If one asks the doctor what causes SD, he will likely tell that no one knows, and give you some topical steroids. This is not a solution but will only weaken your skin.
If you search on the internet, you will mostly find "ITS THE MALASSEZIA FUNGUS ON YOUR FACE/SCALP, KILL IT KILL IT KILL IT" and suggestions on how to get rid of this skin fungus, with often exotic creams and oils which will get you broke. It's true that malassezia is a link in what causes SD, but killing it is not a solution either, because this is a natural inhabitant of the skin, and it will just get back in a few days.
So lets follow the link backward from the flare-up all the way down the the stomach:
1) The malassezia feed on saturated fatty acids from your skin. The leftovers are unsaturated fatty acids and some toxins, which causes an immune response, ie. SD flareups. This is why people with autoimmune diseases such as HIV or parkinson is generally affected by serious SD.
2) There are two major things that will increase the amount of saturated fatty acids in your skin. Its dairy products, which will increase sebum production in general and also causes pimples and zits. The other thing is a dysfunction in the delta-6-desaturase function in the body. What delta-6-desaturase (simplifed) does, is that it convert saturated fatty acids to unsaturated fatty acids before it enter your pores. See study at https://www.ncbi.nlm.nih.gov/pubmed/12713571.
3) The delta-6-desaturase activity is dependant of magnesium, zink and some B-vitamins, mostly B6. The zink and B6 avabillity for enzyme activity such as delta-6-desaturase will be low as long as there are circulating acetaldehyde in your body. Acetaldehyde is whats makes you hungover after some alcohol consumption, so its a breakdown product of alcohol (This is why alcoholics often have SD symptoms). It is also a leftover when some kinds of gut fungus such as candida has done its dirty work. Also, some tobacco products and coffee contains small amounts of acetaldehyde. This is unfortunate because nicotine makes the acetaldehyde even more toxic (https://www.ncbi.nlm.nih.gov/pubmed/15496937).
4) Gut fungus exists in most people but some have more, some less. There are simple ways of lowering the amount of gut fungus. Not eating sugar is NOT a way (which is often stated), beacuse if the fungus is starved from sugars, it can feed from protein, and this is even worse. Go to any low-carber forum, this is where you will find people having major problems with candida. Its literally growing throug their intesines, feeding on tissue.
So, what can we do practically about the SD? Its a complicated topic so there is no guarantee this will work for everyone but for me it did:
1) No diary product, absolutely no milk. Possibly a few cheese doodles now and then

2) Make sure you are not D-vitamin deficient (for immune system), preferably beeing in the sun if possible. Malassezia hates UV light as a bonus beeing in the sun.
3) Avoid alcohol, tobacco and coffee.
4) Make sure your zink, magnesium and B-vitamin levels are good.
5) Eat cold pressed cocnut oil. Candida hates it. (http://www.medicalnewstoday.com/articles/302809.php). Eat 2-3 tbsp a day, but begin _slowly_ with a peanut sized lump a day working your way up to the reccomended level. This is because you can get rally sick from the fungus dieoff if to much at once, this is called the Jarisch–Herxheimer reaction. (https://en.wikipedia.org/wiki/Jarisch%E ... r_reaction)
6) You can use some special schampoos as a facial wash (leaving it on the facial skin for 2-3 minutes) to get rid of the mallasezia. This is not a long term solution but a panic-fix which will heal the face in 3-4 days. The SD will most likely return in 1-2 weeks unless the above points are fixed. For me, Selsun works very well. Some other swears by schampoos containing ketoconazole.
Hope this can solve someones skin-troubles!