The human body is designed to function well into old age. But a lack of essential nutrients sets the stage for poor health outcomes, and increases the risk for adverse conditions such as pneumonia.

I have been checking nutrient levels in patients for more than 25 years. What I’ve found is that, sadly, most people suffer from nutritional deficits.

Here are some of the nutrients you can take not only to live a longer and healthier life, but also to protect yourself from illness:

• Vitamin C. This vitamin is considered an essential nutrient because we cannot produce it in our bodies, yet we can’t live without it. Vitamin C is found in a wide variety of foods, including fruits, vegetables, and animal products. Still, most Americans don’t get enough. The body requires more vitamin C during illness or other times of stress. When stress levels rise markedly — such as during an acute infection — it is imperative to increase dosages of vitamin C, because the immune system can’t function optimally without sufficient vitamin C. As a general rule, I tell patients to take 3,000 mg to 5,000 mg per day of vitamin C as a maintenance dose.

• Iodine. White blood cells can’t guard against infection without adequate iodine. No bacterium, virus, parasite, or fungus has ever been shown to be resistant to iodine. Like vitamin C, iodine is an essential nutrient, but more than 97 percent of our population is deficient of iodine. Average doses of iodine I prescribe range from 25 mg to 50 mg per day. During an infection, I often have patients increase their dose to 100 mg per day. I have done this with COVID-19 patients.

• Vitamin A. Most Americans do not get enough vitamin A, another essential nutrient. Vitamin A is crucial for the function of your heart, lungs, kidneys, and other organs. In times of infection, I ask patients to supplement with high doses of emulsified vitamin A in the form of palmitate, which is easier on the human liver than vitamin A that is derived from fish liver. At the first sign of an infection (sore throat, cough, fever) I have patients take 100,000 IU of vitamin A for four days. Note: Beta-carotene is not the same as vitamin A, and does not have its antimicrobial effects.