Say you have a relatively complete, yet simple meal, e.g., peanut butter and banana on whole wheat, with a glass of milk. And in the interest of frugality, you eat only this all the time. Could this lack of culinary variety harm someone physically?
According to my world food crops professor a person can sustain himself for years on nothing but potatos and beer. However this does come with certain health consequences.
You turn Irish?
Peanut butter, whole wheat bread, a banana, and milk are all good foods and contain many nutrients, but not all of them. You would need to get the other nutrients that they lack or suffer the consequences. They contain many vitamins, but vitamin C is lacking, so you may acquire scurvy. Add an OJ for the C. Many of the minerals that you need in tiny quantities (micronutrients) are also lacking, but I don’t know all of the composition of that diet. You would have to check to see which ones you would need to add.
Yes, it could have health effects, especially from the diet you specified [which falls short of the required level of most vitamins, if you only eat 1500-2000 calories a day] “Vitamin enrichment” of (e.g.) the bread can’t fully counteract that. A limited diet may not make you ill in weeks or months, but it’s not a good idea for years.
FIRST, we do not have a complete catalog of nutrients. Roughly a dozen-odd “major vitamins” and a handful of minerals have an FDA Minimum Daily Requirement. A similar number have a “Recommended Daily Allowance” – i.e. they are necessary, but we can only make a ‘best guess’ as to the requirement. Another couple of dozen are known to be necessary, but don’t have RDA yet.
You’re likely to get some of these trace nutrients “by accident”: we know selenium is necessary in micro amounts because we can see the ill-effects on animals with special feed with all the selenium extracted, but the required amounts are so small that, in The Real World, selenium toxicity is more likely than selenium deficiency-- in fact, this is becoming a problem in some of the “irrigated desert farmland” of southern California (a major agricultural region); the trace selenium from Colorado river irrigation is left behind as the water is sucked into the thirsty air, and accumulates over decades to poison the soil. (accumulated salt is also a problem, but easier to flush away)
How much Selenium do you need? Miniscule amounts - micrograms or less a day. How much is beneficial (e.g. would a little more help many people)? We’re not sure, and it’d still likely be a trace amount. How much are we getting? It varies greatly, but generally decreases as food is processed.
SECOND, There is an ever-increasing catalog of ancillaries (“helper compounds”), often related to, and found in the same source foods as, known nutrients. These help the nutrients to be better utilized or more efficacious in the body. This shouldn’t be surprising: animals evolved eating these co-nutrients together for hundreds of millions of years, and it’d be a little odd if they made an unspoken internal list --vitamin/nonvitamin-- instead of adapting to use them efficiently together, when possible, in the combinations they were usually eaten.
Think about it this way: each living organism has hundreds if not thousands of chemicals in them. Many are simply chains of basic building blocks, but others are small molecules that one organism needs/makes and others don’t. We evolved as omnivores, not just in the sense that we eat meat and vegetable, but in the larger sense that we evolved eating a varied diet (unlike, say koalas eating almost entirely eucalyptus leaves, or pandas that depend on certain species of bamboo) Each living thing we eat brings more chemicals to the party.
THIRD, different people have different needs, which vary over time with environment and circumstance. Any miinimal diet is risky --or at least significantly less than ideal-- for those reasons. The absorption and metabolism of various nutients can vary over time, too (e.g. anemia often doesn’t result from a lack of dietary iron intake, but from a poor absorption, which can sometimes be partly overcome by extra large doses of vitamin B-12)
FOURTH, a varied diet can provide many healthy substances that are not currently known or well characterized – and may not be “nutrients” per se. A nutrient is a substance that sustains life: a source of energy, a biochemical tool for metabolism, or or is a building block for growth and repair. Quite a few health-promoting substances don’t fit these categories, and are not “nutrients”. They may help decrease the risk of cancer or coronary artery disease or defer the onset of other conditions. They aren’t essential, but if you did a controlled study, and the group that didn’t get them died or suffered serious functional impairment in their late 70s instead of their early-mid 80s, I’d call that a “health effect”.
There are a lot of other potential benefits (e.g. exposure to a wide variety of compunds may assist the robustness and tolerance of the immune system, pre-induce enzymes at low levels, so they are needed if you are exposed to toxins etc.) but the bottom line is: medical science doesn’t yet know enough to produce a single simple optimal human diet with only a few ingredients. That wasn’t what we were ‘designed’ to eat. Maybe someday, we’ll have a healthy universal “Purina People Chow”, but I’m not eager for that day
In closing, I should acknowledge that not every chemical we ingest, even in a “organic” or “natural” diet is going to be good for us. Even many international staple foods can actually be harmful if prepared incorrectly:e.g. the root known as manioc, cassava or tapioca must have a toxin squeezed out early in preparation – it is never eaten whole; many raw seeds and beans can be poisonous; many people get sick if they eat fava beans, because they don’t have the enzyme G6PD; the traditional diet of desert south Mexico lacks essential nutrients, unless the corn is ground on stones made of the traditional mineral lime – they even have to grind their corn for their pigs to keep the pigs alive.
Still, on the whole, a reasonably varied diet, prepared in time-tested ways, is the best path through a universe we don’t fully understand