Broster tested positive for the anabolic agent clenbuterol following a test at Macclesfield in July 2014.
The player had blamed eating Biltong for the positive test, but the Rugby Football Union found no basis for reducing the sanction.
The ban, which has been in place since August 2014, will continue through to 15 August, 2016.
UKAD's director of operations, Pat Myhill, said athletes "are responsible for how and why a prohibited substance is found in their system."
Myhill said in a statement: "Meat is an area of particular risk with clenbuterol as a contaminant, having caused adverse analytical findings in the past."
"However, proving meat contamination is not straightforward, not least because the meat that has been contaminated will, in most cases, have been eaten.
"Finding evidence to support this defence can therefore be difficult."
Rugby union player Sam Broster has been suspended from all sport for two years by UK Anti-Doping (UKAD).