Macronutrients, also known as food, must be consumed in bulk, almost every day, whereas micronutrients are needed only in trace amounts. This was not understood until the first vitamin deficiency came to light in 1747, when James Lind, a Scottish surgeon in the Royal Navy, proved, in a clinical trial, that scurvy could be prevented and treated with citrus fruit. Oranges, lemons, and limes contain some chemical, now known as vitamin C, that we just can't do without. Since then we have identified an assortment of vitamins, vitamin A through vitamin K, along with certain minerals, that are essential for good health, but are only needed in trace amounts.
When you realize that "mineral" is just another word for element, then a mineral deficiency makes perfect sense. If, for some reason, your diet doesn't include enough zinc, your body can't make zinc out of iron. Outside of the center of a star, it is not possible to transmute one element into another. If you're short on zinc, you're short on zinc, and that's the end of it. This can affect every organ system in the body - and nearly a quarter of the world's population is at risk. A slight shortfall might not be recognized as such, since it leads to diarrhea and pneumonia, conditions that are often attributed to something else. Zinc can be added to the soil (where it is taken up by crops), or added to food (fortification), or included in a vitamin / mineral supplement.
Iodine deficiency leads to thyroid disfunction, but this is rare in industrialized nations, where iodine is routinely added to table salt. Remember that salt is sodium chloride, NaCl, and iodine is in the same column as chlorine in the periodic table. Potassium iodide, KI, blends right in and isn't noticed at all. Even if it had a different taste, you still wouldn't notice, because it is added to salt in the tiny proportion of one part per ten thousand. Yes, that's all we need for proper thyroid function, 0.01% of a dusting of salt per day. And so it goes for all of these vitamins and minerals.
Vitamins look like ordinary organic compounds, small molecules of carbon, hydrogen, and oxygen. You might think that we could manufacture these from the food we eat. After all, we don't have to transmute one element into another, just rearrange the carbons and oxygens to build the necessary compounds. But no, we don't have the machinery. These vitamins are already part of our daily diet, so there's no need to synthesize them from scratch.
All well and good, but humans put themselves in strange situations, don't they? The Crusaders of the 13th century, for instance, ate army rations for months, and developed scurvy. In fact scurvy was documented by Hippocrates around 400 B.C, though he had no clue as to the cause or treatment. It is quite rare, on the whole, but it became endemic among sailors, starting in the 15th century, as they bravely embarked on intercontinental voyages lasting months or years. They brought food along of course, enough for the journey, but even well fed seamen developed a strange condition marked by lethargy, spots on the skin, spongy gums, tooth loss, jaundice, convulsions, and even death. Magellan, for example, lost 208 men out of a crew of 230, mostly to scurvy. Other explorers endured similar losses, and nearly 2 million sailors died of scurvy from 1500 to 1800, until the connection to citrus fruit was finally established and accepted by navies around the world. With no refrigeration, a sailor's diet typically consisted of dried grains and salted meats. There were no fruits or vegetables in the mess. In the late 1700's, sailors began to carry fruit juices or other plant extracts aboard ship, without realizing which chemical in the fruit was saving their lives. They sometimes believed any organic acid would suffice, and even dilute sulfuric acid was considered.
In 1932, Charles Glen King, of the University of Pittsburgh, proved that hexuronic acid was the long sought prevention and cure for scurvy. As such, hexuronic acid was renamed ascorbic acid, as in "not scurvy" acid, and later, vitamin C. Indeed it is an organic acid, present in many citrus fruits, though it can also be found in certain vegetables (such as broccoli), parsley, rose hips, and liver. People who eat a well balanced diet ingest plenty of vitamin C. Still, it is the most common nutritional supplement, because health benefits have been ascribed (without clear evidence) to high doses of vitamin C.
Almost every animal on earth is able to manufacture vitamin C from other nutrients. If cats ran the world, they wouldn't call it a vitamin at all. They have eaten meat, and nothing but meat for so long, that they can synthesize ascorbic acid from other compounds. A similar story can be told for most animals. The exceptions to this rule can be counted on two hands: bats, guinea pigs, capybaras, certain fish and birds, monkeys, apes, and of course humans. So what's wrong with us? Humans and their primate cousins have eaten fruits and vegetables for so long, that vitamin C is "assumed" to be present in the diet. In technical terms, evolution has no reason to maintain the enzymes that synthesize vitamin C. In fact we still have the genes, as do guinea pigs and monkeys, but they are broken. Mutations changed these genes in small ways some 60 million years ago, yet those ancestors did not die of scurvy, thanks to the fruit that they ate every day. In contrast, a cat with a broken or missing enzyme might die of scurvy before he could reproduce, and that defective gene would not propagate in the feline population. The mutation stops here. Cats are locked in. In contrast, our vitamin C genes were free to drift out of functionality. But wait, there's more to the story. When vitamin C became scarce once again, humans developed the ability to recycle oxidized vitamin C in the blood, thus making a limited supply last longer. This illustrates a key concept of evolution, natural selection solves a problem by taking a small and somewhat random step that may or may not be optimal. The vitamin C gene is still present in each of us, why not just fix it and restore its original functionality? But that's not what happened. Instead, evolution stumbled upon a secondary mechanism to carry us through modest times of want. This, combined with the fruits and vegetables we normally eat, makes scurvy a non-issue. Problem solved! An intelligent designer would not have established this secondary mechanism; he simply would have given us a working gene from the get-go, as he did for almost all life on earth. He surely wouldn't give us broken genes to carry around in our DNA. What's the point of that? The proof of evolution is not in its magnificent perfection, but rather in its poor design. Stephen J. Gould described many examples in his popular book The Panda's Thumb, which should, in my opinion, be required reading for every high school student.
If your religion, or your adherence to Biblical Truth, makes it difficult for you to accept evolution, then allow me to leave you with two parting thoughts. First, Darwin was a deeply religious man. After all, God can create us using any mechanism he likes, can he not? A thousand years is, to Him, as a day, (2 Peter 3:8), so why not create us via 3.5 billion years of evolution? Second, the Bible can pretty much say anything we want it to say, as you select certain passages and interpret them according to the times and the prevailing view. Consider the following passage from Ecclesiastes chapter 3. It's one of my favorites.
"Man's fate is like that of the animals; the same fate awaits them both: As one dies, so dies the other. All have the same breath; man has no advantage over the animal. … All go to the same place; all come from dust, and to dust all return. Who knows if the spirit of man rises upward and if the spirit of the animal goes down into the earth?"