Fructose can only be processed by the liver. In small amounts and along with fibre (eg, in a handful of fruit that is not very sweet, like autumn berries) fructose does not cause a problem… It is, however, also found along with glucose in table sugar and high fructose corn syrup. In large amounts (especially if ingested over a short period of time in fruit juices or beverages/foods containing high fructose corn syrup or sucrose), it:
- Causes stresses on the liver; the excess fructose can also be made into fat, and is stored locally as harmful hepatic fat or visceral fat
- Can be shipped off to fat cells, causing an increase in serum (blood) triglycerides in the process
Fructose in more detail
To understand why added sugars (either as sucrose, fructose or high fructose corn syrup – HFCS) are a problem, we need to see what happens to fructose. Fructose does not stimulate an insulin surge and so bypasses one of insulin’s properties – that of triggering a feeling of satiety.
(Insulin, when working propely, is a good hormone. In diabetes, the process goes wrong.)
Fructose also decreases leptin levels and increases ghrelin levels, thus making a person feel much hungrier than they would have done if they had eaten the same calorific amount of glucose, protein or fat.
Studies show that people (and animals) who consume large amounts of sucrose (28% of energy) have an increase in energy intake, body weight, fat mass, and blood pressure.
In addition:
- Fructose is almost entirely processed by the liver
- Fructose, when eaten as part of a whole vegetable or fruit, gets delivered to the liver a little at a time
- Fructose, when part of processed foods (either as pure fructose or in HFCS) reaches the liver too much at a time and too much over a day, so its delivery is both acute and chronic
- The liver uses some fructose to make glucose and glycogen, and turns excess fructose into fatty acids and triglycerides, some of which are stored in the liver and the rest of which are released into the blood stream
- Too much fructose means:
- The liver is left with excess fat* to store and non-alcoholic steatosis can result
- Insulin resistance and in compensatory hyperinsulinaemia develop
- Increases in blood cholesterol when coupled with a copper deficiency
- A combination of a high-fat diet with fructose results in even more increased circulating triacylglycerol (which does not happen when starch is combined with a high-fat diet)
- Where fructose is not absorbed, it can disrupt healthy gut bacteria populations
*Fructose in higher doses bypasses the major control point by which glucose carbon enters, so whereas glucose metabolism is limited by feedback inhibition by citrate and ATP, this doesn’t happen with fructose meaning the fructose to serves as an unregulated source of both glycerol-3-phosphate and acetyl-CoA for hepatic lipogenesis. Thus, fructose is more lipogenic than is glucose, and studies suggest the effect might be exacerbated in people with existing hyperlipidaemia, insulin resistance or type 2 diabetes.
Even worse, the more fructose a person eats, the more the transporters that allow for fructose uptake in the gut are turned on, so more fructose is absorbed. Studies show lean children tend to only absorb about half of the fructose they consume (and they burn it up), whereas obese children who have fatty liver disease absorb close to 100% and, by also having more fructases, metabolise it more effectively.
It’s also thought (and is definitely the case with mice) that very carb-sensitive people also covert some glucose to fructose in their livers, leading to the problems outlined above.
What to do
Avoid sweet fruits (they have the most fructose) and have sourer fruits (such as blueberries) in moderation – more as a treat than a must-have part of a meal. Do not drink fruit juices and increase the intake of leafy vegetables. Absolutely do not eat processed foods or drinks where fructose has been added. Don’t add sugar to cereals, strawberries or beverages. If very sensitive to carbohydrates drop all sugars and starchy foods until the body is back in kilter.