An Apple a Day — Does a Dietary Supplement Keep the Doctor Away?

We’ve all heard the saying, “An apple a day keeps the doctor away.” Most people don’t take it literally. The idea that an apple a day could make a huge difference to your overall health sounds absurd to modern ears, but it was probably more accepted in Victorian England, where fruit was scarce.

Today, our once-daily collection of nutrients is much more sophisticated. Vitamin and mineral supplements are a $50 billion industry, more like an Apple than an apple. Chances are you’ve taken some, as most American adults take at least one dietary supplement. These are often called “micronutrients” to distinguish them from bulk nutrients like sugar and protein.

So what exactly are people taking? The most popular dietary supplement in the United States and most of the world is the multivitamin (MVI). As the name suggests, multivitamins contain a variety of vitamins and minerals, but the amount varies from one multivitamin to another. There are 13 essential vitamins and 16 essential minerals in the human body. Most multivitamins contain most of the essential nutrients, but not all of them. For example, MVIs usually do not contain sodium because our diets are full of salt.

Unfortunately, there is no evidence that taking a daily multivitamin is beneficial for the majority of adults. A 2022 US recommendation found insufficient evidence that multivitamin supplementation is beneficial for the general public, and a new cohort study in 2024 also showed no evidence of benefit. And that makes sense. In this modern age, we are more likely to suffer from eating too much than from eating too little. We are probably already getting all the essential vitamins and minerals we need. That said, multivitamins are cheap and non-toxic, so there is no harm in taking them even if they are not strictly necessary.

Ironically, it’s not much different from eating an apple.

Vitamin supplementation is much more effective during pregnancy. Folic acid (vitamin B9) supplementation significantly reduces the risk of birth defects such as spina bifida and anencephaly, and pregnancy significantly increases a woman’s calcium and iron requirements. Therefore, women who are pregnant or hoping to become pregnant should take a prenatal vitamin that contains folic acid, calcium, vitamin D and iron.

Speaking of Vitamin D, it is the second most popular daily dietary supplement. This vitamin is normally produced by photosynthesis when the skin is exposed to sunlight. People who spend most of their time indoors and those with darker skin are at higher risk of having Vitamin D deficiency. This can lead to bone diseases such as rickets and osteoporosis, and can also increase the risk of diabetes. However, it turns out that knowing exactly how much Vitamin D you need can be very difficult.

Observational studies have shown a correlation between higher vitamin D levels and younger age, a healthy BMI, and physical fitness. However, there is also an apparent reverse causation: younger and healthier people are more likely to spend time outdoors and therefore have higher vitamin D levels. The question of who can improve their health by taking vitamin D remains largely unresolved. Therefore, the Endocrine Society guidelines state that the recommended daily intake of vitamin D is “not known.” As of June 2024, the guidelines state that children under 18, women who are pregnant or planning to become pregnant, and people over 75 should take vitamin D supplements regularly. Non-pregnant adults aged 19-74 do not need to take vitamin D supplements unless they have a medical condition that requires them.

The third most popular supplement is omega-3 fatty acids. These fatty acids are essential nutrients that are abundant in a variety of foods, but are particularly concentrated in fish, shellfish, some seeds/nuts/legumes, and algae. Studies from the 1960s through to the 1980s found a strong correlation between fish consumption and reduced mortality from heart disease, especially in patients who had already had a heart attack.

However, many recent studies have failed to replicate this protective effect. Advances in the treatment of heart disease may have diminished the benefit of dietary supplements. As with multivitamins and vitamin D, the biggest arguments for omega-3 supplements are that they are inexpensive and nontoxic. Thus, even a small benefit may 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 on them all. But I’ve found that the “apple a day” concept isn’t such a bad way to think about nutrition. If a supplement is like an apple — cheap, well-known, and non-toxic — then you can make a case for eating it every day, even if it’s highly unlikely to improve your health.

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