Fructose sensitivity is a condition most of us may be experiencing to some degree due to the huge amount of fructose that has entered the food chain in the form of HFCS (High Fructose Corn Syrup). Corn syrup (HFCS) is added to most foods that are sweet in the USA, and table sugar (sucrose) is broken down to glucose and fructose.
Humans have a limited ability to metabolize fructose. Our ancestors likely came across fruit and berries on occasion, but we were not designed metabolically to handle large amounts of fructose. The fructose that we eat (HFCS, sugar, fruits) is rapidly absorbed and metabolized mostly in the liver. Fructose provides both the glycerol and the acyl portions of triglyceride, thus stimulating both triglyceride and VLDL (Very Low Density Lipoprotein) synthesis. In addition to reduced hepatic insulin sensitivity, fructose does not affect leptin and insulin in a way to reduce appetite. Add to this our high fat and high cholesterol diet, we have a huge issue with dyslipidemia (high cholesterol, LDL, VLDL, and low HDL) coupled with insulin resistance resulting in metabolic syndrome, pre-diabetes and the development of diabetes, heart disease, stroke, etc.
The accumulation of fructose in the cells (fructose-1-phosphate) depletes ATP in some of us, which interferes with protein, DNA and RNA synthesis, cAMP formation and reduced ammonia detoxification, with resulting elevations in lactate, uric acid, triglycerides. Gout and cardiovascular disease can result.
SYMPTOMS OF DEFICIENCY
Fructose intolerance is more a result of excess than a single nutrient deficiency. Symptoms might include fatigue, headaches, weakness, dizziness, behavioral changes and a depressed immune function. Over time, elevated triglycerides can lead to heart disease, elevated uric acid can lead to gout, insulin resistance, and the metabolic syndrome can develop and interference with copper metabolism has been described.
If you are pre-diabetic, suffering from the metabolic syndrome, you may notice that after a large dose of fructose, sugar, or processed carbohydrate, within an hour or two you get shaky, or feel faint and dizzy. What has happened is that your body, due to insulin resistance, must pump out more insulin than usual, to get the blood sugar back down. Your blood sugar then over-shoots the mark on the way down and it is the low blood sugar that causes your symptoms. This also explains why so many of us feel the urge to eat more carbohydrates (carbs) all the time. You feel hungry or shaky, so carbs work fast and best, but they also trigger the higher insulin production, we make too much insulin, drop our blood sugar too low and repeat the cycle over and over.
The consumption of fruits in their natural form is rarely an issue, and the benefits of numerous micronutrients from fresh fruits far exceed the rare likelihood that you are eating too much fructose. If you are experiencing obesity or the metabolic syndrome or insulin sensitivity, you should limit your fruit consumption to one or two fruit servings a day. The problem is typically with HFCS, syrups, corn syrup, fruit juice concentrates, sucrose (table sugar), and prepared foods that have these ingredients.
Given that 1/3 of the USA population is obese and 2/3 of us are over weight, we should all avoid these nutrient depleted forms of sweeteners.
And NEVER NEVER, EVER consume aspartame (diet sodas and gum etc.).
SAFETY AND TOXICITY
Except for the symptoms of low blood sugar described, and the horrible long term effects of being insulin resistant (metabolic syndrome) and obese, there are few immediate toxicity symptoms of eating too much fructose. This may be why the food industry with their short term studies, can claim that this nutrient is good for you.
There is no RDA or Tolerable Upper Limits set for fructose.
Diagram from reference below
Here are couple links that you may find interesting: