Macronutrient Absorption Times

Okay, maybe some of those folks with more knowldge or access to research can clear up this and some other question. What are the clearance times for various macronutrients?

For example, when carbs are absorbed the resulting insulin release generally clears them from the bloodstream fairly quickly. This has been measured a fair amount and gives us the GI/II values. If we are lucky carbs go to the muscles and if not they find their way to fat cells.

When fats are absorbed, via the lymphatic system, they finally enter the bloodstream a fair amount of time after initial ingestion. How long do they stay there? Since they don’t generally cause insulin release on their own what is the clearance process? How long does it take?

As for protein breakdown into amino acids, I’m not really very aware of the timing involved. Since we eat every three hours or so I’m assuming they will be entering or be in the bloodstream for approximately that length of time. Is the liver processing these if they aren’t being taken up by the muscles? Are they preferentially driven into muscles in the presence of insulin?

I guess a good time graph showing the blood levels of carbs, lipids and amino acids following various types of meals would be great. If we could get that for trained subjects both at rest and during physical effort that would be even better.

[It looks like I might be spending some hours Googling over the next couple of days since I have so many questions.]

Anyway, based on the thread pointed to by Deuce, I’m curious about microtiming the various macronutrients.

I’m also going to step out on a limb and hypothesize about Insulin resistance. Would it make sense that we become more insulin resistant as our cells become filled to capacity with nutrients? Then, in the morning or after a workout when our cells have depleted their reserves insulin is suddenly more effective at driving in nutrients? A dangerously simple interpretation.

Anybody care to pontificate about these issues and or discuss micromanagement of blood levels of these nutrients for various goals?

Man, this is the best post I’ve seen in a while (except my flirting thread of course).
OF course your timing questions depend on the type of food ingested (ie whey hydrolysate vs steak)

Carbs: The glucose solution I give my burned kids is very concentrated. So much so, that the OR is still sucking the liquid from their stomachs 4 hours after they drink. What’s also interesting is that it actually EXPANDS their stomach volume. Quite the case for diluting carbs non? On the contrary, Surge ramps up and drops blood sugar in about 45 min.

Proteins/Amino Acids: AA’s are in and out in about an hour. Check the Surge article for some graphs of different proteins/aa’s. The references therein should also help.

Fat: No clue.
I wish I had better answers…

When fats are absorbed, via the lymphatic system, they finally enter the bloodstream a fair amount of time after initial ingestion. How long do they stay there? Since they don’t generally cause insulin release on their own what is the clearance process? How long does it take?

Okay, I did some Googling and found some research on these areas. The following article shows blood lipid levels rising and then falling over an 8 hour period based on various types of fats consumed.

[b]Effect of 6 Dietary Fatty Acids on Postprandial Lipid Profile



So, going back to the original post, does this imply that we may benefit by spacing a carbohydrate meal well after a meal containing significant amounts of fats?

Also, if I have followed along correctly in my reading, lipoprotein lipase (LPL) is responsible for seperating fatty acids from lipoproteins in the bloodstream. For body composition purposes can we selectively raise LPL levels in skeletal muscle relative to adipose tissue LPL activity?

Weightlessness and sedentary lifestyle both decrease skeletal muscle LPL activity…

And exercise of course seems to ramp it up… not surprising considering that physical effort can be fueled by fats stored in muscles.

Finally, there is lots of research out there showing elevated insulin levels result in higher blood lipid levels. Probably not a good thing if we are trying to drop fat.

Anyhow, researching into lipids and such has actually been interesting. I don’t suppose anyone else has some thoughts or questions around these issues?

This seems worthy of a bump. I’d like to see if anyone else has any input on the topic… ahem JB, LL

I’ve been doing some more poking around on the topic of lipids and thought I’d share some other things as well.

For example, here is a nice powerpoint which among other things simplifies lipid absorption and transport issues greatly.


It discusses chylomicrons (exogenous fats are transported into the body via these) and the progression of VLDL (endogenous fats are transported via these), LDL, and HDL bodies as various processing occurs. It makes “cholesterol” much easier to understand.

If you want a nice overview diagram of various metabolism pathways, here is one you can use.

[I’ve saved a copy of both of these to hard drive in case the links die…]

Here is a cut and paste of an interesting look at the ability of the body to store energy… you can read the entire thing at the following URL if you like.

It is interesting to compare the relative amounts of energy provided by various biochemicals in a typical 154 lb male. [154 lbs?] The free glucose in the blood provides only a 40 kcal energy reserve – only enough to maintain body functions for a few minutes. Glycogen remaining stored in the liver and muscles after an overnight fast, amounts to about 600 kcal energy. Glycogen reserves can maintain body functions for about one day without new inputs of food. Protein (mostly in muscle) contains a substantial energy reserve of about 25,000 kcal. Finally, lipid reserves containing 100,000 kcal of energy can maintain human body functions without food for 30-40 days with sufficient water.

However, all the talk of lipids and cholesterol got me to thinking. The body attempts to maintain levels of VLDL, LDL and HDL lipoproteins (and their contained cholesterol) at all times. In fact, generally you should fast before your levels are tested.

So, time for some questions! Does the body differentiate between fats arriving via consumption versus lipolysis? If you aren’t spiking insulin with a high carbohydrate diet, will your body happily perform lipolysis as needed to keep lipids/lipoproteins at normal levels?

Oh, that reminds me, I also found this interesting study. It shows that fat cells are preferential in their release of lipids.

Basically, for a given chain length, the relative mobilization rate increased with increasing unsaturation, whereas for a given unsaturation, it decreased with increasing chain length.

As you lean down the fats supplied to your body may start to shift and have an impact on your metabolism.

Also, stealing from another study, it seems that as you fat cells shrink their rate of lipolysis will decline.

The maximum lipolysis rate was significantly correlated to hormone senstive lipase (HSL) activity. This is in accordance with the role of HSL as the rate-limiting step of lipolysis. However, adipocytes were 24% larger in the sc than in the omental region, and the lipolysis rate was significantly correlated to fat cell size regardless of either the region of origin or gender. This indicates that the regulation of HSL activity in healthy subjects, which appears to occur at a transcriptional level, is to a large extent dependent on fat cell size.

Might that be another reason why as you get leaner it is hard to lose fat?

Okay, I guess this is getting pretty long winded. So the real question, depending on the answers above, is this. Can you reasonably and safely cycle high fat loss days into your diet by taking advantage of the bodies processes?

For example, eat very low carbs, high protein, high fiber, no fats and drink a ton of water while doing a lot of low intensity aerobic or recovery activities on a non-workout day? If the body is burning fat for energy, lipolysis has to be used to keep lipoprotein levels static and your macronutrients are such that lipogenisis will not be stimulated (via gluconeogenisis), why not?