I am going to approach this post by showing how Woo-practitioners (Quacks) who like to talk about the body being too acid and needing become more alkaline are talking nonsense. In fact, it should be a warning sign, if a treatment or diet mentions “restore your alkaline body pH” or “the secrets of an alkaline body”  it is distinctly woo. This is a big area for quackery. For example, I did a Google search on “alkaline body” and got approximately 24,000 hits.
So I am going to discuss human physiology.
We all are buffered. It is very important that the pH of the various body liquids remains within a tight range. Different body fluids may be at different pHs, but they are tightly controlled at that pH by buffering .
So why is the alkalinity body theory considered woo?
When you eat anything, it is digested [no arguments, right?] passing through the mouth, the stomach, small intestine, large intestine (AKA bowel, colon). Bits, if not most, are absorbed along the way. Different fluids in the digestive system are at different pH.
For example, the mouth typically has a resting pH around 6.3; and the presence of bicarbonate ions in saliva maintains this pH. This is not a true buffered system as the introduction of food can alter the pH and the bicarbonate salts neutralized the acids present (ref).
As food passes into the stomach, gastric juice is released. Gastric juice contains hydrochloric acid with a pH 1-4. Even when the stomach is full, the pH of the stomach will fall that low. It doesn’t matter what you eat, gastric acid will be produced to reduce the pH. Stomach enzymes are ineffective if the pH is higher.
High levels of HCl (pH 3) convert pepsinogen to pepsin, which is active at acid pH and kills ingested bacteria. (ref)
When food leaves the stomach and enters the small intestine, bile and pancreatic juices are released. Both of these are alkaline, partly to neutralize gastric juices, but also to act as a detergent, allowing the lipids to become more dispersed and available to enzymatic activity. Small intestine enzymes require this increase in pH to be able to function.
So given that food goes through all these changes in pH during digestion, it is unlikely that food can alter the pH of the body. Additionally, blood pH is closely regulated with an internal buffering system based around bicarbonate ions as well as serum proteins. I’ve posted on this before using this reference.
Thus, diet has little, if no, effect on the body’s pH. If your body’s pH is out of whack, you probably should check for an underlying illness.
So having sorted out those Quacks, why might we want to use buffers? The body already has buffers especially in the blood. If we want to study body functions outside of the body [known as in vitro] we would need to have a model that mimics the body especially the pH. If we did experiments at a different pH, the enzymes present might not function. For example, salivary amylase, which initiates starch digestion in the mouth, has a pH limit of 5.6-6.9, below and above which it cannot function.
As both plants and animal have natural buffering systems that means that food does too. When I am studying the Maillard reaction, I typically use model systems. This is not ideal, but removes any extraneous factors such as other reactions interfering with the reaction I am want to understand. This model systems are buffered at a pH typically found in food. When the pH is changed, a different reaction mechanism may even take place.
So buffers are important tools in chemistry allowing us to carefully control the pH.
Knowing the pH of a food product is important for food safety, as different micro-organism can or cannot grow when the pH is low. Acid-foods are more resistant to spoilage than low-acid foods, and are regulated differently.
 I cannot bear to put any more links in. Reading the two I posted is giving me a headache. Quick, I had better restore my brain’s alkalinity!
 Frustratingly I cannot find a list of body fluids showing their pH. If any body knows for one, please let me know if the comments.