Why Eat Only the Egg Yolks?
by Jon Sasmor RCPC (Mineral Guide, MinBalance LLC)
Updated August 2, 2022
Runny soft-boiled egg yolks provide super-dense nutrients! They also may help chelate excess iron!
Egg Nutrition: Yolks, Whites, and Shells
Egg Yolks: A Superfood
Egg yolks contain far more nutrients than egg whites do!
The egg yolk contains a gold mine (pun intended) of many valuable nutrients, including:
- Biotin (B7)
- Many other B vitamins
- Healthy fats
- Cholesterol, a valuable dietary precursor of sex hormones, stress hormones, bile, natural sun-made vitamin D, and more!
Nutrients and Anti-Nutrients in Egg Whites
Egg whites contain protein in significant density, though less dense than yolks. Because egg white makes up more of an egg than yolk does, each egg probably contains more protein in the white than in the yolk.
Egg whites also contain denser amounts of sodium, potassium, magnesium, and niacin than egg yolks. Otherwise, egg whites contain lower to much lower density of all measured nutrients, compared with egg yolks. See table below.
Nimalaratne and Wu (2015) state:
"Egg is a complete biological system designed to nourish and protect the growing embryo from various pathogen invasions. As a result, egg shell with membranes and egg white proteins possess physical and biological defense mechanisms such as viscosity, pH, antimicrobial properties, etc."
The whites help protect from predators as well as pathogens. Those predators include humans!
Avidin, a glycoprotein found in raw egg whites, binds biotin (vitamin B7) and prevents biotin utilization. Though yolks are rich in biotin, the avidin in raw egg whites can block biotin metabolism and produce biotin deficiency. Cooking only partially neutralizes avidin, leaving an ambiguous effect of cooked whole eggs on biotin metabolism. (Masterjohn, 2005.)
Ovoinhibitor, constituting 1.5% of egg white proteins, is a serine proteinase inhibitor that reduces enzymatic digestion by trypsin and chymotrypsin, so certain egg proteins are absorbed intact.
Notably, serine, the same amino acid whose protein digestion is blocked by ovoinhibitor in egg white, also comprises more than 50% of the amino acids in the egg yolk protein phosvitin. (Nimalaratne & Wu, 2015.)
Thus, for predators such as us trying to eat whole eggs, ovoinhibitor in the egg white interferes particularly with digestion of phosvitin in the egg yolk, as well as interfering with protein digestion generally.
Given the benefits of phosvitin (discussed more below), it even may turn out advantageous that ovoinhibitor blocks phosvitin digestion.
Cooking seems to neutralize ovoinhibitor only partially. The degree of neutraliation of ovoinhibitor depends on the cooking temperature and the pH of the egg white. (Nakamura & Matsuda, 1984.)
Egg White Allergies
Most allergies and intolerances to eggs arise from the egg whites. In particular, the ovomucoid egg white protein is the most allergenic egg component. Ovalbumin, the most abundant protein in egg white, also is an important allergen. Other egg white allergens are ovotransferrin and lysozyme. Allergens in egg yolk include α-livetin (chicken serum albumin) and YGP42, which cause reactions more rarely than egg white allergens. (Dhanapala et al., 2015; Vapor et al., 2022.)
If you have allergies or food intolerances to egg, you may experience any sort of symptom after eating eggs, sometimes up to a few days after eating them. However, you may still be able to eat egg yolk alone, carefully separated from the white. The yolks may be easier to separate after soft-boiling the eggs.
Calcium and Copper in Egg Shells
Egg shells (as well as the yolks) provide calcium to the developing chicken embryo (Johnston & Comar, 1955; Vieira, 2007).
Egg shells also can provide a calcium source to those who eat egg shells. Egg shell calcium is in the form of calcium carbonate, which may neutralize stomach acid and impair digestion, if eaten. Some people do eat and enjoy egg shells, by boiling, grinding, and drying the egg shells. (Axe, 2018; Bartter et al., 2018.)
However, egg shells have extremely high calcium content (in the range of 2,000 mg per egg shell). A single egg, averaging 60 grams, contains 10% shell by weight or 6 grams of shell. The shell is mainly (>90%) calcium carbonate, providing around one third of the mass of a single egg shell, or 2 grams, of elemental calcium. (Hunton, 2005.)
It's easy to overdose on calcium and induce a relative magnesium deficiency. For this reason, those focused on improving magnesium status probably shouldn't eat egg shell powder, except maybe in small fractions of an eggshell. (For those who need low-dose calcium supplementation, I recommend bone meal powder, which contains phosphorus and other co-nutrients together with calcium. See Bone and Ocean recipe.)
