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I've seen this identical statement quoted in several non-technical publications, without any citation other than the author. For instance:

“Processed sugar is a major source of inflammation in the diet, and it is wreaking havoc,” says Renae Norton, a specialist in the treatment of eating disorders and obesity. “When you eat sugar, you deplete the enzymes that help you to digest protein. So the protein gets into the bloodstream as a partially digested protein, and is attacked by the immune system.” — The Inflammation Epidemic: Your Number One Health Concern (And Sugar's Role In It)

Is this true, and if so, how significant is it and what are the actual details of the metabolic process involved?

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    Sugar is sugar, processed or not. It's a simple chemical. Processing just removes additional chemicals (flavours), so you end up with pure sucrose. Sucrose may or may not have the claimed effect, but processing does not affect it. – hdhondt Aug 25 at 0:38
  • @hdhondt While that's correct, here it most probably translate to 'too much (added) sugar', and that whole sentence seems largely correct, apart from your observation But the central point is really everything in the 2nd & 3rd sentence. There are many grains connected to truth in it, grains like in sand, which forms the foundation of this pile? – LаngLаngС Aug 25 at 8:05
  • The same author is about to release a book on the matter and here she talks remarkably similar yet strikingly different about the subject. – LаngLаngС Aug 25 at 8:45
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In short: There is no evidence to claim that high intake of refined (or other) sugars impairs the digestion of proteins, but may stimulate inflammation via other mechanisms.

The most commonly used refined (added) sugars (glucose or dextrose, fructose, HFCS, invert sugar, corn syrup) are monosaccharides, which do not need to be digested, so they do not require or "deplete" any digestive enzymes; sucrose is digested by the enzyme sucrase and maltose by the enzyme maltase (Medical LibreTexts).

Proteins are digested by other enzymes that have nothing with the digestion of sugars: pepsin, trypsin, chymotripsin and peptidases, and whole or "partially digested proteins" are, in general, not absorbed (VIVO Pathophysiology).

Partially digested proteins and certain toxins may be absorbed in "leaky gut syndrome," a pathological condition with increased intestinal permeability, but there seems to be no evidence that sugars would contribute to it.

Dietary sugar may increase inflammation:

There is some evidence that high intake of refined sugars may increase the risk of inflammatory bowel disease (IBD):

high pre-illness (IBD) intake of refined sugars and low fiber has been observed in numerous studies [4, 21, 43] but two large observational cohorts found no association between baseline sugar intake and IBD [22, 44] (Indian Journal of Gastroenterology, 2018).

Also:

It has been postulated that dietary sugar consumption contributes to increased inflammatory processes in humans. Central to the potentially relevant mechanisms is the fact that dietary sugar promotes de novo synthesis of free fatty acids (FFA) in the liver [17,18,19], which according to the lipotoxicity theory, would produce FFA metabolites that may trigger inflammatory processes and reactive oxygen species (ROS) formation [20,21] (Nutrients, 2018)

Diet rich in saturated fat, trans-fats, or refined sugar is associated with higher production of pro-inflammatory molecules, especially in individuals with diabetes or overweight individuals (StatPearls, 2019).

  • Not that it changes the relevance of your answer, but fructose (HFCS) does have a different digestive process. All the other sugars (and starches) eventually end up as dextrose (glucose), which provides immediate energy to the body's cells, while most of fructose ends up being converted to triglycerides and glycogen. (Even more off-topic: triglycerides are the basic component of fat, and fructose doesn't alleviate hunger, which is why high-fructose diets (most sweetened beverages) tend to lead to obesity.) – Ray Butterworth Aug 26 at 16:14
  • Why the focus on "refined"? Only dates and apples are problematic as well. But more important: Is she conflating things? Co-transport, proteases, glyco-proteins, 'competition' instead of "depletion"…? Currently, this is a tangential. Can you attack this more directly? – LаngLаngС Aug 26 at 17:10
  • @LangLangC, Why does she focus on refined sugar - I don't know, but it's popular to do so. I just went along with the claim, but I did not limit my search to refined, just to sugar in general. I don't have a feeling that "co-transport, proteases, glyco-proteins and competition" are relevant for concurrent absorption of either proteins or sugars, at least not in this context. She does not mention it, but she obviously refers to "leaky gut syndrome," which could result in the absorption of partially digested proteins. As said, I haven't found any association between sugars and this syndrome. – Jan Aug 26 at 17:26
  • No. "Leaky gut" is solely your interpretation. And a quite deliberate one, I'd say. But I was merely spitballing what might be informing her complex theory – not what's 'real', textbook knowledge. It has to be some kind of mixup, and I guess we first need to reconstruct its origins before dismantling it? – LаngLаngС Aug 26 at 17:42
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    @LangLangC, I don't know why she has connected sugar and protein absorption - I'm not aware of any significant interactions, so I don't feel my guesses would help. On the other hand, it's completely obvious to me that by "absorption of partially digested proteins" she refers to leaky gut syndrome, even if she does not do this consciously. The absorption of whole or partially digested proteins that trigger immune response is in the center of leaky gut syndrome idea. I havn't found any evidence that sugars would increase the risk of the syndrome. – Jan Aug 26 at 17:51

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