When too much sugar is toxic

Is sugar really “the plague on the American diet today?”

Doctors are constantly battling each other in the dietary fat versus sugar evil villain debate. Some claim that fat, which is linked to obesity, arteriosclerosis, and heart disease (the number one killer in the United States) it is the most deadly. Others say it is sugar, which is linked to chronic inflammation, hormone imbalances, diabetes, and obesity, is the ultimate plague on the modern day diet. While we can all agree that too much of either is bad, lately sugar is taking the most heat due to the duality of its nature; in excess, sugar is stored as fat. 

Unfortunately, adding sugar to foods to make them taste better is becoming all too common. Even foods you would not expect like pasta sauce and bread are loaded with sugar. Not surprisingly, with this trend non-communicable diseases are also on the rise.

In this review, we examine the role of sugar, specifically added sugar in the development and progression of various conditions including mood disorders, oral health, inflammation, insulin resistance, cardiovascular disease, weight gain, and sexual function. While sugar is not harmful within limits, excessive consumption can have a significant impact on our health. 

What is sugar?

Sugars are sweet soluble carbohydrates that fall into two main categories; simple sugars (monosaccharides) and compound sugars (disaccharides).

Monosaccharides include

· Glucose (dextrose)

· Fructose

· Galactose

Glucose occurs naturally in fruits and plant juices and is produced from photosynthesis. Most sugars in the body are converted into glucose, which is transported around the body in the bloodstream. Fructose occurs naturally in fruits, some vegetables, sugarcane, and honey. Fructose is the sweetest of all the sugars. It is a component of table sugar (sucrose) and is used to make high-fructose corn syrup. Galactose does not exist on its own in nature, it is one of the components that make up lactose (sugar milk).

Disaccharides include

· Lactose

· Maltose

· Sucrose

Disaccharides are created by combining two monosaccharide molecules and removing a water molecule. Lactose naturally occurs in milk and can be found in milk products. Lactose is generated by combining galactose and glucose. Maltose results from the germination of grains like barley and is made by combining two glucose molecules. Maltose is also made in the body through the digestion of starches. Sucrose originates from sugarcane stems and the roots of sugar beets; it also occurs naturally with fructose and glucose in some fruits and vegetables. Sucrose is created by combining glucose and fructose.

Refined Sugar

Refined sugar is sugar that has had its impurities and coloring removed through the manufacturing process. The original sugar, called raw sugar, is dissolved and various components of the sugar are then removed resulting in white sugar. The most common type of white sugar is granulated sugar and it is commonly added to beverages or to baked goods.

Processed foods and sugar

Processed foods are foods that have been altered from their original state prior to consumption. However, when processed foods are referred to, it is primarily in reference to foods that have had ingredients added to add flavor, color, or to improve shelf life. Many processed foods have sodium, fat, and sugar added to them. Added sugars are mainly used to improve the taste and consistency of low-fat foods. Contrary to common belief, sugar is not only added to processed sweet foods, it is also added to staple foods like bread and cereal, as well as savory foods such as pasta or BBQ sauce.

Negative Effects of Sugar on the Body

As we stated earlier, too much sugar in our diets is linked to inflammation, insulin resistance, weight gain, and hormone disruptions. These altered body states are underlying causes of many medical conditions such as depression, diabetes, obesity, rheumatoid arthritis, heart disease, skin rashes, dental issues, and possible impotence. 


Sugar induces transient inflammation in the body. In moderation, this is a normal response which the body is able to handle. However, when a large amount of added sugar is consumed, the body undergoes a state of chronic or persistent low-grade inflammation that can wreck havoc on the body. Many longevity doctors state that aging and many disease process are “the accumulation of low grade chronic inflammation over time.” 

Sugar causes inflammation through multiple mechanisms. One is by combining with proteins and fats to form harmful substances called advanced glycation end products or AGEs. High levels of AGEs promote processes such as inflammation and oxidative stress. Sugar also increases gut permeability, which is a condition known as leaky gut. This is when harmful toxins, organisms, and undigested foods are able to move from the gut into the blood more easily, which can result inflammatory reactions. Sugar may also indirectly promote inflammation. Eating excessive sugar can cause weight gain, which in turn increases inflammation. Lastly, sugar increases LDL or bad cholesterol. High levels of LDL cholesterol are linked to high levels of inflammation marker C-reactive protein or CRP.

It should be noted that chronic low-grade inflammation is induced by added sugar much more than natural sugar. Added sugar is sugar that has been removed from its original source and added to another food to make it sweeter. Added sugar commonly comes in the form of high fructose corn syrup, sucrose, fructose, glucose, and corn sugar. Natural sugars are those naturally occurring in the food such as fructose in fruit or lactose in milk. Natural foods are less likely to cause inflammation because they are accompanied by other minerals in the food that slow its absorption by the blood stream.

Insulin Resistance

Insulin is a hormone made by the pancreas that helps glucose enter muscle, fat and liver cells to be used for energy. The rise in blood sugar that occurs after a meal is eaten causes insulin to be released. Insulin then moves the sugar into the previously mentioned cells to keep blood sugar at a normal level. In the case of insulin resistance, the cells do not respond to insulin and thus can’t take up sugar from the blood as well. Thus blood sugar levels remain high and the pancreas releases more insulin. If the insulin released is no longer enough to maintain healthy blood sugar levels, the individual may develop type 2 diabetes.

