It's common knowledge in Paleo diet circles that sugar, especially the sugar fructose, should be limited and that it can cause a multitude of problems like those categorized under the metabolic syndrome umbrella term. Unlike other major unhealthy foods and non-foods (grains, soy, vegetable oils), sugar is also in foods that are natural and healthy like fruits and vegetables. This can make it hard for us to really understand the dangers of consuming too much sugar and reminders are often a good idea.
The problem comes from the amount of sugar consumed in today's diets. We were probably never in contact with as much sweet fruit before in history, let alone fruit juices, sodas, sweeteners and candies. If some of our ancestors were ever in contact with high amounts of sweet fruits, it surely wasn't year round.
In sucrose (table sugar) and in sugary fruits, the fraction that is problematic is the sugar fructose. The other main fraction is glucose, which can be used by all our cells for energy and is the main fuel for life on earth.
Because glucose is the good sugar, can be used by all our cells for energy and is essential for some parts of our bodies, the best sources of natural carbohydrates are starchy vegetables. Starch is a complex polymer of glucose molecules that are disassembled in our digestive systems and absorbed as glucose. Starchy vegetables have been demonized in the past by Paleo practitioners, but science has shown that starchy vegetables are not only generally healthy, but have also been consumed for a very long time already by our ancestors as a dense source of energy. Of course, many sources of starch like grains and legumes are very unhealthy and over consumption of total carbohydrate is also problematic, especially for the already metabolically challenged.
It's good to keep in mind though that fructose, in small amounts, has been in our diet for a very long time as a species and that we usually handle small amounts very well. Of course, the amount where fructose becomes toxic and damaging varies for everybody depending on a multitude of factors, but a good rule of thumb for most healthy people is at around 50 grams of fructose per day. Keeping in mind that most fruits are half glucose and half fructose, consuming over 100 grams of sugar from fruits every day can become problematic.
We should also keep in mind that by eliminating the other toxic agents in our diets like grains and vegetables oils our bodies probably become more tolerant to a little excess sugar. This shouldn't become an excuse to indulge in high amounts of fruits and natural sweeteners though, but only as a reminder not to stress over your diet if you find yourself eating higher amounts of sugar from natural sources from time to time. The real damage is when the high sugar habit becomes chronic and is repeated day after day.
Without further ado, here are 10 reasons why excess consumption of the sugar fructose, whether it comes from apples or high-fructose corn syrup, is damaging to our health:
10 reasons to limit fructose consumption
- Fructose can only be metabolized by the liver and can't be used for energy by your body's cells. It's therefore not only completely useless for the body, but is also a toxin in high enough amount because the job of the liver is to get rid of it, mainly by transforming it into fat and sending that fat to our fat cells.
- Excess fructose damages the liver and leads to insulin resistance in the liver as well as fatty liver disease. In fact, fructose has the same effects on the liver as alcohol (ethanol), which is already well known as a liver toxin.
- Fructose reacts with proteins and polyunsaturated fats in our bodies 7 times more than glucose. This reaction creates AGEs (advanced glycation end-products), which are compounds that create oxidative damage in our cells and ultimately lead or contribute to inflammation and a host of chronic diseases.
- Fructose increases uric acid production, which, in excess, can cause gout, kidney stones and precipitate or aggravate hypertension.
- While most of your body's cells can't use fructose as a source of energy, the bacteria in your gut can and excess fructose can create gut flora imbalances, promote bacterial overgrowth and promote the growth of pathogenic bacteria.
- In part because of the damage done to the liver, chronic excess fructose causes dyslipidemia, which means that your blood lipid markers tend to shift towards numbers that indicate a risk for heart disease.
- Fructose rapidly causes leptin resistance. Leptin is a hormone that controls appetite and metabolism to maintain a normal weight. Leptin resistant people tend to gain fat and become obese really easily.
- Excess fructose alone can cause all the problems associated with the metabolic syndrome (diabetes, obesity, heart disease).
- Cancer cells thrive and proliferate very well with fructose as their energy source.
- Excess fructose also affects brain functioning, especially as it relates to appetite regulation. It has also been shown to impair memory in rats.
Sugar: The bitter truth
For a very interesting and lengthy discussion on fructose and its effect on our biochemistry, here is a very insightful and popular talk given by Dr. Robert Lustig: