BTW you can still buy Tab in Texas. I bought a 6 pack out of nostalgia - could not drink it - it tasted vile.
He's probably confusing type 1 diabetes with something else. Phenylketonuria, at a guess. People who can't process phenylalanine can have a bad reaction to diet sodas that are made with certain sweeteners. And yeah, diet soda is bad for you — all soda is bad for you, and guitarnut is right about what the phosphoric acid does to bones, which in diabetics is a concern because diabetes ALSO is bad for bones. But specifically in regard to blood sugar, it's not anything to worry about.
So in answer to your question, he's talking out his a**... but that doesn't mean he's necessarily wrong, it IS better to drink water than diet soda. If you have to have something other than tap water (and I hear ya, I've been spoiled by living near Poland Springs, our well water is better than bottled water so I can't drink tap water except at home) then maybe buy seltzer instead? That way you get the fizz.
I know that's a popular theory, but when talking about insulin & blood sugar levels, physiologically it doesn't hold up, because insulin release isn't predicated on what the brain says is coming — it's predicated on the amount of ACTUAL glucose present in the blood. If the glucose isn't passed into the bloodstream (because it's not there in the soda sweetened with artificial sweeteners), then the pancreas doesn't increase its release of insulin. I haven't enough information about how ghrelin (hunger hormone) and neurotransmitter levels could be affected to say one way or the other if either of those ARE affected, but insulin and blood glucose levels aren't. The brain doesn't need insulin to uptake glucose, so eating "phantom sugar" would not trigger it to signal a need for glucose for that reason, as long as blood sugar isn't already low.
Actually, that's not quite true. The relationship between insulin/blood glucose and emotions is significant--and bi-directional. And there's certainly a link with stress hormones, both emotionally and physically--blood sugars run higher when people get less sleep. Saying "the brain doesn't need insulin to uptake glucose" is not the same as saying the brain doesn't need insulin. In the August 2012 issue of Diabetes Forecast, on page 51, it says "Most people are taught that the brain is an 'insulin independent' organ, says William Klein, PhD, professor of neurology at Northwestern University. That means the brain does not need insulin to fuel its cells. Yet insulin is still critical to brain function. 'Insulin seems to play a role in learning and memory,' says Klein. (the article is about the connection between dementia and diabetes) If insulin can't do its job elsewhere in the body, blood glucose levels increase, leading to diabetes. If insulin can't do its job in the brain, learning and memory may be compromised, leading to Alzheimer's disease or, as some experts are calling it, another type of diabetes." ("Diabetes on the Brain," Erika Gebel, PhD, Diabetes Forecast, August 2012)
It's a different issue, but the article is pretty depressing. "People with diabetes are twice as likely to develop Alzheimer's disease as those without diabetes." They are not saying that diabetes CAUSES Alzheimer's and refer to a 2011 study published in The Lancet that "found that intensive blood glucose (with a very tight target A1C of under 6 percent) failed to prevent dementia."
I know I've gone far afield here, but I think it's clear that we really know very little about triggers and relationships and are safer saying we don't know than thinking we do.
Even if the anticipation of eating something sweet somehow triggered insulin release, it would be insignificant or absent in a long term type 1 diabetic whose insulin producing cells are almost all destroyed I.e. a c-peptide negative diabetic. This fact does not negate he possibility that a diet soda might tempt you to add something with carbs in it.
Researchers at Rhode Island Hospital and Brown Medical School have discovered that insulin and its related proteins are produced in the brain, and that reduced levels of both are linked to Alzheimer's disease. The findings are reported in the March issue of the Journal of Alzheimer's Disease (http://www.j-alz.com), published by IOS Press.
"What they found is that insulin is not just produced in the pancreas, but also in the brain. And discovered that insulin and its growth factors, which are necessary for the survival of brain cells, contribute to the progression of Alzheimer's," says senior author Suzanne M. de la Monte, a neuropathologist at Rhode Island Hospital and a professor of pathology at Brown Medical School. "This raises the possibility of a Type 3 diabetes."