The human digestive system is a massively complex system that can fall afoul of any number of diseases and conditions. As if that wasn’t bad enough it also suffers from many misconceptions, false beliefs and myths. We rely on it every day of our lives and take great interest in what goes in one end, and sometimes what comes out the other, but what do we know about what goes on in between?
We all know that the main function of the digestive system is to break down food into its constituent nutrients; sugars, amino acids, fats etc so that they can be absorbed and used by our bodies as either energy or building materials, but how exactly does it go about doing this?
The first stop (hopefully) is the teeth.
The teeth at the very front are called incisors and help to shear the food into more manageable pieces. At either side of the incisors sit the canines which help to both hold your food in place and tear it apart, particularly if the food is too tough for the incisors alone. Next up are the pre-molars or bicuspids which serve to crush the food into smaller particles, though typically today, it’s the teeth at the back, the molars, where much of this grinding tends to occur.
Diet plays a huge role in the evolution of teeth with an animals teeth almost always suited to the types of food that animal tends to eat. Around the web you’ll often find people debating human teeth and what that say about foods that we are adapted to eat. Many vegans and vegetarians argue that we evolved as herbivores, comparing our teeth to other herbivores, some go the other way and say that we are more adapted to a carnivorous diet. The evidence, though, strongly suggests that our teeth evolved to suit a generalist, omnivorous diet, with meat playing an increasingly larger part in our evolutionary history as time went by.
Here’s an interesting talk by Peter Ungar about the teeth of our ancient ancestors the australopiths and what they say about the diet of our early ancestors.
It’s worth noting, too, the role played by both tools and cooking, which help us to process food even before it reaches the teeth. These allow us to soften the food meaning much less work is needed by the teeth, essentially sparing us (along with higher quality, more nutrient dense foods like meat) from having to spend hours a day chewing to get the nutrients we need.
The Art of Mastication
As children we are often told to chew our food properly, but where does this age-old advice come from? Just why is it so important to chew our food well? Recent studies are beginning to shed light on the subject.
When we think about digestion we often think in terms of the stomach and the intestines, but digestion really begins in the mouth. Within the oral cavity salivary glands secrete a number of chemicals and enzymes that help in disinfection, lubrication and digestion. There are more than 2000 proteins and peptides (relatively short chains of amino acids) in human saliva (1) and we have some way to go before we fully understand all the complex chemistry that takes place within the mouth, but let’s have a look at some of the things we do know.
First up is amylase (also known as pytalin [tahy-uh-lin]), an enzyme that begins to break down carbohydrates by cleaving bonds within starches to get maltose, maltotriose and dextrins. The optimum pH for pytalin is around 7 which means it’s quickly denatured by the low pH of the stomach. Despite this, up to 75% of the carbohydrate content is actually broken down by the enzyme in the stomach, this is partly due to the fact that the bolus of food (the ball of food coming into the stomach) protects the enzyme from the gastric juices and thus denaturation and that both starch and the enzyme product protect the enzyme against inactivation by gastric acid (2,3)
Another enzyme, lingual lipase, begins to break down fats (triglycerides), but unlike ptyalin, lingual lipase continues quite happily in the stomach and duodenum because its activity is optimised at low pH.
Yet another enzyme, lysozyme, offers a basic level of disinfection against bacteria by causing bacteria to clump together and by breaking down bacterial cell walls. This together with a multifunctional protein, lactoferrin and immunoglobulin A (IgA) act as the first line of defence against pathogens. Lactoferrin inhibits the growth of bacteria that require iron by essentially stealing it away. It also has antiviral and antifungal properties along with many other functions (4). Immunoglobulin A is an antibody (a molecule that binds to foreign bodies), which then activates an immune response.
Mucin proteins coat the food and act as lubricant to help the food pass along the esophagus. They also bind to bacteria which helps to protect the teeth.
Haptocorrin (otherwise known as transcobalamin I, R-protein and sometimes R-factor) binds strongly to vitamin B12 to protect it from the hydrochloric acid in the stomach.
Potassium bicarbonate is also present in saliva and acts as a buffer, keeping the pH of the mouth around 7, which helps to deal with any acids that might damage the teeth.
The act of chewing thoroughly, massively increases the surface area of the food, allowing all the various enzymes both greater access and more time to act on their respective targets, the lipase on the fats, the lysozyme on any bacteria and the amylase on the carbohydrates etc. All of which means more thorough digestion both by the time the food reaches the stomach and subsequently the small intestine. Not chewing food well enough can lead to food spending more time in the stomach or relatively large particles of inadequately digested food reaching the intestines which can cause problems like small intestine bacterial overgrowth (SIBO) etc.
Less chewing, caused in part by highly processed foods, has also been linked to dental crowding, a phenomenon where the teeth and jaw fail to receive the necessary stimulation to grow properly (5).
Reducing food to pulp and allowing enzymes to do their business is only one aspect of chewing. Another vital aspect is taste; a vastly complicated process that is still the topic of much research. It’s so important, in fact, that the taste organ, the gustatory epithelium, is capable of complete regeneration and is highly resistant to both ageing and damage (6).
