My responses are in bold.
From a more general nutritional standpoint and based upon what the studies show as an insulin increase causing appetite suppression what are your thoughts related to these effects of soda/diet soda:
1. Suppression of appetite due to diet soda consumption giving a false sense of being "full" with a nutrient empty food/beverage in place eating even a moderately "healthy" snack/meal.
First, the false sense of full you're referring to could be a couple of things. It might be the carbonation and the consumer just needs to let loose a burp. They might have just been a bit dehydrated and replenishing fluid levels (diet soda is mostly carbonated water) was just what the body needed to send the hunger packing. If you are poking around at a discussion on cephalic-phase insulin release and/or GLP-1, this is a good article. Let me know if you would like me to discuss it in a separate entry.
Second, but certainly MOST IMPORTANT; the million dollar question regarding the bulk of debates on what is good or bad in nutrition is "What are your goals?". For example, a cancer patient that is experiencing involuntary weight loss due to decreased appetite is not doing themselves any favors by guzzling down the diet soda, especially if it's leaving them feeling full. As you mentioned, there are no nutrients in diet soda. However, what about the obese, type 2 Diabetic with an insatiable appetite? Diet soda will not raise their blood sugar and will not put any additional weight on them and if it gets them to feel full and thus consume fewer calories, that's fantastic. The aforementioned patient needs to create a caloric deficit to generate weight loss and so replacing a zero calorie/zero nutrient drink with a "healthy" snack/a calorie containing food, is not cohesive with their goals. "Healthy" is an individualized term and thus has different parameters for different people (eg what's healthy for person might be unhealthy for another)
2. Using Diet Soda as a thirst quencher instead of good ole fashioned water and subsequently its effect on the metabolic process.
Explaining the thirst mechanism in its entirety goes well beyond the scope of this blog but this is as detailed an explanation as you'll find. The short of it is that increases in osmotic pressure get us parched. I'm not an expert on hydration but I would think that if increases in osmotic pressure are what are triggering thirst that you would want to look for a beverage that is low in osmolality to quench it/not add to the problem. This is purely my opinion; I don't specialize in sports nutrition, I'm not an exercise physiologist, so if a true expert knows, by all means, chime in. I also want to make it clear that I'm talking about literally quenching the thirst mechanism. I fully realize there are advantages to consuming electrolyte-laden drinks such as Gatorade but I'm just talking subjective thirst. As I mentioned, diet soda is mostly carbonated water, keyword, "mostly". So if you have 12 oz of diet soda and 12 oz of water, assuming the water is reasonably purified, the water should have a lower osmolality. Here is a table I found from this online paper from a crime lab.
As you can see diet soda has a higher osmolality than tap water, so IMO, it would not quench your thirst as well. However, tap water does not contain Na and K which you excrete and need to replace during hard athletic adventures to keep that Na/K pump system up and running, so Gatorade/sports drink might be the better choice while you are performing, even if it doesn't subjectively quench your thirst as much. This is a nice piece on Gatorade and the whole subjective thirst vs. rehydration argument. In fact, the Na in Gatorade is actually supposed to prompt you to drink more so you get beyond simple mouth thirst and get more rehydrated. To put this in simple terms, 8oz of water might make you feel less thirsty than 8oz of Gatorade but you really need 12 oz to get rehydrated so the fact that the Gatorade leaves you wanting more is better in the long-run. Let's be real though, we're splitting hairs, if we're thirsty, an ice cold Gatorade seems to do just fine. BUT, what if we have an obese pt that is blasting through a hard workout; should we encourage them to consume a calorie filled beverage like Gatorade? Do the electrolyte replenishment benefits outweigh the caloric costs? I say "no way". Give them water or even a Crystal Light. I don't even have an issue with diet soda as long as the carbonation didn't cause GI stress. Like I said, it all depends on your goals.
Basically under the realm of both points that while Diet Soda (from my point of view) doesn't necessarily directly cause weight gain but rather inhibits the consumption of better alternatives and good nutrient practices to ensure an efficient metabolic process. Thus I refer to the People's Dietitian to set the record straight. Great post though.
The "good nutrient practices" and "efficient metabolic process"are broad strokes. Entire textbooks are written on metabolic processes and some people study a few enzymes and their effects on certain parts of various metabolic processes as their entire career! Again, this comes down to "What are your goals?". For example, please provide me with a better alternative than a zero calorie beverage for an obese pt. Counterpoint: the anorexic model that drinks Diet Coke all day to stave off hunger and is no doubt malnourished, is not doing herself any favors with her diet soda habit. This runs much deeper than diet soda though and is essentially the "guns don't kill people" argument.
Thank you for reading the blog and I hope I answered your questions satisfactorily.
Beverage Osmolality Database [mOsm/kg]
Water; Purified, Mineral, Tap
Diet; Sodas, Teas
Brewed Coffee [Black]
Sugar Containing Sodas, Fruit Drinks
Energy Drinks; Red Bull, etc