorption, which may have obscured an effect.
Why Grade D Route 2 and not Grade C
Grade D Route 2 applies when a compound has been tested in humans and benefit was not shown. Boron has been tested in 3 human trials on endpoints relevant to the claim being evaluated, and the primary endpoint was not met.
A randomized, placebo-controlled trial pre-specifying a meaningful clinical or functional endpoint and demonstrating statistically significant benefit would move this to Grade C or higher, depending on effect size, population size, and whether it is independently replicated.
What this grade covers and what it does not
What would move this grade
A randomized, placebo-controlled trial pre-specifying a clinical aging endpoint and demonstrating statistically significant benefit on that primary endpoint would be sufficient to move this to Grade C or higher, depending on effect size and population.