We close this series where the whole subject of vitamins began historically — with deficiency. It was by studying the diseases caused by missing vitamins that scientists discovered them in the first place. Today, severe deficiencies are far less common than they once were, but milder shortfalls are still surprisingly widespread, and certain groups of people are more at risk than others. Knowing who is vulnerable, and what to watch for, is the practical payoff of everything we have covered.

What a Vitamin Deficiency Looks Like

Because each vitamin has its own jobs, a shortage tends to show up in predictable ways. Some classic signs include:

• Persistent tiredness and weakness — often linked to low B vitamins, including B12, or to anemia from low folate or B12.
• Frequent infections or slow healing — associated with low vitamin C, A, or D.
• Bone pain or muscle weakness — a hallmark of low vitamin D.
• Numbness or tingling — a warning sign of low vitamin B12.
• Dry skin, brittle nails, or hair changes — sometimes reflecting low vitamin A, C, or biotin.

These signs are clues, not diagnoses — many have other causes — but persistent symptoms are worth discussing with a doctor, who can confirm a deficiency with a simple blood test.

Who Is Most at Risk

Some people are considerably more likely to run short of one or more vitamins. The main groups include:

• Vegans and strict vegetarians: chiefly at risk of vitamin B12, which comes almost entirely from animal foods.
• Older adults: who often absorb B12 less well, get less sun for vitamin D, and may eat less overall.
• Pregnant and breastfeeding women: who have higher needs, especially for folate and vitamin D.
• People with limited sun exposure or darker skin: more prone to low vitamin D.
• People with digestive conditions: such as coeliac or Crohn’s disease, which impair absorption.
• Those with very restricted diets or heavy alcohol use: at risk across several vitamins, particularly the B group.

The Vitamins Most Worth Watching

In practice, a few specific vitamins account for most real-world shortfalls in otherwise well-fed populations. Vitamin D is probably the most widespread, because sunlight rather than food is its main source. Vitamin B12 is the key concern for plant-based eaters and many older adults. Folate matters most around pregnancy. If you fall into a risk group for any of these, they are the ones worth paying particular attention to — with food first, and a targeted supplement where appropriate.

Prevention Is Simple for Most People

The reassuring conclusion of this whole series is that protecting yourself from vitamin deficiency is, for most people, refreshingly straightforward:

• Eat a varied, colorful, whole-food diet — the single best protection, covering nearly every vitamin.
• Include foods from all groups: fruits, vegetables, whole grains, legumes, and, if you eat them, animal foods.
• Pay attention if you are in a higher-risk group, and use targeted supplements such as B12 or vitamin D where they are genuinely needed.
• See a doctor about persistent symptoms rather than guessing, so any deficiency can be properly identified and corrected.

And that brings our vitamin series to a close. From the big picture in Part 1, through all thirteen essential vitamins, to cooking, immunity, supplements, and now deficiencies, one message has run through every part: vitamins are essential in small amounts, they work best as a team, and a varied, whole-food diet remains the surest way to give your body everything it needs to turn food into a healthy life.

This article is intended as general nutritional information and is not a substitute for personalized advice from a doctor or registered dietitian. If you suspect a deficiency, please consult a healthcare professional.