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Dozens of sites throughout the internet nowadays, seemingly concerning nutrition, essentially try to convey the same message: alkaline food helps to regulate your internal pH.
Having read some simple notion on the subject, I learned body fluids' pH has to be maintained always in a quite strict range (between 7.35 and 7.45), although in the stomach it goes under 3 because of gastric juices.
Hence, is there any scientific evidence backing the aforementioned statement?
Yes, there is evidence that alkaline food affects internal pH, but it is important to note that the regulation of internal pH (acid-base homeostatis) has other contributing factors, which is why the impact of food is subtle.
I'm mainly aware of evidence from studies on sodium bicarbonate, as an example for a simple way to supplement alkaline food.
This experiment studies the effect of sodium bicarbonate as a pre-workout supplement. Blood pH is measured at baseline, pre-workout (after either placebo or sodium bicarbonate supplementation), and post-workout.
Dashed lines are the placebo group, solid lines the sodium bicarbonate group. This shows that the buffering from sodium bicarbonate has a slight effect on blood pH even pre-workout.
This study has a different focus, but the experimental setup and results in terms of blood pH response are similar (labels "SB" are sodium bicarbonate groups; RSA and CCT refer to two different exercises):
So technically you could say that alkaline food "helps to regulate internal pH", but it is a very weak influence. To emphasize the impact of other contributing factors to blood pH, it is also interesting to look at something like this:
This article studies the effect of the Wim Hof method, in particular its breathing technique. The breathing technique basically alternates between voluntary hyperventilation and breath holds. The supplementary material features a video of one subject performing the breathing technique, including monitoring of blood pH over the course of the exercise. The subject has a basline blood pH of 7.4. During hyperventilation, blood pH increases over 7.6 as a result of the reduced carbon dioxide levels in the blood. During breath retention, carbon dioxide levels normalize and the blood pH falls (almost) back to the baseline level. Note that these effects on pH are very quick compared to nutritional influences.
This example shows why it is difficult to separate the effect from alkaline food from other factors contributing to pH homeostasis. Taking a few deep breaths or holding your breath has a very immediate effect on blood pH, which can easily bias the more subtle effect from an alkaline diet. I'm not an expert, but I could imagine that the respiratory rate is even determined by dietary factors. Thus, instead of studying the effect on internal pH, it might be more sensible to show that an alkaline diet leads to reduced breathing in the long term, because the body could maintain the same pH level even with higher carbon dioxide levels.
As for the studies... I would think the first study actually proves my point. People were given 20+g of bicarb and then exercised nearly to death (as evidenced by their ICU worthy ABGs), and yet they survived. Furthermore, they survived with the buffers on the sigmoid curve, which means these extreme values in ABGs represent a very very extreme assault on the body. With a minor glitch that a change to a more alkaline diet is... no... this sort of system will not register a change, simply because it is built to withstand the extremes of external environment with minimal changes to the internal environment. If the ancestor in my first comment could have their blood pH affected by something as minor as a change in alkalinity of an otherwise non-toxic diet, they would have no chance of withstanding a sustained extreme physical effort that is hunting an animal without firearms or running from an animal hunting them.
