This recent manuscript nearly slid beneath the radar… almost stopped reading at the abstract until the word “nifedipine” appeared (among its widely pleiotropic effects, nifedipine also lowers insulin).
The series of experiments described below demonstrate one aspect of the scientific method reasonably well. None of the individual experiments, when viewed in isolation, really prove the hypothesis. But the researchers tested it with a variety of widely different methods and all of the results went in the same direction. The hypothesis in question: insulin causes fat gain, and hyperinsulinemia per se, not macronutrients or calories, is the root cause.
This group has previously shown that sucrose is more detrimental than fish oil is beneficial toward obesity and glycemic control.
High glycemic index carbohydrates abrogate the anti-obesity effect of fish oil in mice (Hao et al., 2012 AJP)
Divide and conquer
Mouse study. Lots of diets, in brief:
Pair fed: high fish oil (180 g/kg) plus 13%, 23%, 33%, and 43% sucrose (by weight, switched out for casein [a poor choice IMO])
High fish oil (180 g/kg) plus sucrose, fructose, glucose, low GI carbs, and high GI carbs.
That’s a lot of diets. Kudos.
As expected, higher sugar and lower protein intakes enhance weight gain (yes, even when pair-fed similar calories [i.e., a calorie is not a calorie]) and this is at least partly due to reduced metabolic rate (as per the poor man’s energy expenditure test- measuring body weight before and after 24 hours starvation [higher weight loss = higher metabolic rate]):
High sucrose-fed mice also had more inflamed adipose tissue and less thermogenic brown fat, which likely contributed to their glycemic dysregulation and elevated adiposity.
Sucrose is comprised of glucose and fructose, so to determine which component was causing obesity, they fed mice high fish oil diets plus either sucrose, glucose, or fructose. Interestingly, the glucose group gained as much weight as the sucrose group. Since the fructose group gained the least amount of weight, the researchers attributed the sucrose-induced obesity to insulin! (fructose doesn’t elicit an insulin response; and insulin levels were lowest in the fructose group).
Body weight, plasma insulin, and glucose tolerance:
I. Thus far: glucose and sucrose cause obesity by stimulating insulin secretion. Glycemic deterioration is worst in the glucose-fed group because they were consuming most of the most insulinogenic sugar: glucose. It was lower in the sucrose and fructose groups because sucrose contains only half as much glucose as pure glucose, and fructose contains no glucose. IOW, these data suggest hyperinsulinemia per se causes obesity and insulin resistance. Gravitas.
They further tested this by comparing high and low GI diets which cause higher and lower insulin levels, respectively. As expected, the low GI diet led to less weight gain, and significantly lower insulin levels and adipose tissue accumulation compared to the high GI diet:
II. Thus far: high insulin levels, whether induced by glucose, sucrose, or high GI starch, lead to obesity.
They next took a non-dietary approach by artificially increasing insulin levels with glybenclamide in fish oil-fed mice to see if hyperinsulinemia could still cause obesity. The results weren’t robust, but the higher insulin levels tended to increase adiposity even in mice fed the anti-obesogenic fish oil diet. 
In the experiment, the opposite approach was taken: nifedipine was used to lower insulin. The use of octreotide and diazoxide has been used in a similar context with similar results in humans, discussed HERE and HERE.
Again, the results were not robust, but when viewed collectively a picture begins to emerge: raising insulin levels, whether it is with a high glucose or sucrose diet, a high GI diet, or glybenclamide increases adipose tissue growth; and conversely, lowering insulin levels, whether it is with a less insulinogenic sugar diet (fructose), a low GI diet, or nifedipine decreases adipose tissue growth. Oh yeah, and low carb works too.