The Incredible, Edible Egg Yolk

Ten years ago, someone who cared about their health would likely eat egg whites and throw the yolks away because they were afraid their cholesterol content would cause heart disease. Now that the consensus against cholesterol is crumbling, we are no longer so fearful of egg yolks.

But did you know that egg yolks are actually a superfood jam-packed with nutrients? Egg yolks are rich in a broad array of nutrients that makes them a great multivitamin, but there are three vitamins that they are exceptionally rich in. So let's kick off our discussion by looking at these three.

Protect Your Heart With Vitamin K2

Over the last decade, we have learned about the incredibly important role of vitamin K2 in protecting against heart disease. Most heart attacks and strokes occur because a plaque within an artery ruptures, causing a clot to develop that blocks the blood supply to the heart or brain. Plaques are much more likely to rupture when they accumulate calcium deposits, which makes them fragile. Vitamin K2 is the single most important nutrient that prevents our arteries from accumulating these calcium deposits.

Vitamin K occurs in two forms: vitamin K1 is found in leafy greens, while vitamin K2 is found in animal fats and fermented foods. When we eat leafy greens, the vitamin K1 has an easy time making it to our liver, where we use it to make proteins that regulate blood clotting, but it has a difficult time making it to our blood vessels, where we use it to make a special protein that protects our blood vessels from calcification. When we eat animal fats or fermented foods, the vitamin K2 does a great job reaching our blood vessels, and thereby helps us protect our blood vessels from calcification.

The groundbreaking study that elucidated the importance of egg yolks was The Rotterdam Study. The study followed just under 5,000 people from 1990 to 2000 and found that people who had the highest intakes of vitamin K2 had the lowest risk of arterial calcification and were the least likely to suffer or die from a heart attack. Amazingly, the two most important dietary sources of vitamin K2 were egg yolks and cheese!

When was the last time your doctor told you to protect yourself from heart disease by eating egg yolks and cheese?! If it's never happened, you need to tell your doctor about this study!

It's certainly true that there are better sources of vitamin K2 than egg yolks. In fact, the richest sources of vitamin K2 are goose liver and natto, a fermented soy food from eastern Japan that has a very sticky texture and strong odor and flavor that most westerners find off-putting. The reason the subjects in the Rotterdam Study were getting so much vitamin K2 from egg yolks and cheese was because most people in modern western culture find it easy to eat a lot of these two foods. As a result, most people would find it easiest to improve their vitamin K2 intake in a sustainable way by eating more egg yolks and cheese (notably, cheese is richer in K2 the longer it's been aged).

Look for the Deepest Yellow — or Orange! — You Can Find

Not all egg yolks are created equal. Nothing could make this clearer to the data-driven person than the fact that databases report egg yolks in The Netherlands as having more than twice as much K2 as egg yolks in the United States (32 vs 15 mcg/100 g yolk). One potential explanation for this is that chickens may be more likely to be raised on pasture and thus more likely to eat lots of high-quality grass in The Netherlands.

Thankfully, a pretty good indicator of the K2 content of egg yolk is the color of the yolk. Chickens obtain vitamin K1 from the grass they eat (remember grass is a leafy green!) and convert a portion of it to vitamin K2, which they put into the yolk. The reason K1 is in the grass in the first place is because it plays an essential role in photosynthesis. So does beta-carotene, which imparts a yellow or orange color to the yolk. The more rapidly growing grass that a hen eats, the more beta-carotene will accumulate in the yolk to make it a deeper yellow or even an orange color, and the more K1 will be converted into K2 and deposited in the yolk.

So, look for the egg yolks with the deepest yellow you can find. If you can find orange yolks, all the better!

If you order your groceries delivered or shop at Whole Foods, your best choice is likely to be Vital Farms pasture-raised eggs. If you have access to local pastured eggs, use yolk color as your primary trick to evaluate them. If they have a crisp shell that cracks cleanly instead of crumbling, that's a nice bonus. It means the chickens get enough calcium in their diets.

To read more about vitamin K2 and heart disease, check out my in-depth article on vitamin K2, and my article on fat-soluble vitamins and heart disease.

Or, keep reading and we'll move on to the next of the three nutrients: choline.

Protect Your Liver and Brain With Choline

Choline is the single most important nutrient needed to protect against fatty liver disease. Researchers in the field currently believe that about 70 million Americans have fatty liver disease, most of whom don't know it.

