A couple of years ago I saw a BBC Horizon television documentary about sugar and fat. One section mentioned three experiments in which rats were given their ordinary rat food plus and unlimited supply of:
Experiment A. Unlimited sugar
Experiment B. Unlimited fat
Experiment C. Unlimited sugar&fat mixed together (50:50)
The stated results were that in experiment A the rats did not put on any weight, in experiment B the rats put on weight but not much and in experiment C they put on loads of weight and would eat the 50:50 mixture to the exclusion of their ordinary food. I also remember the documentary said the 50:50 ratio was critical and if the ratio was shifted too far in either direction then the weight gain effect reduced sharply.
I have two questions. 1. What was the original research paper. and 2. Have there been any related new results with other mixtures like sugar/salt fat/salt?
EDIT: I have just found this related paper. I don't know if its the exact one relied upon by the documentary, but it's certainly close... perhaps the documentary employed the results from multiple papers.
EDIT: You can see the whole documentary here: https://www.dailymotion.com/video/x1arpze - the rat experiment discussion begins around 46 minutes in.