I do have quite some experience with low HCI as well. I tried ACV, Red wine vinegar, Lemon, Orange juice, Grapefruit juice, a juiced gingershot made with lemon, apple and ginger, beet juice, beet juice with carrot and ginger, raw corn on cob, sparkling water (both, naturally sparkling and added carbonation), certain red wines (syrah and carbernet sauvignon were best), and also bought 2 HCI brands. One from thorne (porcine source) and the other one from premier labs (beetroot source) among some mixes of the above like orange juice with lemon, acv with lemon juice and so forth..
I took some of these things for over a year and here are my observations:
1. Apple cider vinegar and lemon. They seem to help but in my experience they don´t in the long run. Especially ACV as it can surpress hunger and made my stomach bloated. So I didn´t want to drink it anymore when I drank it before a meal (also tried with and after a meal) even though I was hungry a couple of minutes earlier. That´s a problem if you then start to eat, since your digestive juices in the mouth do not prepare the stomach for the foods what´s about to come. So you eat and you cant digest it since the hunger feeling is gone. IMO it can replace HCI, but at the same time represses your own. Bad idea. Same with lemon and red wine winegar.
2. Orange juice and grapefruit juice (g is more acidic and less sugar) help digestion too but you get a lot of sugar and you are then dependent on them. I also observed that they surpress my own HCI.
3. The gingershot was helping as well, especially due to the ginger but same thing here. However, just biting into one ginger and leaving it in the mouth for several minutes after a meal helps A LOT more to enhance digestive juices/enzymes
4. Beet juice on its own did nothing, but juice with beets, carrots and ginger helped quite a lot. But it was such a mess and hazzle to do this that i stopped.
5. Raw corn on the cob. Worked really great, however, it´s expensive to get organic so it wasn´t feasible and practical (ur not gonna carry around raw corn on the cobs all the time ur out)
6. I tried several naturally and carbonated waters. I read that the carbonated ones are acidic (I assume the natural ones as well). They seemed to work since they made the food move through the intestines quickly which makes space in your stomach and therefore you feel you need to eat more. So more hunger more gastric acid is produced (at least in my theory). Only problem is that when you drink sparkling water (even naturally sparkling) all the time it hurts or maybe even inflames your intestines. Bad idea when u got inflamed intestines already.
7. Certain red wine grapes (which are more acidic) can help digestion tremendously. I would say 1-2 glasses were ok. The food got properly digested and the wine grapes themselves have digestive abilities which help digestion further down the digestive tract. Only problem is too much alcohol is no good when ur not fully healthy and it becomes expensive. Sleep however was great on 1-2 glasses as digestion was very fast and smooth.
8. As mentioned i tried 2 types of HCI. The porcine source made me blocked and bloated (maybe the pork source) and the other one I initially cut out too. However, I started the other one again, this time on a different time interval. That´s 30-45 minutes after food. And so far it works well but I would be careful as it has already been mentioned when you don´t have much stomach lining.
Basically what I can conclude at this stage is that most of these substitues are no good. What you want is your own body to produce enough HCI himself, not lower it by taking substitutes. You might think ACV helps, but it makes things worse. Since I cut out ACV and lemon juice for example, my body produces HCI by himself more and more it seems. I just made this discovery a couple of days ago and I still experiment with the HCI supplement, but I want to cut it out asap as well. Even when ur older you should produce your own HCI, not depend on supplements. Do tribes that eat a RVAF diet use supplements when they get older? No.
I think the most important thing to reproduce your own HCI is by NOT taking supplements, but by teaching your body how to produce it naturally again. I think one crucial thing is the need for hunger, since saliva is produced which is the first step, and how fast the digestive process is (let say food moves through the intestines). IMO that can be enhanced with fats and fiber. When I was eating a lot of bread and grains (like pasta which has a lot of sauce and olive oil) my digestion was super fast and great (aside of the accumulating inflammation which then fucked up digestion), but moving food through the intestines fast mainly with the help of fats or fiber, foods high in magnesium and therefore the peristaltic movement is crucial. I recently looked into food challenges. You know, the people which can eat a shitload of food very fast. Their stomach acid must be incredible and the digestion must work super fast. Otherwise there is no way you can stuff that much in such a short time without puking it all out in a couple of minutes. So I think the key is finding out how they do it and apply it to yourself by eating healthy foods. I also noticed that what I once read you should never do actually turned out to be more beneficial than anything. That´s drinking still water with meals. Especially cooked meals (which I eat since they are easier to digest than raw foods on low HCI).
So to recap. I think the key to get your stomach acid levels back to top class is by drinking a big glass of water before food (which increases the activation of saliva and digestive enzymes according to my experiments), then eat bigger portions more frequently (bulking up) step by step with plenty of fats AND fiber (fiber in form of tigernut flour and nuts to replace the fiber in breads) together with water. That would make things move fast and fast digestion means more hunger, means bigger portions which signals the body to produce more HCI naturally. During that time you could add HCI and digestive bitters (which i found very helpful but still didn´t finish experimenting with) and maybe magnesium if food travels too slowly through the intestines and ur not eating enough foods high in magnesium (if u for ex. are intolerant to certain foods), then slowly lowering these 2/3 supplements until you notice that your body produces HCI naturally again. Once that is done you could cut out the starch/fibers of the tigernut and nuts and eat raw fats and meats only. But in my experience its better to eat a cooked paleo diet first, since the collagen is broken down due to cooking which makes digestion a lot easier on low HCI, then once digestion/HCI is back up you switch to raw meats and raw fats, fully heal and u reached ur goal.
I also think that just taking digestive bitters without HCI could help a lot, that would also be healthier than a HCI supplement. I ordered one recently and will provide an update once tested properly. And then there is the issue of candida and parasites. I think if you have an overload of them your body can´t produce enough HCI in the first place. So you need to lower the "bad bacteria" candia level by cutting out all fruits except maybe lemon and papaya for lets say a month, at the same time kill the malicious parasites (i think not all are bad) and then you have the whole package to heal.