Humans who eat low-choline diets rapidly develop liver problems, unless they are lucky enough to have genetics that favor a low requirement for choline. Experimental animals develop fatty liver disease if they consume large amounts of sugar, alcohol, or fat, but in every single case the fatty liver disease disappears if extra choline is added to the diet. Altogether, the evidence suggests that we have an epidemic of fatty liver disease that egg yolks could play a major role in reversing.

Animal experiments suggest that choline is also among the most powerful weapons we have to protect ourselves from age-related declines in cognitive function. When pregnant rats consume three times their minimal requirement of choline, they give birth to pups that have 30 percent better visual, spatial, and auditory memory across their entire lifespan; they grow old without becoming senile; they have a greater ability to multitask; and they have lower rates of interference memory, which is the type of memory problem you have when you forget where you parked your car because you've parked your car so many times before in different places and the memories are all jumbled together.

Egg yolks are unparalleled in their supply of choline. A 100-gram serving of egg yolk contains 683 mg of choline, which is ten times more choline than even the most nutrient-dense vegetables like broccoli and Brussels sprouts have. Only liver, with 300-400 mg of choline per 100-gram serving, comes close.

If measured per calorie, vegetables would come out on top. If whole eggs are compared to liver, liver would come out on top because egg white does not contain any choline. When it comes down to it, however, most people will have an easier time consuming 3-4 eggs each day than consuming a quarter pound of liver or two pounds of broccoli every day. While I believe most people should consume liver once a week and lots of veggies every day, egg yolks are clearly the best strategy for sustainably boosting your choline intake.

For more information on this topic, check out my article on how to get enough choline.

Or, keep reading and we'll move on to the last of the three nutrients: biotin.

Biotin for Beautiful Hair, Skin, and Nails

Biotin is needed for the proper synthesis of fatty acids that are essential to all of our cellular membranes. Its deficiency produces mental problems such as depression, and defects of the skin, hair, and nails.

As can be seen from this table, liver and egg yolks top the biotin charts. Imagine a breakfast of three eggs, two pieces of whole wheat toast, and an avocado. The eggs would supply 75 micrograms of biotin; the toast would supply 12; the avocado would supply 6. In other words, the eggs would supply 80 percent of the biotin in that meal.

Replace the three eggs with three ounces of chicken or salmon and the total biotin content of the meal would drop from 93 micrograms to 19 or 20. That's a whopping four-fold decrease in biotin content.

Replace the three eggs with three ounces of liver, and the biotin content would remain stable. But how often will you eat liver? I believe in liver so much that I've written a post on how to cook it to make it taste great, but most people cannot and probably should not eat liver every single day. If you eat liver once a week and eggs for breakfast on the other days, liver will be an important source of biotin, but egg yolks will be the overwhelming source of biotin in your entire diet.

Marginal biotin deficiency seems to be the norm during pregnancy, when the need for biotin rises to help synthesize the fats that will make up the body of the developing fetus. Whether biotin status becomes limiting enough even during pregnancy to cause skin problems or depression is unclear, but it does make sense to consider eating more egg yolks if those symptoms develop, whether you are male or female, pregnant, lactating, or just chillin'.

Perhaps nothing could demonstrate the effect on hair (or fur, in this case) more clearly than comparing the two rats in the picture below.

Click the picture to view it on Instagram!

In 1974 Fred Kummerow published a study in the journal Pediatrics wherein rats were fed either on real eggs or Egg Beaters. Both mothers and pups fed Egg Beaters developed diarrhea in one week and the pups died three to four weeks after weaning. When rats fed eggs or Egg Beaters were gently washed, the ones fed Egg Beaters lost chunks of hair.

Egg Beaters later added pantothenate, biotin, vitamin B6, and vitamin B12 to their formula. But this simply helped the product come closer to the nutritional value of whole eggs by adding some nutrients found mostly or exclusively in the yolk. Real, whole eggs had these nutrients all along.

The Nutritional Value of Egg Whites Compared to Egg Yolks

While vitamin K2, choline, and biotin deserve special attention, egg yolks are also rich in a broad array of other nutrients. I would consider egg yolks “Nature's Daily Multivitamin” and liver “Nature's Weekly Multivitamin.”

Let's highlight the broad importance of egg yolks by comparing their nutritional value to that of egg whites.

You could compare whites versus yolks in a few ways. If you open up the NutritionData pages for fresh yolk and fresh white, you can select from the drop down menu in the upper left of each page a 100-gram serving, or one large. Since a large egg contains 33 grams of white but only half as much yolk, you will come to different conclusions depending on which comparison you make.

For example, if you want to know what proportion of nutrients comes from the yolk when eating whole eggs, you would compare one large yolk to one large white. But if you wanted to choose whether you should throw your yolks away or throw your whites away, you could compare 100 grams of one to 100 grams of the other.

For some comparisons, it just doesn't matter: egg yolks contain virtually all of the vitamins A, D, E, K, and B6, and virtually all the choline, biotin, iron, zinc, and essential fatty acids.  The white does not contain 100% of any nutrient.

For the other comparisons, let's assume people are going to eat whole eggs and ask what proportion of a nutrient within an egg is supplied by the yolk.

The yolk contains more than 90% of the calcium, phosphorus, and folate (the natural food form of folic acid), and over 80% of the panthothenic acid. The white does not contain more than 90% of any nutrient, but contains about three-quarters of the potassium, over 80% of the sodium, and 80% of the magnesium.

The selenium, riboflavin, and protein are split more or less evenly between the white and yolk (neither the yolk nor white dominates beyond 60%).

There is no vitamin C to speak of within an egg, and some of the B vitamins that aren't listed above are not very abundant in eggs. Nevertheless, whole eggs are a great source of most nutrients, and it is mostly the yolk that accounts for that broad-based nutritional value.

Egg Whites Contain Anti-Nutrients

The story with biotin is slightly more complicated than told above. Egg whites contain an anti-nutrient known as avidin. It's a glycoprotein, which means that it's a combination of a sugar and a protein. It binds biotin and prevents its absorption, so it effectively acts as an anti-nutrient that antagonizes biotin.

In the experiment where Egg Beaters were compared to whole eggs, part of the problem was likely the fact that Egg Beaters contained avidin. The rats not only weren't getting biotin from the yolk, but they were consuming egg white that was actively antagonizing their biotin status.

Cooking egg white neutralizes avidin, but not completely. Frying destroys two thirds of it, two minutes of boiling destroys 60 percent of it, and poaching only destroys 30 percent of it.

In pregnant rats, a diet of five percent egg white without any accompanying yolk decreases biotin status of the mother by 10 percent, decreases  biotin status of the fetus by 50 percent, and increases the risk of limb and palate defects when the pups are born.

It seems clear that raw egg white on its own is anti-biotin, and becomes less and less antagonistic to biotin as it is cooked more fully. It also seems clear that egg yolks are the most incredible source of biotin on the face of the earth. I believe there is a gray area where it is unclear what happens to biotin status when you mix the two together.

Many people rave about the value of whole, raw eggs. Raw egg whites may have a unique ability — shared only with raw milk — to boost glutathione status (glutathione is the master antioxidant of the cell and the most important molecule that supports detoxification), and when paired with yolks may provide a net biotin boost. Despite abundant speculation on the Internet, the net effect on biotin remains unclear. My suggestion would be to listen to your body and pay close attention to the health of your mind, skin, hair, and nails.

What I Do

Personally, I eat four whole eggs on most days. Lately I have been eating them over-easy, but in the past I have often eaten them in other ways, including scrambled, omelettes, and popping raw egg yolks right into my mouth and swallowing. For more on how I incorporate eggs into my diet, check out my article, What I Eat.

Back to Basics: Enjoy Your Tasty Eggs!

The truth is that most satisfying meals one could make with eggs just don't taste right without both the yolk and the white. Most baked goods come out with a richer taste and a better texture when the yolks are included. But then there is always lemon meringue pie. Food should provide good nutrition — for which inclusion of the yolks is necessary! — but it should also taste good.

Food should be fun. It should be rewarding to cook, delicious to eat, and relaxing to indulge in. Whether you're throwing raw eggs in a smoothie, baking a desert, or scrambling eggs for breakfast, the most important thing is to enjoy your food and pay attention to how your body responds to make sure you are eating mindfully and giving your body what it needs to be healthy.

Egg yolks are nutritional powerhouses with exceptional amounts of vitamin K2, choline, and biotin, and broadly rich in many other nutrients. Knowing this, let's put the science down for a bit and enjoy the incredible, edible egg yolk!

Originally published July, 2005; updated July 2016.

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