Fructose is metabolised by glycolysis in the liver, same pathway as glucose. Glucose enters this pathway before the rate-determining step, whereas fructose enters it after the rate-determining step. This means that fructose speeds up glycolysis much more than glucose does. There's lots of clever stuff evolved to prevent cells frying themselves by futile cycling (significant reactions in both directions on a pathway). The reverse of glycolysis is gluconeogenesis, whereby glucose is synthesised from pyruvate derived from amino acids (from protein) or glycerol.
If a lot of fructose is absorbed by the liver this tends to speed up glycolysis with excess pyruvate having nowhere to go but be converted to fatty acids that are turned into triglycerides. These are the triglycerides associated with cardiovascular disease in people on SAD consuming too much sugar (sucrose and high fructose corn syrup).
Someone on a low carb diet will have some gluconeogenesis going on - eating significant fructose in one sitting can obviously interfere with this.
Another problem with fructose speeding up glycolysis is that can use up a lot of ATP, causing a shortage of energy for other metabolic and catabolic processes. A cell has to invest some ATP molecules in order to eventually get a lot more back when the end product of glycolysis is oxidised in the mitochondria. ATP is the universal energy currency of the body.
Because fructose is so toxic in quantity it's absorbed from the gut much more slowly than glucose and other nutrients. This can contribute to candida overgrowth and gut dysbiosis.
Fructose is much more carcinogenic and inflammatory that glucose as much higher percentage of it exists in the highly reactive open chain form than the ring form. Hence fructose damages important enzymes in the liver. The liver usually has to prioritise detoxing fructose over everything else.
Modest quantities of fructose in fruit and vegetables are usually tolerated OK. Toleration levels vary. A lot of the low-carbers report that fruit sets off sugar cravings. I'm not surprised.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.