Surprisingly, egg shells contain a significant portion of the copper in an egg. When eggs were cultured to grow without shells, the embryos:
- failed to accumulate normal amounts of hepatic copper,
- had increased concentrations of hepatic zinc and iron,
- had reduced heart zinc and iron levels,
- and had impaired mobilization of yolk copper, iron, and zinc,
- as well as displaying symptoms that could be indicative of copper deficiency: hyperlipidemia, reduced hepatic lipid accumulation, hyperglycemia, and a diabetic-like syndrome.
Poultry embryos rely on the egg shell as a copper source; and eating a little bit of egg shell powder might provide an unusual copper source for people too.
Nutrient Comparison of Egg Yolks and Whites
The egg white contains many times fewer of most vitamins and minerals, and is a poor energy and nutrient source:
|Comparison of Egg Yolk and Egg White|
|Nutrient||Per 100g Egg Yolks||Per 100g Egg Whites|
|Vitamin A, IU||1440||0|
|Vitamin D, IU||218||0|
|Vitamin K2, mcg||15 - 64||0.4 - 1|
|Vitamin E, mg||2.58||0|
|Lutein + Zeaxanthin, mcg||1090||0|
|Pantothenic Acid, mg||2.99||0.19|
|Vitamin B6, mg||0.35||0.005|
|Biotin, mcg||27 - 73||2.7 - 7.9|
|Vitamin B12, mcg||1.95||0.09|
|Vitamin C, mg||0||0|
|Chromium, mcg||11 - 13||3 - 6|
Sources: Vitamin K2 data from Masterjohn, 2022. Biotin data from Kwon et al., 2019, Staggs et al., 2004, and USDA, 2019a. Chromium data from Bratakos et al., 2002. All other egg white data from USDA, 2019b. All other egg yolk data from USDA, 2019c.
For more analysis of specific nutrients, please see Dr. Chris Masterjohn's article, The Incredible, Edible Egg Yolk.
Note that the crucial fat-soluble Activators — Retinol-A, Semi-Vitamin-D, and K2-MK4-the-Grass-Fed Vitamin — all are richly present in pastured egg yolks, and nearly absent in egg whites.
Egg yolks contain lesser known antioxidants such as lutein and zeaxanthin. Lutein and zeaxanthin protect the eye from light-induced oxidative damage, both by singlet oxygen and radical scavenging, as well as by their ability to absorb blue light before it damages photoreceptor cells. Gastrointestinal digestion of cooked egg yolk has been found to increase antioxidant activity by 5-8 fold. (Nimalaratne & Wu, 2015.)
As can be seen from the table above, an equivalent weight of egg yolks packs far more nutritional value than the same weight of egg whites or whole eggs. This is why to prepare extra eggs, and eat only the yolks, for greater nutritional value.
Egg Yolk Phosvitin's Intriguing Ability to Substitute for Vitamin C
J. Liu et al. (2013) were conducting research with live mouse bone organ cultures. They were able to separate the bone process of osteoblast bone-building from the opposing bone process of osteoclast bone-breakdown (resorption), as follows:
- Osteoblasts require vitamin C to build bone. The vitamin C is needed to build and stabilize collagen to build extracellular matrix and differentiate osteoblasts. By growing the bone organ cultures in the absence of vitamin C, osteoblast bone growth was halted.
- Osteoclasts don't require vitamin C to break down bone. Osteoclast resorption is stimulated by parathyroid hormone (PTH). By providing PTH to the bone organ cultures, osteoclast resorption was stimulated.
- As J. Liu et al. emphasize, live mouse in-vivo research suffers from the flaw that mice, unlike humans, can biosynthesize ascorbate for vitamin C in their livers. Therefore, vitamin C deprivation in diet means something different to a mouse than to a human. J. Liu et al.'s research of ex-vivo live mouse bone organ cultures offered the advantage that, since mouse liver wasn't present, vitamin C biosynthesis wasn't occurring. Therefore, J. Liu et al. were able to deprive the live bone organ culture of ascorbate and halt osteoblast activity.
Amazingly, J. Liu et al. (2013) found that when egg yolk phosvitin was added to the mouse bone organ culture, PTH-stimulated resorption was potently inhibited, and, osteoid and bone growth resumed in the absence of ascorbate! Egg yolk phosvitin was substituting for the function of vitamin C to permit collagen synthesis and formation of new bone!
