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- High fructose diet affects some people more than others.
- Fructose is harmful when consumed from some sources but not from others.
- High fructose food sources.
- Is fructose toxic?
- Why is fructose bad for us?
- Positive effects of fructose.
- Medical conditions that may arise from the excess fructose consumption.
Foods high in fructose, such as refined sugar products, sugar based sodas (soft drinks) and fruit juices, when consumed frequently, have harmful effects in most people.
High fructose consumption is the key factor leading to metabolic syndrome and increases the risk of obesity, type 2 diabetes, cardiovascular diseases, gout, non-alcoholic metabolic liver disease, and some forms of cancer.
However, when consumed in the form of whole fruit and vegetables, it does not present a health risk for most people. In this case, it is beneficial for health.
NOTE: Hereditary fructose intolerance. This article refers to people with no fructose related diseases such as hereditary fructose intolerance. (1)
High fructose diet affects some people more than others
For an athletic, insulin sensitive person on a balanced diet, the effects of fructose metabolism may be negligible. However, for an obese person, who is insulin resistant and with a sedentary lifestyle, the high fructose diet may have severe health consequences.
It should be noted that genetics also plays an important role in how people react to fructose.
While some studies may have shown a negligible impact on some healthy, slim and insulin sensitive individuals, we should be more concerned about the effects of fructose on the overall population, most of which (over 60% in U.S., UK, Canada or Australia) is overweight and to whom the negative effects especially apply.
In summary, the health effects of fructose depend on:
- level of overweight;
- level of insulin sensitivity;
- fasting / glycogen depleted state (e.g. after a marathon, the glycogen energy reserves would be exhausted);
- other medical conditions;
- level of physical activity.
Fructose is harmful when consumed from some sources but not from others
- Fructose from healthy sources.
Dietary fructose in the form of whole vegetables and fruit, as a part of a balanced diet, doesn’t cause any harm due to the relatively low contents of fructose and high amounts of other nutrients that accompany it, such as fiber.
Whole fruit and vegetables have a beneficial effect on our health. (2)
- Fructose from refined carbs and high fructose sweeteners.
Concentrated fructose in refined carbohydrates (see examples below), on the other hand, put enormous stress on our liver and have a negative effect on our health.
The reason is that the metabolic process of an excessive amount of fructose in the liver produces various harmful substances such as uric acid and triglycerides and has a damaging effect directly on the liver and the entire body. This applies especially, but not exclusively, to overweight and obese people.
- Whole fruit and vegetables are the healthy sources of fructose.
- Fructose is harmful in high concentrations, especially from sweeteners and foods and drinks containing refined carbohydrates.
High fructose food sources:
- refined sugar products (e.g. table sugar, candy or sweet pastry);
- fructose-based sweeteners (e.g. maple or agave syrup);
- soft drinks containing sugar (e.g. Coke or Fanta);
- fruit juices (all juices extracted from fruit, with fiber removed resulting in the presence of a high amount of fructose and a medium to high GL.).
Is fructose toxic?
Here is a definition of chronic toxicity taken from Harrison’s Principles of Internal Medicine.
NOTE: it should not to be confused with Acute Toxicity. (3)
“TOXICITY: The degree to which a substance can harm humans or animals.
Chronic toxicity is the ability of a substance or mixture of substances to cause harmful effects over an extended period, usually upon repeated or continuous exposure, sometimes lasting for the entire life of the exposed organism. “
Any substance can become toxic if it is consumed in excessive amounts, even water.
It is not very common for us to over-consume certain nutrients (e.g. selenium) but when it happens we can get seriously sick.
Fructose is not a toxin per se, although we can live without it. However, it is an unusual nutrient, since it is particularly easy to over-consume due to a number of reasons:
- It tricks our mind into thinking that we are still hungry by acting on our hunger and satiety hormones; (4)
- Fructose consumption is linked to an addictive-like behavior, although science is still not clear whether fructose consumption can be considered as addictive in a strict dependency sense or only habitual; (5)
- It is involved in the vicious cycle of obesity where the excess of fructose in the diet leads to weight gain and weight gain leads to craving for more fructose (more on this below).
- It is available in a concentrated form such as in sweeteners, soft drinks or fruit juices, which allows us to have a lot of it in one sitting;
- Clever/misleading marketing – we are led to believe that since fructose is the “healthier sugar” and that fruit juices are healthy, the servings are excessively large.
These points explain why it is common that after drinking a bottle of Coke, the appetite is not suppressed but actually increases.
- Overall, an occasional small dose of fructose causes no harm.
- There is an overwhelming number of harmful side-effects of excess dietary fructose intake, making it toxic.