Some believe that sugar alone does not cause insulin resistance and it is the excess weight produced by eating sugary foods that leads to the resistance. This is along the same debate if high cholesterol without inflammation can cause heart disease or if inflammation is needed for high cholesterol to be problematic. 

While it is not clear whether sugar itself can cause insulin resistance, there is evidence to support this fact. One study found that excess levels of sugar in the liver maybe the cause of insulin resistance. The study determined that a protein called carbohydrate-responsive element-binding protein, or ChREBP, is activated by large amounts of fructose, the sugar naturally found in fruits and added to many processed foods and drinks. The sugar causes the protein to make glucose in the liver resulting in higher blood sugar levels.

Sugar does not only impact the cell response of insulin, but it can have harmful effects on the pancreas as well. When a high level of sugar is consumed, the pancreas has to release more insulin to maintain a normal blood sugar level. The more insulin that has to be released, the more the pancreas has to work. Overworking the pancreas eventually leads to its breakdown, resulting in an increase in blood sugar levels, which leads to diabetes and heart disease.


Because many individuals with depression tend to crave sugar, it is not surprising that there is a great deal of evidence showing that sugar consumption is linked to depression. A six country study that examined sugar consumption and annual rates of major depression found that the more sugar that was consumed, the higher the rates of depression. Another study that included thousands of middle-aged individuals determined that those who ate a highly processed diet were 58% more likely to develop depression. In addition, individuals on a diet of whole foods reduced their risk of developing depression by 26%.

Specifically, foods with a high glycemic index, or GI, increased the risk for depression. A study of tens thousand postmenopausal women looked at the GI index of their diets and found that the higher the GI index and more added sugar consumption, the higher their risk for developing depression. Foods with high GI index include highly processed foods and soft drinks like soda.

The GI index is the number of carbohydrates contained in food and the bodies response to those carbs after ingestion. Foods with a low GI score (less than 55) are digested more slowly, allowing for the sugar to be released into the blood slowly over time. This is typically because these foods have other components such as fiber and specific proteins that slowdown digestion. High GI foods enter the blood quickly causing major spikes in blood sugar levels. High GI foods are thought to contribute to the risk for depression because they result in sudden spikes and then drastic drips in blood sugar levels. Low blood sugar can cause the body to have a hypoglycemic reaction, which causes stress hormones to be released. This will bring on feelings of anxiety, nervousness, and depression. In addition, these stress hormones take a while to clear out of the body and can therefore have lingering effects.

Sugar also impacts depression because of its effects on nutrients in the body. Once inside the body, sugar depletes stores of nutrients important to our mood such as B vitamins and chromium. Diets low in sugar have been found not only not to cause depression, but to protect against depression.


As one would expect, sugar is also harmful to teeth. But why? It is primarily due to the presence of bacteria that feed off that sugar. The mouth contains several strains of bacteria and there are nearly 1-100,000 bacteria present at any given time in an individual that practices good hygiene. While some have positive effects on the teeth, others contribute to their decay. These harmful bacteria include those that feed on sugar and release acid in response to the sugar. The mouth should be kept around a neutral pH of 7. However, these acids can bring the mouth to dangerously low pHs (around 5.5 or lower). The acids breakdown the minerals in tooth enamel—the white tissue coating on the outside of the visible portion of the tooth that protects the tooth from harmful substances. This breakdown of the enamel is called demineralization, which can potentially lead to the development of a hole in the tooth called a cavity. The cavity can spread to deeper layers of the tooth causing greater pain and even loss of the whole tooth.

The bacteria most responsible for this acid build up in response to sugar are Streptococcus mutas and Streptococcus sorbrinus. These bacteria ingest sugars left in the mouth and form a sticky film called plaque. In addition, plaque allows for the mouth to become more acidic, leading to tooth decay and cavity formation.

Sugar consumption has been linked to the development of cavities. One study found that children who ate sugary snacks were 4x more likely to develop cavities than those that did not. Another study found that reducing sugar intake to 10% of daily calories consumed can significantly reduce the risk of tooth decay.

One of the most harmful sugary foods are sugary drinks such as juice, soft drinks, and energy drinks. This is because, in addition to the sugar, these drinks already contain high amounts of acid that contribute to the bacterial acid breaking down the teeth. Just drinking an occasional sugary drink can increase the risk of losing 1 to 5 teeth by 44%, according to one study. Sticky sugary candies are also harmful because they provide a long-lasting source of sugar for the bacteria.

Rheumatoid Arthritis

Rheumatoid Arthritis (also known as RA) and other inflammatory joint pain can be made worse by ingesting sugar. There is some evidence that the risk for RA increases in response to sugar. One study found that drinking 1 or more sugar-sweetened sodas each day increases the risk of developing seropositive RA by 63%. Individuals with RA have also been found to have high levels of AGEs.


