Tums are calcium carbonate, not sodium bicarbonate. Calcium carbonate is often used as a placebo in bicarb research.
I've researched bicarb over the past 35 years and have published about 6 papers and a book chapter on the topic and find it a bit ironic that it has recently made a comeback. Supposedly putting it in the hydrogel eliminates the gastrointestinal distress. However, I have only had one subject in all of my published research that experienced this, and that was after the fact, not before. The 100% key to eliminating the gastrointestinal issues is to make sure that you are well hydrated beforehand and that you consume it with a lot of water. This dilutes it enough to minimize/eliminate the gi problem.
A few proposed mechanisms for how it exerts its effect. 1) it acts to enhance the extracellular buffering capacity (arterialized plasma pH is increased by about 0.1-0.15 units). Too much bicarb and the pH goes up too high and this definitely causes negative side-effects. 2) it enhances the efflux of lactate out of the cell, thereby maintaining optimal muscle pH. Almost all bicarb studies will demonstrate higher plasma lactate levels with bicarb loading.
Save yourself the money and prepare your own loading dose by using 00 gelatin capsules (get them on Amazon) at a precise dosage of 0.3g/kg of body weight (This literally costs about 50 cents compared to $20 for the gel) and consume it 90 minutes before you start your 30 min warmup. Any activity that has a high intensity, high acid producing component over 1-4 minutes is a candidate for bicarb. This can be 400/800m or it could be a 10k with a sustained 2-3 min at the end. My kid does MMA at a fairly high level and bicarb fits his scenario perfectly with 3, 3 min rounds. Especially in the last 3 minutes. Olympic boxing and wrestling...absolutely!
I would be stunned if 80% of the top 10 800m runners were not using it. It is way to cheap, it's legal, and if you do it correctly it has minimal to no side effects. Does it work?... it is debatable; however, the evidence from 40+ years of research is pretty solid. For those elites that have not used it, I'm looking forward to when they do. May only give them 0.1, 0.2 or 0.3 second improvement but...