I?ll be honest, I?ve never really cared but then I saw a strange statement in something I read online (that nuts somehow specifically took 55 hours to digest) and it made me want to look a little closer.
Save for some medical research on special medical conditions like constipation and IBS, it seems that the research world doesn?t really care that much either.
While I discovered it?s not true that nuts specifically take 55 hours to digest, it is true that food in general can take that long to get from your pie-hole to the bottom of a toilet bowl.
Photo by DesignClass on Unsplash
And in some cases actually longer; In other cases shorter; Usually shorter.
I still don?t really care, but I did a bunch of research, so I may as well share what I found. Some of it is actually interesting.
The Mayo clinic is one of the top hits on Google when you look this up and they cite a couple of research papers where the average was 53 hours, though it?s noted that this number is overstated (read: probably on the high side).
This data uses the more standard radiopaque method (basically slightly radioactive tracers that are ingested with your food).
Then however the Mayo clinic article switches to kid research that used a completely different method of detection: Juice with a Red Marker.
Liquids digest more quickly, so naturally these numbers seemed a lot faster, and make a lot of people reading that article assume that digestion rates in kids is a lot faster than adults ? when you use the same methodology of radiopaque tracers that?s not the case. A more recent accurate technique that newer research uses is a smart-pill. Oh how far we?ve come!
I can tell you without a doubt that the number is not absolute and there is a wild variance from person to person and even day to day in the same person. I?m going to take it a step further to say that the Mayo article is just wrong.
Digestion vs Absorption
Before I continue I want to take a moment to distinguish between two things that some people often confuse when talking about digestion.
Gastrointestinal transit time (GITT) is the total amount of time it takes from eating to pooping it out. That?s what I?m referring to in the rest of this article. GITT includes stomach transit time, small intestine transit time and colon transit time. Colon transit time is by far the most researched component and the other two are thought to be pretty consistent between people.
Absorption of the nutrients that you?ve ingested is an entirely separate thing.
So a lot of people will say things online that use the two terms as if they are interchangeable; For instance you might read on the interwebz that, ?it takes 5?6 hours to digest your food? and what they really mean is that it takes about that long for most nutrients to get processed/absorbed by most people after a meal.
Note: The stomach is where all nutrient breakdown really hits critical mass but very little absorption actually happens here. You?ll see amino acids and sugars hit the bloodstream at different rates depending on type but it?s like an hour or more after eating before you?ll really see most things hit the blood stream.
After about 2?5 hours, food clears the stomach and moves to the small intestine. About 95% of absorption happens in the small intestine over the next 2?6 hours before that all empties in the large intestine (colon). This process of 4?11 hours to the colon is where almost all nutrient absorption has occurred. That?s the 5-6 hour figure people are referring to above? really sometimes more, sometimes less.
It turns out that the longest part of digestion is in the large intestine, with tracers showing that some food can sit for more than 40 hours here depending on the person. The large intestine doesn?t absorb much of anything save a few vitamins that result in bacterial reactions and it?s role is largely to suck water out of your stool so you stay hydrated.
If the whole process takes longer than 59 hours, it is medically considered ?delayed.? From front to back in ?healthy? people, it can take anywhere from 10?59 hours basically.
The reason I think people use the terms somewhat synonymously, is because the majority of absorption has occurred in that timeframe of 4?11 hours. The waste product sitting in your colon isn?t really doing much of anything for the roughly 6?47 hours it can sit there. Carbs, fats, proteins and most mineral/vitamins have all long been absorbed by this time.
Note: I?m just using rough times here as described in the literature, if you want to know where the numbers are coming from click through to some of the research I?ve linked to.
Fiber and Fat
Fat and fiber both slow gastric emptying (how long it takes for food to exit the stomach), and healthy people even without meeting typical fiber requirements have faster transit times than people who experience constipation in spite of adequate and even high fiber intake. Suggesting that there is more to constipation than just a lack of fiber.
That?s not to suggest that people shouldn?t strive to hit a minimum (typically 25 for women and 35 for men is recommended) but that additional fiber above those rough recommendations doesn?t necessarily mean ?better poops.?
Basically high fiber foods and high fat foods sit in your stomach longer and probably help you feel fuller for longer. That?s good incentive to have some of each in a reasonable quantity at mealtimes. Though fat in particular isn?t necessarily satiating at the time of consumption that?s a whole other thing.
