If by science you mean policy of NHS? Current scientific consensus is that it is caloric surplus - confounded by a bunch of poorly researched factors. For weig

If by science you mean policy of NHS? Current scientific consensus is that it is caloric surplus - confounded by a bunch of poorly researched factors. For weight gain source does not seem to matter too much. This does not say anything about dietary specifics or how they affect intake and metabolism. Technically nutrition science does not provide strong support for such statements as this carb vs fat one based on interventional studies. Epidemiology alone does not tell you "therefore this change will be beneficial"...