The Ultimate Detox: Your Kidneys

The specter of unseen, unspecified toxins eroding our health is worth many millions of dollars in the United States and abroad.  Companies offer "detox" supplements, beverages, and creams that supposedly rid us of supposed toxins, despite a complete lack of evidence that these products do anything at all*.  This comes from an industry that excels at creating boogeymen and offering costly solutions for them.

If your wallet needs to lose weight, then these products are highly effective, otherwise it's probably best to save your money.  Here's why.

The body is equipped with an extremely advanced system for excreting toxins.  The kidneys are part of this system, and their design is genius.  The basic functional unit of the kidney is the nephron, and the average kidney contains about a million of them.  Nephrons have two major parts: a renal corpuscle and a renal tubule. 

A nephron.  In this image, the Bowman's capsule and glomerulus make up the renal corpuscle, and the proximal/distal tubules and the loop of Henle (#1-3) make up the renal tubule.  Note the network of blood vessels (capillaries) that allow the transfer of water and other goodies from the tubule back into the blood.  Image source.

The renal corpuscle is the interface between the blood and the fluid that will eventually become urine.  Blood is filtered by a fine "sieve" of cells that prevents everything larger than a small protein from passing into the renal tubule.  Red blood cells, platelets, and most proteins stay on the blood side, while small proteins such as albumin, minerals, urea, glucose, water, and almost anything that would be considered a toxin** are allowed through into the renal tubule.

The renal tubule is a long tube that re-absorbs everything in this filtered blood that the body wants to keep.  Water, minerals, albumin, glucose, amino acids, and other useful molecules are re-absorbed.  Everything else ends up as urine and is excreted. 

Can you see the genius of this design?  Urine is blood, minus all the good stuff.  Everything that isn't specifically recognized by the body as useful is excreted by default, no matter what it is.  The body doesn't have to recognize each of the thousands of foreign compounds that make their way into our circulation each day.  These substances are all out the door, by default.

Are you impressed by your kidneys yet?  If not, consider this.  Your kidneys filter your entire blood volume roughly 70 times per day.  The reason you don't have to pee a liter a minute is that urine volume is reduced by 99 percent due to water reabsorption in the renal tubules.

This is why most drugs have to be taken on a regular basis, often several times per day.  In concert with the detoxification enzymes of the liver, which tend to make drugs easier for the kidneys to excrete, the kidneys rapidly reduce the circulating concentration of drugs simply by excreting everything they don't recognize as useful.

Can a detox product improve upon 500 million years of kidney evolution***?  I have my doubts.


* Exception: chelation therapy offered by a licensed medical practitioner for actual, diagnosed heavy metal poisoning.  Second exception: strategies that use the word "detox" loosely to refer to removing unhealthy foods from the diet.

** Toxins tend to be very small-- either small organic molecules or minerals such as arsenic.  Larger toxins such as proteins are uncommon in the circulation because proteins are generally not absorbed by the digestive tract.  Toxic proteins have to be injected or otherwise directly introduced into the circulation, e.g. by a snake bite or a bacterial infection.  But if you're bitten by a rattlesnake, I hope your first line of treatment won't be a detox kit from your local supplement store.

*** Kidneys are present in hagfish and lampreys, the most "primitive" living vertebrates.

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