There isn?t a ton of data here but I managed to find a few papers that look at a Korean population (averaged 20?30 hours in colon) and one that looked at the Chinese population (averaged 24.5 hours in the colon).
They all seem to suggest that transit times in these populations is a bit faster than it is in Westerners (who tend to average 30?40 hours in the colon), so it could be related to ethnicity and it could be related to diet. Hard to tell for sure. Researchers have even suggested that spicy food might speed everything up!
Remember roughly 4?11 hours is the normal assumption for the stomach/small intestine in everybody so colon only data can still be utilized.
All the data seems to suggest that women take longer to digest than men do. Though things are pretty much the same until everything hits the colon, which is where the female delay becomes a lot more obvious.
One paper found that GITT (total transit time) in healthy male subjects took on average 45 hours, but in women the average was 57 hours.
A different paper that seemed to focus on men (compared to those with spinal injuries) found that healthy subjects averaged about 42 hours of total transit time.
This paper found that the high end of digestion just through the colon (large intestine) for men was 44 hours but for women (regardless of menstrual cycle, which contradicts newer research I mention below) is was up to 70 hours!
And that?s generally a problem looking at a lot of research, a great deal of it looks solely at colon transit times and not total gastrointestinal transit times. At least for comparison sake, I had to stop and think. Colon times seem more relevant to medical research, which is why so much of the data seems to focus on it but most of the time we can assume the rest of digestion is a relative constant (4?11 hours).
It?s interesting to also note that the female menstration cycle influences digestion. At least one study found that women in their luteal phase vs follicular phase delayed colon transport time to nearly double the length of time (40.9 hours vs 20.6 hours).
Unlike gender, ethnicity or the Mayo Clinic paper using juice, colon transit time using the same method of measure does not seem to actually differ much based on age.
Colon transit times are roughly the same for children and adults, though young children probably digest more quickly than adolescents who are maybe a little faster than adults until they reach about 17 or 18 years of age.
Other data between young and old adults reveals roughly the same thing.
Smoking and Other Conditions
Male smokers (though surprisingly not female smokers) experience longer digestion times. No one seems sure why.
Other conditions are probably obvious. People with spinal cord injuries take longer. People with constipation, obviously take longer. Irritable Bowel Syndrome flare ups? Shorter?
I want to keep this focused on the average healthy person so I won?t get into the details of all the ailments associated with the papers I?ve linked to. Suffice to say that most of the research is about comparing healthy subjects to people with medical issues. If you have such a medical issue, you should probably be seeking medical treatment for it anyway, so nothing I could say here should be news to you.
Now that I?ve bored you to death with subset data let?s finish up, shall we?
I still don?t care, and I don?t think anyone else should really unless you experience really big problems pooping over a period of time. It seems that all healthy people will generally complete digestion by the 59 hour mark and times longer than 70 hours are not unheard of.
What annoys me, and really the main reason I did all this research, is that there seems to be an abundance of people online using bullshit nutritional reasoning to suggest that food sitting in your system for too long ?is bad.?
I couldn?t understand why nuts taking 55 hours to digest would even be relevant to discuss?
Most of the time perfectly normal people will be 59 hours or less, regardless of food type intake ? though as mentioned you should probably eat your veggies and get enough fiber!
If you?re over that numbers here or there, it might not be a concern. It?s the chronically elevated time frames that could be associated with some kind of other condition (like those mentioned) or you may even just be mildly sick or a medication you?re taking could be the cause. Even then without a tracer or a smartpill, who could tell for certain?
If you?re constipated (or the opposite) for long periods of time, you should seek medical treatment.
So the next time you see this argument used in an article on a food or types of food, scream fraud. To suggest that nuts (or whatever other specific food) sit your GI tract for X number of hours/days and that is bad, is ludicrous. Specific foods (outside of specific macronutrients) don?t even seem to be studied in this regard.
What is clear is that some food from the same meal will get ahead in the process and some of it will lag. In the tracer studies, 100% tracers were cleared in healthy subjects within 5 days, while 50?80% passed was used to determine the average time for total digestion depending on the study.
So it?s not unusual to say see red in your poo within 24 hours of eating those delicious beets. Even if the beets take another another day or 4 to fully make their way out of your system.
At the very least you now have very interesting dinner conversation.
For the geeks out there who care, a good summary of many of these papers can be found here.