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When cooking for diabetics it is important to keep the amount of carbohydrates and sugar low. So I am wondering how cooking food - particularly vegetables - changes the amount of carbohydrates and sugar.

For example when simmering tomatoes slowly for a long time they gain a lovely sweet taste - but does really the amount of sugar increases? And if so, does the amount of other carbohydrates decrease proportionally? Or do slow cooked tomatos become through cooking more dangerous to diabetics?

How to calculate it? Are there lists somewhere available showing carbohydrate and sugar content for vegetables in raw an cooked state?

ps: I did find a similar question about onions - From the answer I deduct that in the case of onions the sweeter taste is a perception effect not a real change in sugar or carbs.

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    I would like to point out that we have a strict policy where questions about health are off topic. This question is on topic, but to answerers: please do not write answers which go beyond the literal discussion of chemical changes and start discussing what is dangerous for diabetics and what isn't. Also, halloleo: you may want to try finding more information elsewhere, as there are aspects which are relevant, but we cannot cover them here. – rumtscho Sep 9 at 13:22
  • Besides what was mentioned in the onions question, heat can change the composition of sugars in food, so you might end up with sweeter tasting compounds ... or less sweet, depending on what you're cooking, and how far you cook it. The real question for diabetics is if the changed sugars have a different absorption rate into the bloodstream. Although health questions are generally frowned upon here, I would think that asking how the glycemic index of foods changes with cooking is still cooking. – Joe Sep 9 at 15:34
  • @rumtscho I didn't want to give the question a particular health angle; I'm just interested in the nurtitional changes through cooking (which of course in turn might inform health-related cooking decisions...). – halloleo Sep 11 at 7:10
  • @Joe To reply to the health-related part of your comment: %-) I am loosely aware of the GI index, but I do think the absolute carbohydrate/sugar intake is important too. – halloleo Sep 11 at 7:14
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This is a rather general question, and I will try to answer it in a general manner.

First, the total amount of carbohydrates will not change, or maybe there will be a neglible reduction due to things like burning tiny bits of surface carbohydrates to carbon, or cleaning the vegetables.

Second, the ratio of all carbohydrates to everything else can easily change, due to evaporation. So if you start with a vegetable that is 10% carbohydrates and 90% water, you can end up with one that is 30% carbohydrates and 70% water, rounded (the numbers I am using are pretty arbitrary, the real range is very large). It is easy to find out the direction of change yourself (the carbohydrate density will go up if you evaporate water, and down if you add water), but calculating the actual amount can be a pain. You'd have to know the initial density to start with, and then monitor the initial and final weight of the food.

Third, the ratio of sugar to nonsugar carbohydrates can change. It will always go from more complex to less complex carbohydrates - from polysaccharides to oligosaccharides to monosaccharides. So you can sometimes produce conditions under which starch turns into glucose, or sucrose turns into glucose+fructose, although this won't happen every time you cook. I cannot think of any cooking process that will convert sugars in the other direction. The magnitude of that conversion will vary wildly in practice, depending on the very specific chemical conditions in your cooking pot.

Fourth, there is another effect, but it is likely to be very small in magnitude. The browning of food happens during the Maillard reaction, which is a family of reactions between a carbohydrate and an amino acid. While it is best known from meat, where I think it involves mostly glycoproteins from the animal cells themselves, vegetables brown too, and I suppose some carbohydrates from the vegetables "disappear" in this kind of reaction. It is a thing that happens mostly on the surface of roasted food, and in a specific temperature range, so while it contributes a lot to the aroma, I don't think it actually converts much carbohydrates away.

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    Wow, this makes all total sense. Thank you! It still would be nice to have a table with some indication how much carbs are in 1kg uncooked tomatoes and to how many grams they condense to after, say, 1 hour of cooking - assuming all/most carbs/sugar stay in the tomatoes. – halloleo Sep 9 at 13:32
  • @halloleo you can of course go with standard nutrition data, the usda has a database of it. It will have values for the raw vegetables for sure, and then you can try comparing to values for canned vegetables. But they don't publish actual ranges (e.g. they may have 5-6 samples of X having from 3 to 28 g carbohydrates, but just publish the arithmetic mean, which can be, say, 7 g) and you cannot be sure how well your cooked food approximates the canned food they measure. – rumtscho Sep 9 at 13:38
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    I think 'you can easily have starch turn into glucose' is not really correct. It depends on the presence of suitable enzymes, in particular amylase. A related question was answered (by you!) here: cooking.stackexchange.com/questions/95592/…. – Mark Wildon Sep 9 at 14:11
  • @MarkWildon thank you for catching that - indeed, it is not all that common, so it was very misleading wording. Does it read better now? – rumtscho Sep 9 at 15:23
  • Yes, it's now much more accurate. (+1). – Mark Wildon Sep 9 at 15:36

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