Yes, I think diet changes can effect melasma. I happen to think that a copper overload can cause hyperpigmentation. Women especially can be prone to this problem because estrogen has a strong connection with copper. When estrogen goes up in the body so must copper. So any women who have an estrogen dominance, any women who have given birth (pregnancy means high hormone levels including estrogen), any women who use or have usedBC pills or any type of snythetic estrogens, any women who has used a copper IUD or other IUD that contains estrogen hormones, or any women who has too much copper in their body from environmental exposure could be high in copper and low in zinc.
A zinc deficiency is also possible from eating a vegetarian diet and not supplementing zinc. The best sources of zinc are animal proteins.
There are many sources of copper in our environment. Tap water (copper pipes), soda fountain machines (copper tubes), smoking cigs or weed puts copper in the body or depletes if of zinc, drinking booze also depletes zinc and other nutrients, spending a lot of time in a pool or a hot tub can also expose you to copper which is used an an antibacterial in pools and hot tubs, and some veggies are sprayed with copper solutions to help kill can all provide excess copper that most people don't realize.
I strongly recommend that you buy Zinc Status which basically tells you your zinc status or if you have a zinc deficiency. This product works and is sold in most health food stores for about $15. That is the first step to realizing that there are some nutritional steps you can correct which will help your melasma and your health.
When you figure out if you have a zinc deficiency then you need to work with a doctor who knows how to treat nutritional deficiencies. But having too much copper in your body can mean that you have heavy metal toxicity. If you have too much copper then you have too little zinc. That low zinc can make you prone to storing other metals beside just copper that we can all be exposed to everyday, like mercury. Without zinc your body may not be able to excrete the mercury we can all ingest in foods like tuna.