Though we often get much pleasure from the sense of taste its primary purpose, particularly in omnivores, seems to be analysis and screening of food and any toxins that may be present and readying the body for the food that’s about to be ingested.
The Evolution of Amylase
Another interesting aspect of human digestion that takes place in the mouth, which could have huge implications for nutrition and disease, has to do with the amount of amylase an individual has in their saliva.
All mammals produce amylase within the pancreas, but a few like the great apes and rodents also produce amylase in their salivary glands (The mechanism evolved independently in both apes and rodents which in itself suggests that pre-digestion of starches is important for omnivores).
Humans take this one step further in that we have multiple copies of the amylase gene, AMY1. This is thought to have arisen from the change in human diet to include more starchy foods like tubers after we diverged from other members of the ape family. Subsequently, the advent of agriculture is thought to have led to a greater increase in the number of copies of the AMY1 gene. Populations that eat a higher starch diet tend to have more copies than populations that eat lower starch diets. The number of copies differs from between 2 and 15 between both individuals and populations (7).
More copies of the gene means more amylase in the saliva. A study carried out in 2012 to look at the difference between people with many copies (High Amylase; HA) and people with few copies (Low Amylase; LA) revealed some unexpected results(8). The research team had hypothesised that since the HA group had more amylase, which would lead to quicker and more complete cleaving of bonds and therefore better digestion, they would have higher blood glucose levels after ingesting starch than the LA group. What they found instead was the opposite, that the LA group had higher blood glucose levels following the ingestion of starch. Raised blood glucose levels have been implicated in many medical conditions and disease states and again are a current topic of intense research.
What happened in the HA group is that the higher level of amylase present in the mouth was able to cleave enough starch for receptors within the mouth to stimulate what is known as the preabsorptive insulin release (PIR; also known as the cephalic phase insulin release; CPIR). Whilst PIR forms only a tiny percentage (around 1%)(9) of overall insulin secretion it’s thought to play an important role in glucose tolerance. Loss of PIR is associated with glucose intolerance which is a pre-diabetic state of hyperglycemia, essentially too much sugar in the blood. In the study, the LA group failed to exhibit PIR in response to the starch and so had higher levels of blood glucose (though oral intake of glucose resulted in the same PIR response in both groups)
Why is this important? Well, in today’s society starchy foods make up the bulk of most western diets. Predominantly, the majority of starches eaten today are highly processed and refined, similar to the starches used in the HA/LA study. The study itself found no difference between the foods that both groups consume though each group clearly showed a different glycemic response to such foods. They concluded that
“The imbalance between genetic background and evolutionary optimized diet may also have potential implications for the development of non-insulin dependent diabetes and obesity. The reasons why some individuals develop these conditions while others do not are not currently understood. In light of our current findings, we suggest that AMY1 gene copy number may play a role in the development of insulin resistance and diabetes. Both high and low amylase individuals in this study were young and healthy, with a mean BMI, 22 kg/m2, yet the groups had different glycemic responses following starch ingestion. Although overall insulin concentrations did not differ between the groups, it is possible that chronic high blood glucose concentrations induced by high starch intake may elicit a number of hormonal, receptor, and physiological changes that will indicate that individual differences in salivary amylase may considerably contribute to overall nutritional status.”
Though only a small study it hints at the possibility that our genetics, along with other factors, play an important role in the foods we eat and the diseases we may suffer from a particular diet. For me, this is particularly interesting as all too often I come across the mantra ‘everyone is different’, but with little explanation as to how and why we are different. It’s small studies like these that are beginning to unravel some of those differences that will eventually shed light on the issues of diet and just what is ‘best’ for us.
Next up I’ll take a look at the stomach, how it works and some of the misconceptions about it.
- Loo J a, Yan W, Ramachandran P, Wong DT. Comparative human salivary and plasma proteomes. J. Dent. Res. 2010;89(10):1016–23.
- Myers, E.N, Ferris, R.L. (2007) Salivary Gland Disorders (p.11) Springer, 2007 edition
- Rosenblum, JL, Irwin, CI, Alpers, DH. Starch and glucose oligosaccharides protect salivary-type amylase activity at acid pH. American Journal of Physiology 254 (Gastrointestinal and Liver Physiology 17):G775-780 (1988)
- Adlerova L, Bartoskova A, Faldyna M. Lactoferrin : a review. Vet Med (Praha). 2008;2008(7):457–468.
- Cramon-taubadel N Von. Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies. 2011:7–12. doi:10.1073/pnas.1113050108
- Breslin PAS. An evolutionary perspective on food and human taste. Curr. Biol. 2013;23(9):R409–18.
- Perry GH, Dominy NJ, Claw KG, et al. Diet and the evolution of human amylase gene copy number variation. Nat Genet. 2007;39(10):1256–60. doi:10.1038/ng2123.
- Mandel AL, Breslin PAS. High Endogenous Salivary Amylase Activity Is Associated with Improved Glycemic Homeostasis following Starch Ingestion in Adults 1 – 3. 2012;(14). doi:10.3945/jn.111.156984.AMY1.
- Teff KL. How neural mediation of anticipatory and compensatory insulin release helps us tolerate food. 2012;103(1):44–50. doi:10.1016/j.physbeh.2011.01.012.