Diet determines someone's body weight. If you don't eat, you don't gain. Once you gain it's incredibly hard to lose. Once you lose it's incredibly easy to gain.

I have lost 21 pounds in 2 months from calorie counting. I gained 60 pounds in 3 years by ordering a ton of delivery. I yoyo'd many times in the past 2 years trying to lose the weight using various programs before finding a routine that works for me. All weight gain was due to me making choices that increased calorie intake. No one forced me to eat anything, I did it to myself.

Genetic factors may make weight loss more difficult but it is never impossible. There is no possible way that if you do not eat you will gain weight. Unless you have a handful of extremely rare conditions it's going to be the things you choose to eat and how much of them you eat that determine weight gain, alongside exercise. Some people swear that they do everything right with calorie counting but still gain weight but the reality is that if the average XXX pound person needs 1800 calories to maintain and you eat 1600 and still gain, perhaps your unique body really needs 1400 or 1200 to lose. Or maybe you're blessed with a great metabolism and 2500 calories will be burnt for you so you can eat up to that without any issues.

Mental health challenges make it hard to stick with a routine diet for some but it's still technically possible to lose weight even if your brain won't let you stop eating. If someone gave you an appropriate balanced diet and there were no other possible source of calories for you, you're gonna lose weight. You might go crazy because you can't have dressing on your salad and perhaps even wish you were dead instead of eating less, but you would lose weight.

Beyond weight

In terms of height, diet can influence height somewhat (especially in terms of deficits causing stunted growth) but your maximum height is almost entirely due to genetics. You get very little choice in that. You can't choose to drink more milk and gain a foot of height. Once your body stops producing the right growth hormone you just stop getting taller.

In terms of skin color, there's very little you can do about that beyond genetics. You can paint your body with fake tan or sit in the sun to get darker or hide indoors out of sunlight to be "lighter" than if you went outdoors but there's no way to will your body to stop producing melanin.

In terms of "cultural behaviors" that you seem to call race based on how you describe it, you can choose to behave like someone from any walk of life if you learn how to do so. You can learn other languages, social norms, customs and beliefs and act on them. You are not born any more "white", "italian", "irish", "kenyan" or "thai" culturally than anybody else. You learn the local culture from your experiences. Switching between the cultures has nothing to do with genetics. We're just most likely to be born into cultural groups that have the same genetics as us, the exceptions tend to be when you're adopted or your parents happen to already be a different culture than their genetic heritage's or your parents have different cultures.

I can't believe I had to write any of this out. I'll probably get downvoted because no one wants to hear the absolute truth of whatever thing they can't do that they want to blame something else to feel better for their lack of discipline, but we can choose to behave however we really want to. I don't give a shit how fat or skinny someone is, I care about the content of their character, their beliefs and their actions. I'm not going to point this shit out in polite company because it's not my business and I believe everyone should be able to choose to do to their bodies and their lives however they wish. Doesn't change the facts though.