The “guideline thresholds” have been taken to refer to the recommended or reference nutrient intakes for vitamin B12.
In 1991, the Committee on Medical Aspects of Food Policy (COMA) set the Reference Nutrient Intake (the amount of a nutrient which meets the needs of almost all individuals) for vitamin B12 at 1.5 micrograms vitamin B12 per day for UK adult men and women.
In 2004, the World Health Organisation (WHO) revised its recommended nutrient intakes for vitamin B12 upwards, from 1.0 microgram per day (set in 1988) to 2.4 micrograms per day for adults. The WHO recommendation appears to be largely based on the USA Institute of Medicine's (IOM) Dietary Reference Intakes for vitamin B12 published in 2000. The Institute of Occupational Medicine recommendation was based on a review of evidence some of which was not available to COMA in 1991.
The most recent National Diet and Nutrition Survey (NDNS) of adults aged 19-64 years reported mean daily vitamin B12 intakes exceed both COMA and WHO recommendations at 6.8 micrograms and 5.1 micrograms for men and women respectively.