A few thoughts about carbohydrates and fat gain.
The enzymatic pathway for converting dietary carbohydrate to fat is called de novo lipogenesis (DNL). Contrary to popular wisdom, carbs don't get stored as fat as easily as people like Gary Taubes, Tim Noakes et al. would lead you to believe. These low-carb advocates tend to use rodent models to support their notions that carbohydrates are uniquely fattening, with their explanations usually involving the hormone insulin. While there are situations in humans when large quantitative amounts of DNL occurs, these situations are rare and tend to involve unrealistic conditions, particularly for those looking to reduce body fat (the very people who are most concerned with such issues in the first place).
For example, one scenario in which a significant increase in DNL would occur is during chronic overfeeding of carbohydrates. By this I mean 700-900g per day for several days. Another scenario in which DNL would occur to any meaningful degree would be during an exceptionally low-fat diet (<10% total calories); the same diets many (not all!) vegans and people stuck in the 80s try to promote.
For anything in between, DNL is relatively minor, especially during an energy deficit in which glycogen stores are never likely to be saturated. This doesn't mean that carbs CAN'T make you fat; they CAN if you eat too many, just predominantly in an indirect manner.
To let you understand what I mean, let me run you through a quick example. Let's say we have two identical twins with a maintenance caloric requirement of 2500 kcal. They both consume a 3000 kcal diet consisting of 200g of protein (800 kcal). One twin splits the remaining 2200kcal between 75g of fat and 380g of carbs (twin 1), whereas the other goes for 155g of fat and 150g of carbs (twin 2). Both are in a caloric surplus of 500kcal and are likely to gain almost identical amounts of fat. Twin 1 is storing less dietary fat on a daily basis compared with twin 2. However, they twin 1 is also using less fat for energy due to their higher carbohydrate intake. Conversely, twin two is storing a greater amount of dietary fat on a daily basis but is also burning more of it because they are consuming less carbohydrate.
In other words, when you eat more carbohydrate, you oxidise more carbohydrate for energy and therefore oxidise less fat. When you eat less carbohydrate, you burn less carbohydrate and more fat. However, if your calorie intake is identical, you also eat more fat so things cancel each other out. Therefore, carbs WILL make you fat if you overeat, but they do so indirectly, by blunting fat oxidation, rather than being directly converted to fat per se (the way many carb-phobic gurus would like you to believe).
So, after taking protein into account, it all comes down to total calories in the end; who'd have thought?
De novo lipogenesis in humans: metabolic and regulatory aspects. - PubMed - NCBI