An Apple a Day — Do Nutritional Supplements Keep the Doctor Away?

We’ve all heard the saying – an apple a day keeps the doctor away. Most of us don’t take it literally. The idea that one apple a day will make a big difference to your overall health sounds silly to modern ears, thought it likely played better in fruit-starved Victorian England.

Nowadays we have a far more sophisticated collection of one-a-day nutrients. Vitamin and mineral supplements are a $50 billion industry. They’re more like Apple the technology company than apple the fruit. Most American adults take at least one dietary supplement, so chances are you have personal experience with a nutritional supplement of some kind. These are often called “micronutrients” to distinguish them from bulk nutrients like sugar or protein.

So, what exactly are people taking? The humble multivitamin (MVI) is the most popular nutritional supplement in the U.S. and most of the world. As their name implies, multivitamins contain a broad spectrum of vitamins and minerals, which may vary from one multivitamin to another. There are 13 essential vitamins and 16 essential minerals in the human body. Most multivitamins contain a majority of the essential nutrients, but not every single nutrient. For example, MVIs don’t typically include sodium as there’s plenty of salt elsewhere in the diet.

Unfortunately, there is no evidence that daily multivitamins are beneficial to the vast majority of adults. A U.S. recommendation from 2022 found insufficient evidence that multivitamin supplementation benefits the general public, and a new cohort study from 2024 again showed no evidence of benefit. And that makes sense. In this modern era, we struggle with eating too much rather than too little food. Chances are we’re already eating all of the essential vitamins and minerals that we need. That said, multivitamins are cheap and nontoxic, so you won’t hurt yourself by taking one even if it’s not strictly necessary.

Ironically, that’s not too different from eating an apple.

Vitamin supplementation is far more beneficial if you’re pregnant. Folic acid (vitamin B9) supplementation greatly decreases the risk of birth defects such as spina bifida and anencephaly, and pregnancy greatly increases a woman’s calcium and iron requirements. Thus, women who are pregnant or trying to become pregnant should take a prenatal vitamin that includes folic acid, calcium, vitamin D, and iron.

Speaking of Vitamin D, that’s the second-most-popular daily nutritional supplement. This vitamin is normally produced by photosynthesis when our skin is exposed to sunlight. People who spend most of their time indoors, and people with dark skin, are at increased risk of Vitamin D deficiency. This may cause skeletal diseases like rickets and osteoporosis, and may also increase the risk of diabetes. However, it turns out that it is very difficult to tell just how much Vitamin D you need.

Observational studies have shown a correlation between high Vitamin D levels and younger age, healthy BMI, and physical fitness. But there’s obviously some reverse causation here. People who are younger and healthier to begin with are more likely to spend time outdoors, thus boosting their Vitamin D levels. The question of who can improve their health by taking Vitamin D remains largely unsolved, leading to an Endocrine Society guideline suggesting “we don’t know the correct daily allowance” for Vitamin D recommendations. As of June 2024, guidelines say that children age 18 and below, women who are pregnant or trying to become pregnant, and people over age 75 should take routine Vitamin D supplements. Non-pregnant adults age 19-74 do not need to do so, unless they have a medical condition that requires Vitamin D supplementation.

The third-most-popular supplement is omega-3 fatty acids. These fatty acids are an essential nutrient that are present in a vast number of different foods, but occur at especially high concentrations in fish, shellfish, some seeds/nuts/beans, and algae. Studies from the 1960s-’80s showed a strong correlation between fish consumption and decreased death from heart disease, especially in patients who already had a heart attack.

However, many newer studies have failed to replicate this protective effect. It is likely that improvements in medical treatment of heart disease have decreased the magnitude of benefit from dietary supplementation. Like multivitamins and Vitamin D, the strongest argument in favor of omega-3 supplementation is that it is cheap and nontoxic. Thus, even a small benefit could be worth it for many people, especially those who already have heart disease.

There are hundreds of other micronutrient products out there so I can’t go into detail with all of them. But it turns out that the concept of an “apple a day” is not so bad of a way to think about your nutrition. When a supplement resembles an apple – cheap, well-understood, and harmless – then you can make an argument for eating it daily even if there’s only a very small chance that it improves your health.

If a supplement is more expensive, newer, or less well studied, and has a higher rate of side effects, then you should demand a higher level of evidence before putting it in your body.