It's difficult because for *some* people that might be fine. But for many others that advice might be a
complete disaster. Aside from a little vitamin C there is little in a potato apart from starch. A form of starch that breaks down to glucose in the bloodstream faster than table sugar or golden syrup in most people. It is unlikely that the nurse you spoke to would suggest that you ensured you sprinkled your meat and vegetables with a good few tablespoons of sugar and a squeeze of lemon, but in a sense that was the suggestion.
The 'eatwell' plate that I think you were shown is based in no small part on the 'food pyramid' which came out of America in the 50s as was driven by the USDA (that's department of agriculture, not health). This may be absolutely fine for people with a fully functioning metabolism, but raises some difficult questions for people who have a problem digesting carbhydrate properly. Part of the initial idea was that fat in the diet was directly linked to heart disease (a notion that is increasingly undermined more recent research). Many Drs and nurses still recommend very high carb intakes for people with diabetes despite knowing that this will have a disastrous effect on blood glucose levels. We can try to go for carbs which absorb more slowly, but almost all bread and breakfast cereals, for example, will absorb at a very similar speed to granulated sugar.
For my money 'everything in moderation' is the key. I have difficulty (not least practically/socially) with extremely low carb approaches. But
moderate to low carb 'eat to your meter' methods seem absolutely logical.
It is recommended that as as adult I eat 250-300g of carbs a day. I
could not eat that much with any regularity without gaining a significant amount of weight and having extremely erratic blood glucose levels, which would do my long term health no good whatever, no matter how little fat I was trying to eat at the time.
This page makes interesting reading too:
http://www.drbriffa.com/2012/03/05/...y-advice-diabetes-uk-dishes-out-to-diabetics/