Few jobs in the body are as important as what the immune system does to protect you from sickness- and disease-causing agents. Yet medical research shows that "immunocompetence" - the readiness of your body to cope with invaders - often starts to decline by age 35 to 40.
Keeping up with the nutritional requirements of the immune system is hard work. Nutrients named after the whole first section of the alphabet - vitamins A, B6, B12, C, D, E - as well as folic acid, iron, zinc, copper, and selenium - are all required to maintain the production of millions of immune cells on a daily basis.
Even a slight deficiency can tip the balance of the entire system, leaving the body vulnerable to infection. And most people's diets are short of one or more important immune system micronutrients.
There is one nutrient-dense superfood you could consider, though: beef liver.
Chris Kresser, licensed acupuncturist and author of the popular Healthy Skeptic blog, is among many health professionals starting to educate people on the value of eating liver on a regular basis.
"When you look at the actual nutrient content of vegetables and fruits and other plant foods versus beef liver," says Mr. Kresser by phone from his office in Berkeley, Calif., "you'll see that beef liver in particular - but other kinds of liver, as well - has orders of magnitude more of each micronutrient in almost every case."
Gram-for-gram, for example, cooked liver has six times the iron of kale. Liver is also one of the best food sources of vitamin B12, containing almost 30 times the amount in cooked ground beef. Famously high in vitamin A - and a good source of folic acid, zinc, copper, and selenium - liver has the potential to be a powerful booster of the immune system.
Mr. Kresser also points out the economic argument for eating liver.
"A pound of grass-fed beef liver will give you five weeks worth of super nutrient content," he says. The total cost of that bundle of micronutrients at your average grocery store? About $5.
When Mr. Kresser recommends that people eat three to four ounces of organic, grass-fed liver every week, he says the difficult part is overcoming some common misconceptions about the food.
"Often when I tell people to eat liver, they say, 'That's gross! That's like eating the toxic dump of the body!'" he laughs. "The liver does process toxins, that's true ... but the liver doesn't store toxins. Toxins are stored in the fat tissue."
The other common worry he hears is about vitamin A toxicity.
"There's a lot of concern about over-consuming vitamin A, and since liver is so high in vitamin A, people think they should avoid it," he says. "[But]vitamin A is only toxic in the presence of vitamin D and vitamin K2 deficiency."
Mr. Kresser cites a study showing that having adequate amounts of vitamin D in the body significantly raises the threshold for vitamin A toxicity. So as long as you get your "sunshine vitamin," you'd have a hard time consuming enough liver to achieve a negative health effect.
As for ensuring you have enough vitamin K2, dietary sources include several dairy products, meats, and fermented foods: everything from butter to probiotic sauerkraut. A small amount of vitamin K2 is also present in the liver itself.
In case you thought you could eschew liver for an equivalent cocktail of vitamin supplements, consider the scientific evidence that says liver's health benefits are hard to match in other sources. For example, the vitamin A in a meal of beef liver is metabolized differently than the equivalent amount taken as a supplement. This means that supplements bring a greater risk of vitamin A overconsumption - perhaps because they fail to capitalize on the ways all the different micronutrients in liver interact with each other.
"I'm not a huge fan of taking a shopping bag full of vitamins and minerals," says Mr. Kresser. "I prefer to get nutrients from food."
And what if the thought of liver makes you recall those miserable meals as a kid, when you sat pushing around an unappetizing pile of liver-and-onions on your plate?
Mr. Kresser suggests consuming it in disguise - by mixing a tiny bit of chopped liver in with ground muscle meat, such as hamburger. His blog readers have chimed in with their own suggestions, including a tasty purée of sautéed liver, bacon, apples, cinnamon, and sugar.
The truly resistant, he says, can seek out freeze-dried liver capsules, which preserve most of the nutritional value of the original fresh product.
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