J. Liu et al. (2013) performed several additional experiments involving the effect of both ascorbate and phosvitin on a synthetic collagen peptide analog and on cultured bone osteoblasts. The results confirmed the capacity of egg yolk phosvitin to stimulate:
- differentiation of osteoblasts,
- collagen synthesis,
- hydroxyproline formation, and
Q. Liu et al. (2017) performed further experiments with cultured mouse osteoblastic cells. They reported that phosvitin and ascorbic acid affected the cultured osteoblastic cells in similar ways:
- both caused a similar rise in alkaline phosphatase (ALP) activity, a marker for the early stage of osteoblast differentiation,
- both led to bone nodule formation, shown by Alizarin red staining and inverted fluorescent microscopy, and
both induced similar mRNA expression of gene markers induced during osteoblast differentiation:
- collagen type I (Col-I),
- osteocalcin (OCN),
- runt-related transcription factor 2 (Runx-2), and
- bone morphogenetic protein (BMP-2).
Thus, Q. Liu et al. (2017) repeated the finding of J. Liu et al. (2013) that egg yolk phosvitin could perform a similar role to vitamin C to permit osteoblast differentiation and mineralization.
You may have noticed in the table of nutrients above that egg contains no measurable vitamin C! Indeed, neither egg yolk nor egg white contains any vitamin C.
Egg yolk is a rich source of almost every nutrient, whereas egg white contains little if any of most nutrients — but still no vitamin C in either part of egg.
How do embryos of birds, fish, and reptiles make collagen to build bones, organs, and connective tissue, in the absence of vitamin C in the egg yolk or white? The reason may be that egg yolk phosvitin replaces vitamin C to perform the same functions in the developing egg embryo. The embryo doesn't need vitamin C to develop bones, organs, and connective tissue.
J. Liu et al. (2013) discovered, and Q. Liu et al. (2017) confirmed, that phosvitin in egg yolk can play the same role as vitamin C to permit collagen biosynthesis! This miraculous function of phosvitin has permitted the survival of certain egg-laying species after their loss of ability to biosynthesize vitamin C, millions of years ago.
Interesting questions follow from the discoveries of J. Liu et al. (2013):
- In addition to collagen and bone formation, what additional functions of vitamin C can phosvitin perform?
- Can phosvitin from dietary egg yolks substitute for vitamin C in our gut, or in the gut microbiome?
- If absorbed intact or partly intact through a leaky gut, can phosvitin substitute for vitamin C in the bloodstream or even throughout the human body?
The Whole Egg or Just the Yolk?
What Egg Yolk Phosvitin and its Digests May Do in the Human Body: Some Hypotheses
Phosvitin from dietary egg yolks may be affected minimally by digestion. Phosvitin resists digestion. Its digested products retain much of the iron-chelating function.
Most phosvitin research has been done in test tubes and with lab animals. The effects of dietary egg yolk phosvitin intake can be extended by hypotheses, not yet proven:
- bind and eliminate excess iron in the gut?
- reduce oxidative stress in the gut by oxidizing iron from ferrous (+2) to ferric (+3) form?
- reduce the activity in the gut of the Fenton reaction involving ferrous iron?
- reduce absorption of unneeded excess iron, both from egg yolks and from foods eaten together with egg yolks?
- sequester iron from undesirable gut biome residents, to prevent overgrowth?
- act as a substitute for vitamin C to permit collagen synthesis in the gut even when vitamin C is lacking?
- liberate stored iron from intestinal cells and mobilize it by oxidizing +2 ferrous iron to +3 ferric iron, which may be transferred to transferrin? (See Osaki, Sexton, Pascual, & Frieden, 1975.)
- in cases of leaky gut (common), where undigested materials such as proteins enter the blood stream from the gut, does phosvitin do all of the above, not only in the gut, but also in the bloodstream and even throughout the entire body?
Recommendations to Benefit from Egg Yolks
Egg yolks contain a gold mine of nutrients! Egg yolks also contain phosvitin, one of Nature's strongest iron chelators!
- Eat 3 to 5 egg yolks per day to benefit from superfood nutrients, to help chelate and remove excess iron, and to protect from the damaging Fenton reaction.
- Egg whites are optional, and may be discarded with the shell. If eating whole eggs including whites, eat 2 to 3 whole eggs per day, and perhaps an additional yolk or two.
- Eat yolks away from fruits, veggies, and other foods high in vitamin C.
- Soft cook eggs (such as soft-boiling) to keep phosvitin and many other nutrients intact, while taking advantage of phosvitin's pathogen-killing abilities in mild heat.
- Enjoy your egg yolks! You are eating a nutrient-dense superfood, while benefitting from the superpowers of one of Nature's most extraordinary proteins, phosvitin.
How to Soft-Cook Egg Yolks
Soft cooking eggs means cooking them lightly enough so that the white usually hardens, but the yolk remains runny. The eggs may be soft-boiled, fried over-easy, or poached.
Soft-cooked egg yolks approach their hardening temperature of 64-70°C, but don't remain hot enough for long enough to harden. (Lersch, 2009; Vega & Mercadé-Prieto, 2011).