- It is easy to over-consume fructose.
Why is fructose bad for us?
The following is the list of known effects of high fructose consumption (6):
- Energy production
Part of the fructose consumed is turned into energy by the liver cells. (7)
- Glucose production
Part of the fructose is turned into glucose. The proportion of that conversion depends on fitness, health level and on gender. Part of this glucose is stored as glycogen. (8)
- Visceral fat accumulation
Fructose metabolism (similar to alcohol but in contrast to glucose) promotes the accumulation of fatty acids in the abdominal area, resulting in visceral obesity.
Visceral fatty tissue is located around the organs and its visible signs are commonly referred to as “beer gut”, since it is usually associated with alcohol. (9)
Visceral fat is associated with metabolic syndrome, cardiovascular diseases and type 2 diabetes. (10)
- Uric acid production
Fructose intake rapidly increases the production of uric acid (by the liver), which reduces the production and availability of nitric oxide.
Nitric oxide (NO) protects the organs (e.g. liver) from vascular damage, increases the blood flow by widening blood vessels (vasodilation) and reduces blood pressure in people with hypertension. In order to be able to perform its function of distributing glucose to the muscle cells, insulin needs nitric oxide to increase the blood flow to these areas.
If the nitric oxide is reduced, insulin action is affected. The result is that there is a circulating insulin that cannot do its job which is referred to as “insulin resistance”.
- Triglyceride production
Part of the fructose that is metabolized in the liver is converted into triglycerides, leading to increased levels of triglycerides in the blood, a condition called dyslipidemia/hyperlipidemia. This affects those who are overweight and obese more than athletic and slim individuals (9)
- “Bad cholesterol” increase and “good cholesterol” reduction
Excessive fructose consumption increases the amount of atherosclerosis’ forming particles: cholesterol, apoB, LDL, small dense LDL and oxidized LDL, and reduces HDL lipoproteins, which leads to heart disease. (10, 21)
- Increase of VLDL particles
Excessive fructose leads to elevated triglycerides production which, together with cholesterol, are packaged inside VLDLs particles and released to the bloodstream.High levels of VLDLs are associated with metabolic syndrome and cardiovascular diseases. (10, 17, 18, 19)
- Increase of insulin and insulin-like growth factors
Insulin resistance is associated with an increased risk of colon cancer and metabolic syndrome. Metabolic syndrome is linked with cancers such as: liver, colorectal and bladder cancers in men and endometrial, pancreatic, breast postmenopausal, rectal and colorectal in women. (7, 24, 25)
- Increase amount of blood glucose
Fructose increases fasting blood glucose (measure of blood glucose after a period of fasting) which leads to insulin resistance. (10)
- Increase in lipid droplets in the liver
Part of the lipids produced from fructose remains in the liver and are called lipid droplets. These particles cause a non-alcoholic fatty liver disease.
- Increase part of fructose not absorbed
The amount of absorbed fructose by the body is limited. The unabsorbed fructose passes to the colon where it is fermented by gut bacteria.
- Increase leptin production
Leptin is a hormone generated by fat cells and signals our brain to stop eating. The more fat that our body has, the more leptin is produced. Too much leptin in circulation leads to leptin resistance. Since excess fructose leads to obesity, the resulting increased leptin production leads to leptin resistance. Leptin resistance is also caused by another effect of fructose: insulin resistance.
- Production of inflammatory substances
“JNK1” is a substance produced in the liver from fructose breakdown that causes inflammation.Chronic inflammation in the liver is associated with insulin resistance in the liver but also as secondary insulin resistance in muscles, and fatty cells. (7, 35)
- Long term high fructose consumption affects many physiological processes that lead to increased risk of diseases, such as obesity, cardiovascular diseases and type 2 diabetes.
Positive effects of fructose
Fructose is not required for any processes in the body. However, in small amounts it can be useful (but not essential) for two functions:
- After strenuous exercise fructose speeds up the replenishing of the glycogen stores; (8)
- For the male reproduction system – since fructose is useful as a preferred energy source for the semen.
Please note, however, that dietary fructose sources are not essential, since our bodies are capable of converting glucose to fructose for this purpose. (36)
- Fructose can be useful after strenuous exercise and for the male reproduction system.
Summary of medical conditions that may arise from the excess fructose consumption.
- Insulin resistance, and related increased insulin production and presence in blood (hyperinsulinemia);
- Leptin resistance, and related increased leptin production and high levels of leptin in blood (hyperleptinemia);
- Obesity, especially central and visceral obesity;
- Type 2 Diabetes;
- Fructose intolerance;
- Metabolic syndrome;
- Non-alcoholic fatty liver disease;
- Heart disease