Back acne for the last six years?

So I get back acne (mainly lower near the waistline and middle of back, typically along my spine and on the side of it. Anyone know what this could be? When I clean my diet up, it seems to alleviate it a bit, but then it's like it will magically reappear if there's no acne. The clearness lasts maybe a day or two at most, but then acne starts right back up. It's the weirdest thing I've ever seen. Why would it start again as soon as it goes away. I don't eat vegetables or fruit really. I eat like 10 things and that's it. Could acne be caused by a nutrient deficiency? Is that possible? Is it possible that going out in the sun for 15 minutes twice a week can cause acne? I'm not really sure what's going on with my body, but it's definitely not normal to have body acne. I've seen obese people who don't even have any acne at all, so it's definitely an indicator of a serious health issue in my opinion, especially when someone with six pack abs and a sculpted body has it. I try to stay away from dairy, recently I've picked up eating yogurt once a day. When I used to eat a lot of pizza, my entire back from neck to lower waistline was covered in giant pus-filled acne spots, it was pretty severe. But since I cut back on pizza to once a month and replaced it with oatmeal and vegan protein, that has made a 90% difference, but I still can't get rid of the mid back acne and lower waistline acne. Any ideas? I know people have recommended soaps with salicyclic acid, but that doesn't work on me