Why Eat Only the Egg Yolks?

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:

• Retinol-Vitamin-A
• Semi-Vitamin-D
• Vitamin-K2-MK4-The-Grass-Fed-Vitamin
• Choline
• Biotin (B7)
• Many other B vitamins
• Healthy fats
• Minerals
• 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.

(Richards, 1997.)

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:

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 Whites Promote More Iron Absorption than Egg Yolks

Egg yolks contain much more iron than egg whites. (See table of nutrients above.) However, egg yolks contain a special protein called phosvitin that binds iron tightly and protects from excess iron. (Ishikawa et al., 2007.)

In contrast, egg whites promote iron absorption and increased iron body load. Egg whites themselves don't contain much iron or lead to iron absorption. Instead, egg whites support relatively increased absorption of iron from other foods eaten together with egg whites.

In rat studies by Kim et al. (1995), intestinal iron solubility and apparent iron absorption increased according to dietary protein source: egg yolk < soybean < egg albumin [from egg white] < casein < pork.

Pork protein diet had highest intestinal iron solubility (42%) and iron absorption (42%). By comparison, egg yolk protein diet had lowest intestinal iron solubility (17%) and lowest iron absorption (13%). Egg yolk protein diet also resulted in lowest plasma concentration of iron in the portal vein shortly after eating, which indicated lower absorption of iron from egg yolk meal.

Rats eating egg yolk protein showed lower iron content in liver, spleen, and kidney compared with rats eating other protein sources. Though egg yolk protein resulted in lowest organ iron content in all measured instances, only the following were statistically significant:

• Lower liver iron content with egg yolk protein diet compared with casein or pork protein diet.
• Lower spleen iron content with egg yolk protein diet compared with pork protein diet.

Additional rat studies by Kobayashi et al. (2015) showed similar results about how egg yolk protein diet opposes iron absorption.

Kobayashi et al. found that rats' iron levels (including liver iron content) increased less with egg yolk-supplemented casein diet compared with egg white, ovalbumin from egg white, or casein-only diets containing matched amounts of iron. In contrast, egg white or ovalbumin led to even greater liver iron content than yolk-supplemented casein or casein alone.

In addition to a lower hepatic (liver) iron content, the egg yolk-supplemented diet group of rats showed other indicators of lower saturation of iron-absorption mechanisms:

• slower recovery of hemoglobin, hematocrit, serum iron, and transferrin saturation after a period of extreme iron deprivation,
• lower hepatic hepcidin mRNA expression,
• higher DMT1 mRNA in small intestine, and
• higher ferroportin mRNA in small intestine.

Some but not all of the above changes were statistically significant in various comparisons with other groups. Please see Kobayashi et al. (2015) for specifics.

The yolk-eating rats continued to upregulate iron absorption because iron absorption wasn't yet saturated by the yolk-supplemented diet.

Like Kim et al. (1995), Kobayashi et al. (2015) similarly found that egg yolk phosvitin may have combined with iron in egg yolk and/or other dietary iron to form insoluble complexes in the small intestine. These insoluble iron complexes may have been excreted without being absorbed and used. Therefore, yolk-eating rats had lower iron measurements including lower hepatic iron content.

Kobayashi et al. also found that ovalbumin in egg white may form a soluble complex with dietary iron that promotes iron absorption. Therefore, rats eating egg white protein or isolated ovalbumin had higher iron measurements.

Kobayashi et al. concluded (boldface emphasis added):

our findings suggest that phosvitin, contained in egg yolk protein, likely reduces the bioavailability of dietary iron. Further research is required to investigate the role of egg yolk phosvitin in the recovery of IDA [Iron Deficiency Anemia] and its possible usefulness in iron removal therapy. . . . [E]gg yolk protein may be useful in iron removal therapy without the need for medicine and may potentially be used in the treatment of diseases such as haemochromatosis, which require control of iron absorption.

My interest in egg yolks arose when I read Kobayashi et al. (2015). Then I realized egg yolks' powerful value as a natural food-based anti-excess-iron agent.

