Okay, it's only an animal study, and of course rats aren't humans, but even so. If the rats in the experiment became more muscular and slimmer by eating less-easy-to-digest carbohydrates, we want to know all about it.
Researchers at Harvard Medical School published a remarkable study in The Lancet in 2004, in which they reported on the effects of a low-glycaemic diet on obesity. The diet basically consists of replacing carbohydrates that reach your blood fast as glucose – like sugar, glucose and fructose syrup, potatoes and white bread – with 'slow carbohydrates'. These are found in whole grain products and beans.
The researchers wanted to know whether a diet like this would help people who are on their way to becoming diabetic. So they fattened rats and then surgically removed sixty percent of their insulin producing cells. In the figures below the arrow indicates when the operation took place. After the operation the researchers gave half of their rats a high-glycaemic diet [refined sugar and carbs], and the other half a low-glycaemic [whole grains] diet.
The rats in the low-glyc group were allowed to eat as much as they wanted; the rats in the high-glyc group diet were put on a restricted diet. They put on weight so the researchers restricted their intake by several percent for the duration of the experiment. This way they made sure that both groups maintained approximately the same weight.
Although the rats in the high-glyc group were really on a diet and the rats in the low-glyc diet weren't, the animals in the low-glyc group built up less fat and gained more lean body mass.
The insulin and glucose levels were lower in the low-glyc group, and the adiponectin level was lower too. Adiponectin is a hormone that is secreted by healthy fat cells. It helps muscles to absorb more nutrients from the bloodstream.
Lancet. 2004 Aug 28-Sep 3; 364(9436): 778-85.