Discusses the digestive process and the importance of fiber to protein cycling.
The digestion of food involves several distinct steps. First it is chewed and enzymes in the saliva work on starches to free some short glucose polymers.
Next it enters the strongly-acid stomach where the acid kills any bacteria and denatures any protein not specifically designed to survive it. The lining of the stomach releases acid peptidases that work on proteins to chop them up into shorter polypeptides. Some small fat-soluble molecules such as alcohol, caffeine and aspirin can enter the bloodstream directly from the stomach but the fat, carbohydrate and protein fragments are still too large to be absorbed.
After 3 or 4 hours in the stomach, the food mixture, now called chyme, is released through the pyloric valve into a one foot long portion of the small intestine called the duodenum. Here it is mixed with bile produced by the liver and bicarbonate from the pancreas and duodenum lining that together neutralize the acid from the stomach. The bile further emulsifies the fat into microscopic droplets suspended in the watery flow. The lining of the duodenum and the pancreas release a wide range of specific enzymes to digest fat, protein, starch and sugar polymers to the simple molecules that can then be absorbed by the lining of all the segments of the small intestine. Aerobic bacteria normally found in the intestines may also aid somewhat in the digestion process.
The digested chyme then passes into the next section of the small intestine called the jejeunum. This section is about 6 feet long and is specialized for absorption. Most of the small molecules from fat digestion (fatty acids and glycerol and the bile salts), carbohydrate digestion (simple sugars) and protein digestion (amino acids) enter the bloodstream here and are transported to the liver.
The remainder of the small intestine is the ileum which is characterized by areas of lymph tissue that become progressively more common down its length. As described in the previous chapter, so-called M cells in these areas ingest large molecules and solid particles including whole bacteria by a process called phagocytosis. It is currently unclear if this is significant nutritionally.
From ingestion to final absorption takes about 6 hours and this likely accounts for the normal daytime 6 hour periods between meals.
What is not absorbed passes into the large intestine, the colon, where bacteria are mixed in and water is removed. Anaerobic bacterial action then results in further digestion and absorption by the walls of the large intestine, accounting for some vitamins and as much as a 100 calories per day. Finally the solid remains, the feces, are eliminated by defecation.
The term fiber can be applied to the material in the chyme that normally enters the colon. It consists largely of the cellulose and other highly-branched carbohydrates of fruits and vegetables. The grams of fiber in a serving are listed on the standard nutrition labels now required in the USA. We can see on these labels that 25 to 30 grams of fiber per day is recommended.
We see that for the digestion process to complete the body must invest its own proteins in the form of enzymes and mucus, usually with the expectation of a lucrative positive return. For a protein-free meal, the body might hold back on protein digesting enzymes but would still be obliged to provide fat and carbohydrate digesting enzymes as well as lubricating mucus. For a calorie free meal such as one would eat on the ADCR diet, the body would at least supply the mucus proteins required to lubricate the passage of food.
Though some supplied protein is recovered, a portion of total protein is always lost, held inside the bacteria that form about a third of the weight of feces. Normally this lost protein is of little significance. Studies have shown that, speaking roundly, even for a conventional high fiber diet (45 g fiber/day + 100g protein/day), only about 10% (10 grams) of the protein is not recovered48. With 7g fiber/day + 100g protein/day, only about 7% is not recovered.
Protein is also lost from the shedding of hair and skin. The cells on the outer layer of our skin dry up and flake off at a rate estimated at 20 mg/minute49or (20*60*24/1000)= 28.8 grams of cells per day. If around half that is protein (largely keratin), then then you might think we lose about 14 g protein/day in this way alone. Actually there is still water remaining and the actual value is more like 3 g protein/day. If we add the losses from all sources, we lose about 25 g protein/day even on a total fast. If our protein input was chronically less than 25 g/day, we would eventually starve to death even with plentiful calories.
For the protein cycling diet, however, this seemingly unavoidable 25 gram per day protein loss gives us some wiggle room. As we have seen, meals totally lacking in protein are nearly impossible to devise. But any day with 25 or less grams of total protein is effectively protein free or even protein negative and certainly sufficient to require the cells to commence autophagy. 25 grams of protein represents about 100 calories or a bit less than 5% of the standard 2000 calorie diet. Under 5% is then the target we want to hit to guarantee autophagy induction.
On a low fiber diet (7g/day), however, that 25 gram margin, is reduced to maybe 22 grams. Though this may be insignificant small change, a high fiber diet does work a bit better than a low fiber diet. A no fiber diet would be even worse, so eating no calorie foods (salads, spinach, pickles, etc.) would be better than skipping meals entirely when following the ADCR diet.
High fiber also has been seen to reduce the incidence of colon cancer and, as we can see in the mortality table, colon cancer deaths alone exceed those from all neurodegenerative diseases below age 75.
Additionally fiber makes you feel full even when protein or calories are lacking. For all these reasons, sufficient fiber should be maintained when changing your diet for protein cycling or ADCR.
Fiber can be added with fruits and vegetables but, for those who dislike them, fiber can be supplemented from brans, guar gum, psyllium (as in Metamucil®), or any number of sources. Some are water soluble, some are not. Guar gum and psyllium are soluble, well-tolerated, taste-neutral, inexpensive, and readily available. They can be added to any beverage to make it filling yet still palatable. This is especially useful when on the ADCR diet.