If you're mineral balancing with Root Cause Protocol, you probably have found you have too much iron which is causing you oxidative stress. (For more on why we don't want excess iron, see Iron Deficiency or Iron Recycling Glitch? and Dietary Iron Restriction.)

Due to their content of the potent iron chelator phosvitin, egg yolks are an outstanding food to help avoid adding more excess iron.

"Egg Yolk Phosvitin Inhibits Hydroxyl Radical Formation from the Fenton Reaction"

Ishikawa, et al. (2004) published an article whose title is worth repeating: "Egg Yolk Phosvitin Inhibits Hydroxyl Radical Formation from the Fenton Reaction".

Ishikawa, et al. (2004) compared egg yolk phosvitin with 16 metal-binding proteins, iron chelators, and hydroxyl radical scavengers.

Hydroxyl radical formation was compared in two ways:

• By ESR spin trapping, phosvitin was more effective (56%) than all other agents in reducing hydroxyl radical formation, except certain hydroxyl radical scavengers: mannitol was equally effective (55%) and thiourea was more effective (73%).
• By deoxyribose degradation, phosvitin was by far the most effective in blocking hydroxyl radical mediated degradation of deoxyribose. Phosvitin provided 93% protection; whereas apo-ferritin provided 60% protection, apo-lactoferrin provided 57% protection; thiourea provided 51% protection; and all other agents provided even less protection.

The tryptic (enzymatic) digest of phosvitin also was compared; it retained almost all of phosvitin's ability to protect against hydroxyl radical formation.

Since phosvitin's antioxidant effect depends on iron concentration, Ishikawa et al. concluded that "phosvitin acts as an antioxidant by chelating iron ions."

By accelerating oxidation of ferrous (+2) to ferric (+3) iron, egg yolk phosvitin reduces the availability of ferrous iron to participate in the Fenton reaction and cause oxidative stress.

Indeed, Ishikawa et al. (2004) confirmed by plasmid DNA strand breakage assay that phosvitin protected DNA from oxidative damage when exposed to ferrous iron and hydrogen peroxide.

Ishikawa et al. (2005) further found that phosvitin also protects from oxidative damage to skin by iron ions in the presence of ultraviolet light.

Special Properties of Phosvitin

• Phosvitin is found in egg yolks of birds, fish, and reptiles (J. Liu, et al., 2013).
• Phosvitin comprises around 7% of hen egg yolk proteins (Mecham & Olcott, 1949).
• In hen eggs, more than 50% of the amino acid residues (components) of phosvitin are the single amino acid serine (Byrne et al., 1984; Mecham & Olcott, 1949).
• The serines are almost fully phosphorylated (bound to phosphorus) (Goulas et al., 1996). Phosvitin contains 10% phosphorus (Mecham & Olcott, 1949).
• "Phosvitin" was named "phos-" for its high phosoporus content and "-vitin" for its source in the egg (Mecham & Olcott, 1949).
• Phosvitin provides one of Nature's best metal chelators! Phosvitin especially excels at chelating iron! (Greengard et al., 1964; Lu & Baker, 1986; Taborsky, 1963).
• Each phosvitin molecule can inhibit the oxidant activity of 30 ferrous (Fe⁺²) iron ions (Lu & Baker, 1986).
• Each phosvitin molecule can bind around 60 ferric (Fe⁺³) iron ions — one ferric iron ion per pair of phosphoserine residues, with 123 total phosphoserine residue components (Byrne et al., 1984; Castellani et al., 2004; Hegenauer et al., 1979; Taborsky, 1963; Webb et al., 1973).
• Phosvitin contains all or almost all of the iron in egg yolk, tightly bound (Albright et al., 1984; Greengard et al., 1964; Ishikawa, et al., 2004).
• In the presence of oxygen, phosvitin accelerates oxidation of iron from the ferrous +2 state to its safer ferric +3 state (Ishikawa, et al., 2004; Osaki, Sexton, Pascual, & Frieden, 1975; Taborsky, 1963).
• Phosvitin can hold much more iron than that contained in yolk. It may uptake iron from other sources if eaten together with egg yolk. (Björn-Rasmussen et al., 1972; Ishikawa, et al., 2004.)
• Phosvitin holds much less copper than other metals (Lu & Baker, 1986, 1987; Samaraweera et al., 2013). Phosvitin seems to grasp iron tightly and leave copper alone — exactly the right properties to chelate and remove excess iron.
• Phosvitin has been proposed to help the developing chick embryo mobilize and transfer iron (Osaki, Sexton, Pascual, & Frieden, 1975), as well as protecting the chick embryo from iron-mediated oxidative stress (Ishikawa, et al., 2004).
• Phosvitin assists with "nutritional immunity", sequestering nutrients from pathogens and cancer cells. Phosvitin shows anti-bacterial (Sattar Khan et al., 2000), anti-biofilm (Kim et al., 2020), and anti-cancer-cell (Moon et al., 2014) properties. By chelating excess damaging iron, phosvitin helps in countless ways!

Phosvitin Holds Iron Tightly

Phosvitin resists digestion by stomach acid and intestinal enzymes. The core of phosvitin remains intact after digestion, and retains its metal-chelating abilities. Phosvitin and its digested phosphopeptides form insoluble complexes with iron that may be excreted in feces. (Goulas et al., 1996; Ishikawa et al., 2004; Ishikawa et al., 2007; Kim et al., 1995; Kobayashi et al., 2015.)

Phosvitin begins to change structure under heat around 70°C (Chung & Ferrier, 1995; Dixon & Cotterill, 1981). This is an important reason to soft-cook eggs, so they don't reach hotter temperatures that would denature phosvitin.

Egg yolks tend to solidify around 64-70°C (Lersch, 2009; Vega & Mercadé-Prieto, 2011), so look for runny egg yolks to keep phosvitin intact. Phosvitin kills gram-negative bacteria E. coli at 50°C, so light cooking is enough to activate phosvitin's antibacterial properties. (Sattar Khan et al., 2000.)

Pasteurization at 61.1°C for four minutes didn't change antioxidant activities of phosvitin; but autoclaving at 121.1°C for ten minutes decreased the amount of antioxidant activity against ferrous iron by around two-thirds (Lu & Baker, 1986). Phosvitin retained its iron-binding ability after heating to 90°C for 60 minutes, but showed some changes to its structure. (Castellani et al., 2004.)

Phosvitin-bound iron, once bound, may not be easily released. Phosvitin retains previously bound iron even at temperatures up to 110°C for 20 or 40 minutes (Albright et al., 1984), though phosvitin seems to lose some of its functional abilities at such high temperature.

Vitamin C releases the iron from phosvitin (Morris & Greene, 1972). For this reason, vitamin C-rich foods should be eaten separately from egg yolks to reduce iron burden.

Interestingly, methylene blue seems to act similarly to vitamin C, releasing the iron from phosvitin (Morris & Greene, 1972).

Strong chemical chelators such as EDTA or o-phenanthroline release iron from phosvitin. However, phosvitin is a stronger iron binder than many other chelating agents, such as citrate and nitrilotriacetate. (Albright et al., 1984; Castellani et al., 2004; Hegenauer et al., 1979.)

How Egg Yolk Phosvitin Protects You from Iron

Due to its stellar iron-chelating abilities, phosvitin lowers absorption of dietary iron from egg yolks and foods eaten together with yolks. (Kim et al., 1995; Kobayashi et al., 2015.)

Phosvitin protects from the oxidative stress of hydroxyl radicals made in the Fenton reaction. Phosvitin protects DNA from iron-induced oxidative damage. (Ishikawa, et al., 2004.)

Phosvitin contributes to the antioxidant properties of egg yolk. It also has antibacterial and antibiofilm properties. (Kim et al., 2020; Sattar Khan et al., 2000.) These arise partially from phosvitin's abilities to sequester metals including iron from pathogen invaders (nutritional immunity: maintaining control over precious nutrients), and partially from other specific properties of phosvitin's structure.

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:

Phosvitin may:

• 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.