That seems to be the go to cause of quite a few problems. Oh, you have acne? It's the spicy food. Oh, the corners of your mouth are dry and cracking? It's the spicy food. They probably don't know what causes the stuff and since spicy hurts your mouth, maybe they think it can cause bad things to happen...? I mean, do you really expect a logically sound reason?

Yeah...some sort of Chinese medicine-based belief I think. "Your body is too hot" is what I hear most often. My best friend here got a big weird ulcer on her neck awhile back and then had some swelling for a couple weeks and her father is a doctor. His advice on the matter? "No more spicy food until it goes away."

My theory? She'd been wearing a cheap necklace her friend gave her for her birthday every day. Exactly where the pendant lay was where the ulcer was, and my friend already has sensitive skin. But when I shared that thought with her she didn't believe me...though she no longer wears that necklace.

On the other hand, though, when I first arrived here I got these big weird zits all over my neck (more like boils actually), which lasted a week or two and then went away.

I don't believe that at all.

Considering when I was in the UK I ate Phall curries regularly and even now in China I have very spicy Chilli sauce (homemade) with every dish of rice (at home), I should have acne on top of my zits if that were true! Whereas in reality I have a fairly clear complexion. 

I would say It most assuredly is not true, although I too have been told it is the cause of everything from gout, to diarrhoea, to the common cold!

I don't think spiciness alone causes acne. Just like when people say, chocolate and peanuts can make acne worse.

The severity of your acne can depend on a lot of things, and I think your diet is usually a big factor. And don't forget stress as another big factor.

Also depends on the person. Some people are just lucky they don't get acne at all!

It is not the spiciness. It is just that some foods does cause your body to heat up and may result in pimple over your body. For instance take an example of cashew nuts. Over consumption of them can cause you pimples.
You may find this interesting home remedies to get rid of acne.

What about the food you eat spices WITH? Who's to say it isn't the oils within the (mostly salty and hearty) foods that contribute to acne? I seriously doubt it's been properly tested. Chinese pass on these voodoo wisdoms, and they are believed or disbelieved absolutely. They are rarely considered or questioned. People will either believe them and scoff at those who don't, or disbelieve them and scoff at those who do. I was advised not to eat pork, chocolate or peanuts when I was 15, and my acne did seem to recede after I altered my diet. I maintained that diet for 14 years until I moved to China permanently, but I'd never state with absolute certainty that avoiding those 3 foods reduces acne or benefits health. I'm eating them all now, and besides a temporarily upset stomach from being unaccustomed to pork, there seem to be no clear health issues. I love spicy food, and I rarely get any pimples. But I am 31 now.

In TCM, edibles and beverage are separated in two categories : the "hot" ones and the rest. A better translation for this "hot" (上火) would be "inflammatory". Inflammatory food is believed to lower your overall body strength and make you more vulnerable. They include : everything spicy, everything fried or deep-fried, some bakeries, coffee, orange juice (probably for their acidity), pineapples and durians.

About the spicy food, they are not tagged inflammatory in every part of China. The provinces that are well known for eating spicy food do not consider them as such. Sichuan, for example.

I usually do not believe much in TCM and superstitions, but in this case, I'll have to make a case for it : I do have pimples popping all over my body every single time I do eat spicy now I live in Guangdong, and this never happened to me when I lived in Sichuan.

I think its the humidity. Its no coincidence tha in the dry north they almost never talk about hot foods, many have no idea about it at all. And the spicy food places are in sothern china. Guangdong dont generally eat spicy foods but they believe it more than anyone. Guangdong sichuan hunan chongqing. They are humid as hell. Soaking in sweat all day is never good for the skin.

The rest i think is a mixture of okd dumb grandmas passing the nonesense down and large pharmaceutical companies who we all know are the most ethical, pushing the idea. No spicy food in Guangdong? well your still screwed , cuz everything you like will make your body hot and make you an ugly horrible person. But to help drink this! (王老吉， wang lao ji rivals any drink around, which isnt a drink company at all but a pharmaceutical one) plus the pills, herbal teas and all other industries that profit from this dumb idea of ying and yang for foods.