I'm not sure exactly what in the books you are referring to. But I think it may be the fact that we need 10g a day of fructose to assist glucose to penetrate the cell walls. Any in excess of that goes directly to the liver where it is converted to fat.
You can get the 10g from two pieces of fruit AND you get the numerous benefits if the fibre in the fruit, including amelioration of the effects of fructose.
Cane sugar is 50% fructose so a teaspoon (5g) contains about 2.5g of fructose. So, including the sugar in all the processed food we eat, you can see that it doesn't take much to reach the necessary 10g per day. That's why it is safest to avoid it all together, as far as is reasonably possible.
