I had acne for 20 years, and tried everything from benzyl peroxide (Oxy10 type stuff) to antibiotics to Roaccutane. Also spent £££ on skin care products. Some things worked to a certain extent, but was not treating my root cause.
In desperation two years ago I gave a gluten free diet ago, and with in 3 months I was clear for the first time since I was 14. If I have any gluten, with in 2-3 days I am having an outbreak. In fact last week I had some rye bread and 3 days later......one volcanic eruption on my chin and lots of smaller spots. Fortunately for me, dairy is not a trigger. I take my fish oils and eat lots of veg and fruit, and healthy fats.
For years, I was told by GPs and dermatologists that food does not affect acne, however recent research is showing that specifically, dairy and high glycaemic index foods (sugar, white bread, white rice, fruit juice etc) can affect acne. There is little research on any link between acne and gluten per se, however, recent research showing 1 in 5 have gluten sensitivity. Gluten sensitivity can manifest in problem skin and other undiagnosable health 'niggles'.
Everyone is different with what causes their acne. Like Caroline Hirons says, it is usually a mix of hormones, inflammation and skin care. Diet influences hormones and inflammation. For many people an overhaul of their diet may not be worth it if you just have a few spots that don't bother you that much, but acne can be soul destroying for some. By the time my patients get to me (I'm a dietitian) they are usually really down about their skin, and changing what they eat/drink not only improves their skin, but their over all well being too.