Inflammation resulting from sugar can also have a harmful effect on the skin. Inflammation results in the release of enzymes that breakdown collagen and elastin. Collagen is­­ a protein in the body that serves as a scaffold to hold bones and muscles together. It is also the structural component to joints and tendons and promotes skin elasticity. Elastin is an elastic protein in connective tissue that enables organs including skin to resume their shape after stretching. Sugar also binds to collagen in the skin, making skin conditions like acne worse.


A high sugar diet can also increase the risk of heart disease. A 15-year study found that individuals who consumed 25% or more of their calories from sugar were 2 times more likely to die from heart disease than those with a diet in which 10% of the calories were from sugar.  Added sugars are empty calories and the more empty calories we eat the less healthy nutrient foods we consume.

It is not clear how sugar increases the risk of heart disease. There is some evidence that too much sugar raises blood pressure, a risk factor of heart disease. Another study also found that high sugar diets stimulate the liver to release harmful fats into the blood. The American Heart Association recommends that no more than 100 calories or 6 teaspoons of added sugar are consumed a day by women. Men should consume no more than 150 calories or 9 teaspoons.

A recent study found that just 3 months of a high sugar diet can cause changes in metabolism that increase the risk of heart disease. In the study, it was determined that healthy men had higher fat content in their blood liver after ingesting a high sugar diet for 3 months. Their body metabolized sugar in a way similar to individuals with non-alcoholic fatty liver disease (NAFLD). In another study with men with NAFLD, a high sugar diet increased their risk of heart disease.

Kidney Damage

Sugar has several harmful effects on healthy individuals. However, added sugar mostly likely will not have harmful effects on the kidney in healthy individuals. However, in people with diabetes, consuming high amounts of added sugar can lead to kidney damage. The kidneys contain small blood vessels that filter out small waste products from the blood to become part of the urine. In the case of diabetes, high levels of sugar in the blood create too much sugar for the kidneys to filter out. Thus, the kidneys have to work harder than in a healthy individual. After several years of working hard, the blood vessels in the kidneys develop leaks that allow large particles such as essential proteins and red blood vessels to leak into the urine. Sugar can also cause high blood pressure which can also damage the kidneys.


Fructose is thought to be a prime culprit in weight gain. This is because fructose is thought to alter satiety, which leads to the intake of more food. Fructose does not stimulate insulin and leptin secretion in humans. Leptin is a hormone made in fat cells, it is also known as the satiety hormone. Leptin tells the brain when enough fat is stored in the body and thus food does not need to be consumed. It also ensures that starvation does not occur by making the individual feel hungry. Consumption of high amounts of fructose induces leptin resistance leading to greater food consumption. It also stimulates dopamine and the hypothalamus. In addition, fructose depletes ATP in the liver, which is a signal for the body to consume more food. There is evidence that fructose reduces metabolism compared to glucose in overweight and obese individuals.

Fructose can also increase the risk of metabolic syndrome and increase fat in the liver without increasing calorie intake. This is thought to be due to the way fructose is metabolized compared to other sugars. The only difference in the way fructose and glucose and other carbohydrates are metabolized is in the initial step. Fructose is broken down by fructokinase in the liver resulting in a rapid decrease of ATP in the liver. The decrease in ATP stimulates the conversion of AMP into uric acid. Uric acid increases in the bloodstream, which induces oxidative stress that leads to fat synthesis in the liver cells through various mechanisms. It is also thought that uric acid contributes to the development of insulin resistance once it is taken up by fat cells.


Like kidney damage, excess sugar can have negative effects on other functions in individuals with diabetes, such as sexual function. Erectile dysfunction is nearly 3 times more common in diabetic men. In addition, sugar can indirectly lead to erectile dysfunction by causing obesity. 

A recent study found that high blood sugar levels, in an indirect result of high sugar consumption, contributes to erectile dysfunction in males with diabetes. A preceding study found that erectile dysfunction was due in part to an interruption of the mechanism involved in blood flow related to an erection. An erection forms in response to a sexual stimulus that activates neuronal nitric oxide synthase or nNOS. nNOS transiently releases nitric oxide in the penis at nerve endings. The release increases blood flow and relaxes the muscles in the penis which allows for more blood to flow into the penis. The blood flow stimulates eNOS in the blood vessels of the penis to release NO continuously to keep the penis erect.

A simple blood sugar called O-GlcNAc blocks activation of eNOS which reduces the release of sustained NO and prevents the penis muscles from relaxing. The study found that diabetic rats achieved smaller erections than non-diabetic rats. The erections of the rats with diabetes also took 70% longer to generate.

Sugar may also affect sexual function in other indirect ways. Ingesting sugar can cause a spike in blood sugar and result in a short-lived sugar high. Once the rush wears off, the individual often feels fatigued which can hinder sex drive. Sugar also can promote resistance to leptin, a hormone that regulates eating habits and sexual behavior. Leptin resistance, in turn, can lead to a lower sex drive. As mentioned with depression, sugar can promote the release of stress hormones which can also impact sex drive.


Sugar can have a significant impact on health both directly and indirectly. To help prevent the development of sugar-dependent conditions, one should consume sugar in moderation.

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