Well if you have a ton of weight to lose - you wouldn't increase your fat. It's useful to, if you need to put on weight though.
A well balanced diet is what you need to achieve whoever you are - whether D or not - and moderation in all things except laughter!
People do have terrible trouble with portion sizes and don't realise for starters that the protein element of any meal should not exceed the size of your average flip-top 20 fag packet. Sadly with the vilification of the cigarette, people who know what that size actually is automatically, are literally dying out! Haven't spoken to a dietician for years and years - whatever are they using as a size comparison these days? (always used to be the fag packet cos absolutely everybody knew that size)
Anyway, small fillet steak size. So even if you fry the steak in butter - it won't actually have all that much on it because it isn't huge. Plus fat is very useful if you aren't on insulin, to slow down the action of 'too fast' carbs in a meal. (People on fast acting insulin before meals have great difficulty with slow-acting carbs, since the insulin often hits their bloodstream quicker than the carbs do, sending them hypo - then hours after a massive spike in BG. Pasta + fat (eg masses of cheese and/or cream) does this for me - the spike from the pasta happens 5 or 6 hours after I eat it!)