March 26, 2009

You Ask, I Answer: Egg Yolk

I heard somewhere that you should keep the yolk when eating eggs as you don't absorb the protein without it.

I know the yolk has the highest concentration of protein but I always assumed that egg whites are also a source of protein, albeit less than a whole egg.

Can you clarify?


-- Lori (last name withheld)

Ottawa, Ontario


Although egg yolks contain some protein (approximately 42% of an egg's total protein content), egg whites contain more.

Additionally, whereas egg yolks are a mix of protein and fat, egg whites are almost entirely made up of protein.

You do not need to eat egg yolk in order to absorb the protein in egg whites.

That is not to say the egg yolk is useless. It's a wonderful source of folate, vitamin A, choline, and the antioxidants lutein and zeaxanthin.

6 comments:

Lori said...

Thank you Andy.

Danielle said...

Thanks!

Lori said...

Andy,
How unhealthy are egg yolks? Is it true that some people have more of a chance (due to genes) of producing more LDL cholesterol and only these individuals should eat egg yolks in moderation? Thanks.

Brandon said...

Speaking of cholesterol (Lori's Q), I have a somewhat complicated question, to satisfy my curiousity. How significant would increasing (soluble) fiber intake be on preventing weight gain from fat? Here's my logic.

Let's say I eat 35g fiber/day, and let's say roughly 15g is soluble. Soluble fiber binds out bile acids (made from cholesterol), preventing it from being reabsorbed, right? Is that a 1:1 ratio? Will 1g soluble fiber bind out 1g bile? Let's assume it does for the rest of my example.

Liver makes cholesterol from Acetyl-CoA. Acetyl-CoA that could've ended up being used to make fat. It takes roughly 15 acetyl-coa to make cholesterol? and roughly 8 to make fat (palmitate)... so by eating 15g soluble fiber per day, I'm preventing roughly 30g weight gain from fat? Which is roughly one ounce. 16 ounces in a pound. So by eating 15g soluble fiber/day, two weeks later I just prevented 1 pound of weight gain from fat.

How does my logic sound?

Andy Bellatti said...

Brandon,

You are correct that soluble fiber helps excrete cholesterol by binding bile acids.

Since these bile acids are excreted (rather than absorbed), the liver draws cholesterol from the blood to make more bile acids. Consequently, blood cholesterol levels are lowered.

So far, so good.

You lose me, however, in your last paragraph. Remember that acetyl co-A is made from pyruvate dehydrogenase (an enzyme related to carbohydrate metabolism).

I'm not understanding the link you're making between cholesterol being excreted, acetyl-coA amounts, and weight gain.

A more logical explanation for weight loss as a result of diets high in soluble fiber is that, since soluble fiber acts like a sponge and slows down gastric emptying, it helps people feel satiated (and therefore, not hungry) for longer periods of time.

Brandon said...

I was thinking something like:

Fatty Acid Synthesis
8 Acetyl Coa + 14 NADPH + 7 ATP -> Palmitate + other stuff

Cholesterol Synthesis
Acetyl Coa + Acetoacetyl Coa -> HMG CoA, and then a lot of other stuff happens

So if you had 'extra' Acetyl CoA, your body could either make fat or cholesterol. The more cholesterol you make from it, the less fat you would be able to make from it.

However, I wasn't thinking about the fact that the liver will just pull LDL from the bloodstream if bile is 'low', which pretty much makes all of my thoughts futile. Problem solved, thanks.