I wonder why is it that some people who are very concerned with fructose do accept vegetables as sweet potatoes or beets whose sugar content is high and is mostly sucrose...
Am I missing something?
Yes, you're missing the fact that sweet potatoes have less fructose than almost all fruits.
You have to look at the actual numbers.
To calculate the total amount of fructose, we add half the sucrose to the free fructose.
Sucrose / 2 + free fructose = total fructose
Here's how we apply that formula to sweet potatoes. According to the USDA database, sweet potatoes contain 2.52 g sucrose and 0.70 g free fructose per 100 g. Here's the arithmetic.
2.52/2 + 0.70 = 1.96
This means sweet potatoes contain 1.96 g of total fructose per 100 g.
This is less total fructose than almost any common fruit. The only common commercial fruits that contain less total fructose are cranberries and limes, and people don't like to eat them because they are sour.
Let's put sweet potatoes in perspective by comparing them with fruits. The numbers show total fructose g per 100 g.
(I didn't include beets because the USDA database doesn't show how much sucrose and fructose they contain.)
FOOD TOTAL FRUCTOSE ---------------------------- Apricots 3.87 Apples 6.93 Bananas 6.04 Blueberries 5.02 Cantaloupe 4.04 Cherries 5.44 Grapefruit 3.02 Grapes 8.20 Kiwi 4.42 Mangos 7.16 Papayas 3.73 Pears 6.62 Pineapple 5.11 Raspberries 2.45 Strawberries 2.73 Sweet potatoes SWEET POTATOES 1.96 Watermelon 3.90
Sweet potatoes don't look so bad now, do they? :)
The reason I didn't mention beets in this answer is because I can't find data anywhere on the web about its sugar composition. (I'm talking about red beets, the kind that people eat. Plenty of info is available about sugar beets, but that's a different thing.)
Source: USDA database