It is important to note that fructose has 3 metabolic outcomes: 1) Most is turned directly into glucose by gut organs that have an enzyme that turns fructose into glucose. It is then released as glucose with a slight delay (as the enzymes work). 2) Most of the rest goes straight to the liver and gets turned into glycogen. It is estimated that about 1/3 of the glycogen burned during exercise can be replenished from fructose. At least 25 grams of fructose plus 1/3 of carbs burned during exercise can be managed by the liver each day. 3) Excess fructose (as in 64 oz soda, or in some cases where I know people who live on basically 750 grams of fruit “carbs” a day (about 300-350 net fructose) will be turned into triglycerides and liver fat. This will lead to metabolic syndrome, insulin resistance, and progressively increasing blood triglycerides levels. Also at that very high level, there is some fructose spillage into the blood, maybe 1-2 mg/dl. That is small compared to say 100 mg/dl of glucose in the blood but fructose has 10x the capacity to cause glycosylation of molecules that end up in blood vessels, kidneys and the retina and brain. That is enough to raise one’s A1C by .3-.6% which is significant, for example someone with a blood sugar that would net an A1C of 5.5 can have a 5.8-6.1 (borderline diabetic) A1C if they consume more fructose than the liver can process.
One thing that many nutritionists don’t understand is that triglycerides are made largely from fructose because it is less “disposable” and more harmful in the blood than glucose. If your triglycerides are high, it is not from eating fat, it is from either overeating fructose/sugar or from being insulin resistant (and overeating fructose causes insulin resistance by turning into liver fat (ie fatty liver). The effects are compounded by alcohol consumption.
There has been no demonstrated rise in A1C or triglycerides from less than 4% daily calories from fructose which would be about 25 grams on a 2500 calorie diet. Also, again, about 1/3 of carbs burned during exercise can be replaced with fructose (it clears room for liver glycogen).
Most fruit contain are about 50% of their carbs in the form of fructose. A banana, which has more starch gets about 40%, so a 25 carb gram banana will only have about 10 grams of fructose, meaning that even a sedentary person on a 2500 calorie diet can have 2.5 bananas a day without reaching the level where there is any evidence of higher triglycerides or A1C. An hour of mild aerobic exercise (300 cals) will also burn about 25 grams of carbs (the rest from fat) and about 1/3 of that can be readily replenished from fructose as liver glycogen before there may be an excess that is turned into triglycerides or nets fructose leakage into the bloodstream. At higher exercise intensities, the amount of carbs burned rises dramatically and makes it very hard to overeat fructose. An hour at 600 cals an hour for example is burning about 75 grams of carbohydrate which would allow 1/3 x 75=25 more grams of fructose a day (another 2.5 bananas).
In comparison though, a “Super Big Drink” soda bay have 250+ grams of sugar and up to 150 grams of fructose (HFCS is 55-60% fructose). To manage that much, you would need to exercise off about 600 calories an hour for 6 hours a day. Sugar/fructose IS a major dietary problem, but fruit is not.
Note that alcohol and fructose are additive in their effects because they require some of the same enzymes and both get turned into triglycerides and liver fat. An alcoholic beverage has about 15 grams of alcohol, more than the amount of fructose in a banana. 2 alcoholic drinks a day will put a sedentary person into production of liver fats and triglycerides. Again, it can be made up for with activity around the time that the beverage is consumed.