I get a strong reaction from my patients when I tell them to stop drinking fruit juice. Their reply is often, “I thought fruit juice is good for me”.
Why isn’t fruit juice good for you?
Fructose is found in fruit, in sugar (half of table sugar is fructose, a sugar molecule is made up of one fructose and one glucose molecule), in honey (about 50% of the sugars in honey is fructose), in Agave syrup (a whopping 90% of the sugars is fructose) and of course in high fructose corn syrup (HFCS) (which may contain between 42-90% fructose). Fructose is often thought of as a natural sugar. Whilst the fructose found in natural fruit is slightly healthier than the fructose found in processed table sugar or HFCS, humans are not meant to consume a large amount of sugar. Fructose is absorbed in the intestines through transport proteins. In young children, consuming apple or pear juice (these fruit contain a high fructose to glucose ratio) may result in diarrhoea, as the unabsorbed fructose draws water into the intestinal lumen causing watery stools. Fruit juices have all the fibre removed (which slows down the absorption of sugar and provides the feeling of fullness). Some juices which are marketed as being naturally squeezed with no added sugar, have as much as sugar as 3 ½ doughnuts.
Toxic effects of Fructose
If fructose is consumed as part of sugar, the absorption is improved (which also means consuming fructose in the form of table sugar will increase the absorption of this potentially toxic sugar). With chronic ingestion of large amounts of fructose, the intestinal linings get more adept at absorbing fructose as the amount of transport proteins increase.
Once fructose gets into the blood stream, it is more effective than glucose in sticking to proteins. Sugars sticking to proteins causes damage to the protein affecting the function and resulting in the production of AGEs (advanced glycation end products). AGEs not only increase photoageing, cataracts, but are also responsible for damage to kidneys, macular degeneration, and dementia. These are the types of disease that we see diabetic patients being more at risk for.
Fructose and triglycerides
Fructose is transported to the liver to get metabolized. Its metabolism is different from that of glucose (the other half of a sugar molecule). It requires more energy stores from the liver (something known as ATP) and this depletion of energy stores is what causes the liver to protect itself by producing uric acid. Uric acid is a powerful antioxidant and in fact 50% of the antioxidant effect of blood comes from uric acid. This is why consuming large amounts of sodas and other sweets can increase one’s risk of developing gout. If you exercise intensely and use up your glycogen stores, the fructose can contribute towards refilling these stores. However, if you consume fructose when your glycogen stores are filled, the fructose is metabolized into triglycerides and excreted into the blood stream to be stored as fat.
Lack of appetite control and satiety, insulin resistance
Fructose does not stimulate the production of insulin which is the hormone that is secreted by your pancreas in response to an increase in glucose levels. Insulin causes the channels in cells of the liver and skeletal muscle to open up to allow glucose to move into the cells to be used for energy or converted into fat. Insulin also stimulates the production of leptin and ghrelin, two hormones that cause satiety, reduce appetite and increase metabolic rate. These are checks in your body to prevent you from overeating and from getting fat. This lack of stimulation of insulin by fructose, means that your body does not get the “feel full” signal and continues to consume more food.
What is insulin resistance?
I remember all the years when I would have an apple instead of the snack I really wanted. Very often, I ended up eating both since I never felt satisfied after the apple, now I know why! This very lack of stimulation of insulin was the basis for recommending fructose consumption to diabetics. However, the negative effects of fructose are often insidious and take years to manifest. Chronic consumption of fructose leads to an increase in uric acid production. This increase in uric acid not only causes gout but also exerts effects on the muscles and fat cells causing insulin resistance. When the body is resistant to insulin, it means that it no longer responds to insulin as well. For the same sugar load, the body now needs to produce a greater amount of insulin. This increase in insulin not only stops your body from utilizing the fat (making fat loss more and more difficult), it also increases blood pressure (a fructose based diet increases your risk of being sensitive to the blood pressure increasing effects of salt), it also tires the pancreas, resulting in a rise in blood sugar levels over time and diabetes mellitus.
Next time, before you reach for that next fruit juice, low glycemic index cereal (often sweetened with fructose) or low fat yoghurt (when they take the fat out of something, they often put sugar in it), think about the fructose that might just be lurking to wreak slow havoc on your body.