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Food · Animal Foods · Poultry & Eggs
food source · 4
One egg has about 147 mg of choline, almost all of it in the yolk. That is the biggest hidden loss when you eat only the whites.
Mechanism · Choline lives in membranes
Choline sits in cell membranes mainly as phospholipid, so foods packed with membranes carry more of it. The yolk is one of the densest everyday sources, while the white has almost none. Choline matters especially for the developing brain of a baby in pregnancy.
Numbers · What two yolks cover
One egg has about 147 mg of choline, almost all in the yolk. The adequate intake (AI) for choline is 550 mg a day for men, 425 mg for women and 450 mg in pregnancy; two yolks a day supply about 290 mg, and with some beans and lean meat added, getting close to that level becomes much easier.
Egg is often used as the yardstick for protein quality, because its amino acid pattern is close to what the body needs. About 91% of cooked egg protein is digested and absorbed, against only about 51% of raw.
Mechanism · Heat lets enzymes cut
The amino acid pattern of egg protein is close to what the body needs, so egg is often used as the yardstick for judging protein quality. Heat unfolds the protein's structure so that digestive enzymes can cut it. That turns the idea that raw eggs are more nutritious on its head: you swallow the same grams, but nearly half as much is absorbed.
Numbers · The protein in one egg
One large egg has about 6.3 g of protein, about 3.6 g in the white and 2.7 g in the yolk, so eating only the white misses part of it. A human study using stable-isotope tracers (Evenepoel 1998) found that about 91% of cooked egg protein was digested and absorbed, against only about 51% of raw.
Egg is one of the few natural food sources of vitamin D. It is concentrated in the yolk, though the amount is far below that in fatty fish.
Mechanism · The vitamin D is in the yolk
Vitamin D dissolves in fat, and almost all of an egg's fat is in the yolk, so its vitamin D is there too, and egg whites alone supply none. It is still one of the few natural sources, just not as concentrated as fatty fish.
In practice · Yolk fat is a carrier too
The fat in the yolk also helps fat-soluble nutrients such as lutein get absorbed: the nutrient and its carrier come in the same bite, which is where a whole yolk differs from a single-nutrient supplement.
B12 comes almost only from animal foods, and an egg's B12 is in the yolk, about 0.6 μg per egg. For lacto-ovo vegetarians it is one source among several, but one egg falls well short of a day's need.
Mechanism · Plants do not make B12
Plants do not make B12. The yolk carries a share, alongside the choline and vitamin D in the same yolk. Lacto-ovo vegetarians can count it as a source; people who eat no animal foods at all still need fortified foods or a supplement.
Numbers · One egg is a share, not a day
One egg has about 0.6 μg of B12, while the US adult Recommended Dietary Allowance (RDA) is 2.4 μg a day, so an egg is only a share and should be counted together with milk, fish or liver.