Here is a basic recipe for soft-boiled egg yolks. It involves placing eggs in water, heating them, and boiling for 2-3 minutes.
Some who trust their local egg source will eat raw runny egg yolks.
Advanced Soft-Cooking of Egg Yolks: The 6X°C Egg
Advanced chefs may wish to attempt the famous 6X°C egg yolk (Vega & Mercadé-Prieto, 2011). In the precise range between 60 and 70°C, cooked egg yolk texture varies not only with temperature but with cooking time. By holding eggs for the exact correct time at an exact temperature in this range, it's possible to achieve nearly any desired nuance of texture. (Fooladi, 2011; Lersch, 2009, 2011a, & 2011b; Vega & Mercadé-Prieto, 2011.)
Chefs frequently prepare the 6X°C egg yolk at temperatures of 62 to 65°C for 60 minutes or more. A temperature of 62°C for 30 minutes tested sufficient to inactivate bacterial contamination of eggs. (Lopes et. al, 2018.)
For those wanting to try the 6X°C egg at home without the benefit of a thermostat-controlled water bath heater, you can use a giant pot in the oven to maintain the correct temperature (Fooladi, 2011).
Personal Note: My Experience Eating Only the Egg Yolks
To me, egg yolks always seemed the most appealing part of the egg. When eating a runny soft-boiled egg, I'd always eat the yolk first.
In early 2021, I read the research of Kobayashi et al. (2015) about iron absorption from eggs. Then I read Dr. Chris Masterjohn's article, The Incredible, Edible Egg Yolk, about the incredible nutrition in egg yolk.
I switched from eating 3 whole eggs a day to eating 4-5 egg yolks per day. It was really strange at first to discard the egg white, but I quickly came to see the white as akin to the shell, and enjoyed the yolks better than the whole eggs.
I continue to eat 4 soft-boiled egg yolks a day (or more if the eggs are small). My body seems very happy to eat more egg yolks instead of eating the whites.
Upon writing this article, I tried egg whites again. I probably have a mild egg white allergy. If I eat 3 egg whites now, by the second day of eating them, I notice inflammatory symptoms returning. Allergy may be part of why I always preferred the yolks. I was never aware of the allergy when eating whole eggs, but it probably was affecting me.
The "Bad Egg" Myth
Eggs aren't bad! The propoganda campaign against eggs has unnecessarily removed a delicious and affordable superfood from the diet of many people around the world.
Eggs now have been vindicated, and dietary limits of cholesterol removed in many places. You still may not yet have heard that the myth you were taught about eggs growing up has since been debunked!
Starting around 1968, health organizations and governents began suggesting limits on dietary cholesterol and egg consumption, based on incomplete and flawed data. However, by 2015, USDA dietary guidelines finally took into consideration epidemiological information and data from clinical interventions, and eliminated the upper limit for dietary cholesterol. (Fernandez, 2016; McNamara, 2015.)
There's a classic case study example of the adaptive mechanisms involved when one eats cholesterol in food. Someone who compulsively ate 25 soft-boiled eggs per day for 15 years or more, still showed normal blood cholesterol levels and longevity to age 88, without significant atherosclerosis. He had adapted to the enormous dietary cholesterol with (a) lower cholesterol absorption, (b) greatly increased bile acid synthesis, and (c) reduced endogenous cholesterol synthesis. (Kern, 1991.)
Dietary cholesterol from eggs and other sources doesn't raise blood cholesterol for 75% of the population. Even when dietary cholesterol does raise blood cholesterol, HDL may rise together with LDL, resulting in no net increase in health risk. (McNamara, 2015.) Beneficial phospholipids in egg yolks help raise levels of anti-inflammatory HDL cholesterol (Andersen, 2015). LDL cholesterol too, in addition to HDL, may turn out to be protective in older people (Ravnskov et al., 2016; Ravnskov et al., 2018).
As noted above, cholesterol is a valuable dietary precursor of sex hormones, stress hormones, bile, natural sun-made vitamin D, and more! Many people are beginning to consider cholesterol as a beneficial and protective compound, and a key part of healthy brain, hormone, and immune function. (Campbell-McBride, 2008; Kresser, 2019; Masterjohn, 2009; Ravnskov, 2004; Ravnskov et al., 2016; Ravnskov et al., 2018.)
In a study of 4,852 Chinese adults, Sukik et al. (2021) found higher egg consumption associated with better cognition. Those who ate little or no eggs showed greater cognitive decline. The correlation was stronger at lower iron intake levels.
As reflected in the 2015 change in the USA dietary guidelines, there no longer is any need to limit eggs based on dietary cholesterol.
Eat your egg yolks!
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