Ofcom said that, according to the Loughborough University website, Dr Reyner received a research grant of £52,962 from Red Bull in 2002.
As well as extolling the virtues of Red Bull, the programme included disparaging remarks about coffee such as 'you can't tell how much caffeine is in it' and 'the instant stuff from the machine ... is horrible'.
Ofcom said the item 'gave the distinct impression that the programme had come under external commercial influence'.
Channel 4 - which is paying the fine - admitted the show had breached the broadcasting code.
'Despite its best efforts, some viewers clearly had received the impression of external commercial influence on the editorial of the programme,' said a spokesman.
Ofcom said the incident ' demonstrated uncharacteristically poor judgment (by Channel 4) resulting in what appeared to be, at the very least, a loss of editorial control.'