Sugar will make you sick over time if you eat enough. It does that because of the way your body processes the sugar, not because sugar contains calories that your body supposedly counts. Ancient mechanisms in your body evolved to turn sugar into fat as quickly as possible. Your brain responds to sugar the same way it responds to cocaine and opioids. Evolution wired you to crave sugar and consume it until it’s gone. That’s because in nature, the sweet sugar treats were ripe fruits and berries, which your ancestors gorged on and turned into fat they used later. But how does sugar make you sick? And what is “sugar”?
The three classes of foods your body uses to produce energy are fats, sugars, and proteins. Animals also use certain fats and proteins as building blocks for the cells of your body. Plants build cell walls with the sugar glucose converted into cellulose, but in animals it’s only used as an energy source. There are three sugars you may eat a lot of– glucose, fructose, and galactose. Here’s what they look like–
Sugars are “carbohydrates”. Carbon (C), Hydrogen (H), and Oxygen (O). These are chemical diagrams. The angles of the shapes are unlabelled carbon atoms.
Table/cane/beet sugar is sucrose, a “disaccharide.” A glucose and fructose are stuck together by the plants that store sugar that way. Milk sugar, lactose, is a glucose and galactose stuck together in cow and human milk. Starch is a storage form of sugar plants make by sticking a lot of glucose molecules together. Potatoes are a famous starchy vegetable. As the name would suggest, fruits and berries are very high in the sugar fructose. A glass of orange juice has more fructose than a Coke.
Regardless of how the sugars are stuck together, your digestive tract breaks them up into the simple sugars, glucose, fructose, and galactose. Those simple sugars are absorbed by the intestine, and sent to the liver for processing, with an important exception for fructose. Unless you’re a baby or consume a lot of cow’s milk, you don’t consume a lot of galactose, which gets converted to glucose anyway. So let’s talk about what your body does with glucose and fructose.
Glucose can get stored in the liver as glycogen, which is a bit like starch in plants. It also passes on into the blood stream, going out to your tissues. Your body wants to control glucose levels pretty carefully, so if glucose goes too high, the pancreas produces the hormone insulin. Insulin signals your cells, particularly the fat cells, to take up glucose. In the fat cells it’s turned into fat. While insulin is elevated, the fat cells are holding onto fat as well. These cells are mostly under your skin, subcutaneous fat. By itself this process may make you gain some weight, but it won’t make you sick, at least according to Dr. Richard Johnson and his collaborators.
Fructose is the metabolic bad boy in our diet. Dr. Robert Lustig gave an important lecture about fructose that was watched by millions. Remember it’s half of sucrose, which is used in a lot of foods. Worse, in the mid 20th Century chemists figured out how to turn corn syrup, which is all glucose, into a mixture of fructose and glucose. The resulted in High Fructose Corn Syrup (HFCS), which goes into all sorts of foods and beverages. Fructose is actually sweeter than glucose or sucrose, so HFCS is an easy cheap way to sweeten foods and beverages. All the fructose is processed in your liver or the wall of the intestine, never making it into your bloodstream. Johnson et al say that if you get enough glucose quickly enough in the liver, some of that gets converted to fructose by the glycol pathway. Think modern wheat and corn that were bred for high yields and high sugar content. A slice of bread has a higher glycemic index than table sugar. Corn is used to “finish” (fatten) cows in feed lots.
In the liver, fructose is metabolized in a way that depresses ATP levels and produces uric acid. This stresses liver mitochondria in a way that promotes fat production in the liver. Over time, that can result in Non-Alcoholic Fatty Liver Disease (NAFLD), one of the signs of Metabolic Syndrome. Metabolic Syndrome is the medical term for the complex of problems that include obesity, insulin resistance (which can become Type II diabetes), heart disease, and elevated blood pressure. Fructose also causes elevated levels of a hormone called vasopressin, which Richard Johnson also implicates in fat deposition.
Besides the liver, fructose is also metabolized to a degree in the wall of the intestine. That causes the “tight junctions” between the intestinal cells to loosen, allowing some food molecules to get into the blood stream. Your immune system responds. Before you know it you can have a food allergy. I first heard about this in Richard Johnson’s book The Fat Switch. Gluten sensitivity is a huge issue public health issue of late. Rick didn’t mention gluten in the book, but when I asked him if the problem included gluten he said “Yes, it’s the fruit juice they give the kids” i.e. the orange juice I mentioned earlier. Or apple juice, etc. Turning fruits or berries into juices is a really bad idea because they absorb quickly and hit your liver hard. Just eat the fruit, albeit not too much.
Johnson points out that what physicians call Metabolic Syndrome is actually an ancient mechanism by which animals turn fructose into stored fat, not a disease. However, the extremely high levels of fructose containing sugars in our diet drives the process into the realm of disease, instead of being a normal mechanism by which we store a bit of fruit and berry energy as a break from hunting. That also relates to why we’re wired to be less active when we’re gorging on the fruits and berries.
I’ll work on page or two in the Biology section that explains the details. Here’s a short talk Richard Johnson gave a few years ago explaining these ideas–