Depends how nutritionally diverse the one food source is. Some, typically called “superfoods” contain a wide range of nutrients that allow them to cover a lot of bases. Others are so nutritionally poor that all they’re really providing is calories.
It also depends on the nutrients missing. Some deficiencies manifest much faster than others. Compare for example vitamin C deficiency, which is scurvy, to vitamin D deficiency, which is fragile bones (something you can go never knowing you have if you just never happen to fall down the stairs).
Another thing to consider is that we’re all living with a bunch of adverse side effects of being alive already. Just because you experience them doesn’t mean you necessarily realise that’s what you’re experiencing or how to fix it.
It’s possible to survive on a single food as long as it has enough micro-nutrients (vitamins) and microminerals (iron, maganese, zinc, etc). Otherwise, yes, there will be adverse side effects.
It also depends on genetics, for example [the Inuit](https://www.sciencedaily.com/releases/2015/09/150917160034.htm) can survive eating only meat because of genetic adaptations.
Adverse side effects, like death and being really sick and in bad health is a major issue. We don’t see this much in modern times in developed countries, but in human history (and even in poor areas now) malnutrition is a major issue. Living off a single food source, or food sources poor in necessary vitamins or such can cause massive issues and death, we’re built to get a lot of stuff in our diet and need it to survive. If you’re not able to get a proper diet, things aren’t gonna go well over time.
You could live off a single source, if that contained everything you need. There are many “meal replacement” products on the market that you can live off fine, and people do (its basically a drink packed with vitamins).
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