50:50 sugar/fat mixture

It is easier for the body to use carbohydrates as energy source rather than lipids. When the food is full of fat, the mouse will use fat as a source of energy and only a little part of it is stored. Furthermore, when the food is full of sugar, the mouse will use the sugar as source of energy and as above, only little part of it is stored. When the food ratio is 1:1 sugar and fat, the mouse uses sugar as energy source and stores fat so it gains more weight than the other two.

Salt is not an energy source and cannot be stored in the way sugar and fat is stored.

I have to apologize at first, we had that phenomena in a lecture but I am not a native speaker and some of the processes are maybe wrong translated.

Despite the other answers, I have learned that the primary cause has a chemical reason :

Your adipose tissue is made out of triglyceride. When you eat Triglyceride your body won't use them like this. Your body breaks them down and is synthesizing its own triglyceride to be saved as fat tissue. Triglyceride synthesizing needs glycerol - this is where the phenomena appears : Your body can not use glycerol from food, it has to use its own synthesized glycerol from sugars and amino acids. So only if you consume fats and sugars/proteins simultanously your body can transform it into fat tissue.

Short way of fat when you eat it : Triglyceride > Monoglyceride+2 fat acids > if energy is needed those fat acids will be used for metabolism, if no energy is needed your body wants to save those acids for a later moment by resynthesizing them to triglyceride. Your body needs sugar or amino acids for this process.

It seems worthful to mention that other factors propably play a role like preview answers already mentioned. But the biochemical principle is this one. Another aspect is the efficiancy - it is not mentioned where the amino acids or sugars come from, if they are saved ressources or fresh molecules from food. But if the phenomena doesnt appears - like my professor said - when you seperate those, it seems reasonable to assume that the body won't use stored sugars.

Edit/Sources: I was trying to get some sources due to request in the comments: The problem that appears to me is that the medicine books, where this pathway is described, are not in english at our local library. Since this is a common known pathway in medicine you should be able to find it in every standard biochemistry book for medical students. But I can give you some keywords:

Lipogenese on Wikipedia, in the introduction the pathway is already mentioned https://en.wikipedia.org/wiki/Fatty_acid_synthesis?wprov=sfla1

This is the pathway in form of a picture from a great german publisher for medical books

I have mentioned that aminoacids can also be used for synthesizing glycerol for triglyceride : https://en.m.wikipedia.org/wiki/Glyceroneogenesis?wprov=sfla1

this is an article I havent read but it seems accurate https://www.sciencedirect.com/science/article/pii/S1043276008001379

hope these keywords help :)

1. What was the original research paper?

This one appears close: Hyperphagia in rats produced by a mixture of fat and sugar (PubMed, 1990)

Experimental groups were fed sugar (sucrose), fat (corn oil), or a sugar-fat mixture as an option to chow; options were in the form of water solutions or emulsions. The control group was fed only chow. The sugar-fat group displayed a robust hyperphagia (greater than 36%), relative to the control group; the hyperphagic response was greater than that observed in the fat group but not in the sugar group. The sugar-fat group selected more calories from the option than the other two experimental groups. Body weight gains were also greater in the sugar-fat group than in the fat and sugar groups. Addition of saccharin to the fat emulsion increased fat and total intakes to levels close to those of the sugar-fat mixture. In a second experiment, the relative palatability of the plain and sweet fat emulsions was assessed with two-bottle preference tests. The sugar-fat mixture was preferred to the saccharin-fat mixture, which in turn was preferred to the plain-fat emulsion. These results suggest that the sweetness of the sugar-fat mixture contributed to the pronounced hyperphagia and obesity obtained with this diet option.

2. Have there been any related new results with other mixtures like sugar/salt fat/salt?

A free-choice high-fat high-sugar diet induces changes in arcuate neuropeptide expression that support hyperphagia (PubMed, 2010) (a study in rats)

Our data suggest that the specific combination of saturated fat and a 30% sugar solution results in hyperphagia-induced obesity.

Snack food intake in ad libitum fed rats is triggered by the combination of fat and carbohydrates (PubMed Central, 2014)

Thus, it can be concluded that the combination of fat and carbohydrates is a major molecular determinant of potato chips triggering hedonic hyperphagia.

Fat/carbohydrate ratio but not energy density determines snack food intake and activates brain reward areas (PubMed Central, 2015)

We conclude from our behavioural data that the ratio of fat and carbohydrates, but not the absolute energy density, is the major determinant of the palatability and intake of snack food during short-term two-choice preference tests in rats.

Which Foods May Be Addictive? The Roles of Processing, Fat Content, and Glycemic Load (PubMed Central, 2015) (a human study)

In summary, the current study found that highly processed foods, with added amounts of fat and/or refined carbohydrates (e.g., sugar, white flour), were most likely to be associated with behavioral indicators of addictive-like eating. Additionally, foods with high GL [glycemic load)] were especially related to addictive-like eating problems for individuals endorsing elevated symptoms of “food addiction.”

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In conclusion, it can be either a combination of fats and sweetness (due to sugar or non-sugar sweeteners) or a combination of fats and high glycemic carbs (sugar, white flour, potatoes) that can induce overeating.