An individual told me in an email that I was wrong about dietary supplements and urged me to read a bag of dog food to get better informed. I get my information from peer-reviewed, randomized, controlled scientific studies, not from dog food bags. I don’t have his permission to share his email, and I don’t want to embarrass him, so I won’t reveal his identity or quote the email in its entirety, but I will try to explain his objections, paraphrasing as much as possible. 

First, let me set the stage. I write a regular column for Skeptical Inquirer magazine under the title “Reality Is the Best Medicine.” In a recent column, I wrote about the science and pseudoscience of aging. The column was published in the January/February 2022 issue and is available on my website at https://www.skepdoc.info/the-science-and-pseudoscience-of-aging/. My critic wrote a letter to the editor, but they declined to print it and instead just forwarded it to me.

In my column, I said: 

“Ray Kurzweil believes that science will soon discover the key to immortality, and if he can just stay alive until then, he believes he will be able to live forever. He optimistically takes 250 supplement pills a day, gets weekly IV infusions, uses acupuncture and Chinese herbs, and does other things that he thinks might help keep him alive. His approach is nothing but hope and speculation.”

My critic objected to the words “nothing but hope and speculation,” calling them uninformed and snarky. He went on to write a paean to supplements. His arguments:

1. Our bodies are composed of seventy minerals.
2. Our soil has been farmed to death.
3. We require supplements to prevent the 900 diseases that dogs don’t get any more because of the supplements in commercial dog food.
4. I should read a dog food bag to learn what supplements are added to dog food.
5. He says it is better to supply the nutrition your body needs than kill yourself with pharmaceuticals and suffer their thousands of side effects.

I had reason to question each of his arguments.

1) Is our body composed of seventy minerals? Not really. Every source I looked at cited a different number, up to 102! A government source says, “A number of minerals are essential for health: calcium, phosphorus, potassium, sodium, chloride, magnesium, iron, zinc, iodine, sulfur, cobalt, copper, fluoride, manganese, and selenium.” Wikipedia says, “The five major minerals in the human body are calcium, phosphorus, potassium, sodium, and magnesium. All of the remaining elements in a human body are called ‘trace elements’. The trace elements that have a specific biochemical function in the human body are sulfur, iron, chlorine, cobalt, copper, zinc, manganese, molybdenum, iodine, and selenium.”

Except for iron (which menstruating women may need because of blood loss), these trace elements are needed only in very tiny amounts. To illustrate, a teaspoon of selenium would satisfy the daily needs of over 90,000 adults. Incidentally, our need for some vitamins is also tiny: our daily B12 requirement is equivalent to 1/67^th the weight of one grain of salt. Sure, supplements can prevent disease in people who are deficient (for instance vitamin C prevents scurvy in people who are not getting enough vitamin C in their diet), but taking supplements of nutrients we aren’t deficient in will do nothing but give us expensive pee. Most people will get all the vitamins and minerals they need by eating a healthy, balanced diet and eating a variety of foods.

And overdoing the supplements has been known to cause harm. Iron is contraindicated for men and post-menopausal women, beta carotene is contraindicated for smokers, and other supplements have been known to harm people in high doses.

2) Our soil has been farmed to death? Nutrient depletion in our soil is almost certainly a myth. See https://soils.wisc.edu/facstaff/barak/poster_gallery/minneapolis2000a/  and https://www.gardenmyths.com/soil-fertility-decreasing/. A point that is often overlooked is that plants won’t grow if soil is depleted of the nutrients they need, and crop yields are increasing.

3) 900 diseases that dogs don’t get any more? What diseases is he talking about? Diabetes? Nope, dogs get diabetes. Cancer? Nope again. Dogs get cancer at the same rate as their owners, and cancer is the leading cause of death in older dogs. What about heart disease, stroke, arthritis, kidney disease? Nope, dogs get all those. Anyway, there’s no evidence that supplements can prevent any of those conditions.

4) Read the dog food bag? Not having a bag of dog food handy, I searched the internet and learned that dog foods commonly contain added multivitamins, minerals, and preservatives. Other additives that are recommended include L-carnitine, DL-methionine, omegas, other antioxidants, probiotics, etc. What I didn’t find were credible scientific studies showing a need for each of these. But that’s really beside the point. Humans are not dogs. For one thing, dogs don’t need vitamin C; unlike humans, they can make their own. For another thing, chocolate is poisonous to dogs but a yummy treat for people. What’s good for dogs may not be good for people, and vice versa. Animal research is only a starting point. It can’t be used to support recommendations for people. Any finding in animals must be confirmed by research on humans. Can my critic possibly believe we would all be healthier if we ate dog food?

5) Better not to kill yourself with pharmaceuticals and suffer thousands of side effects? Sure, some pharmaceuticals have killed people, but you have to weigh that against the lives saved and all the other good they have done for most people. Side effects? Well, anything that has effects is bound to have side effects. “Thousands” is an exaggeration.

Conclusion: now who looks uninformed and snarky? I think Skeptical Inquirer was right not to print his letter to the editor. As Christopher Hitchens said, “That which can be asserted without evidence can be dismissed without evidence.”