Or maybe this should be titled, “Why Are Carbohydrates Bad?”. Carbohydrates take the form of either sugars or starches (“complex carbohydrates”); but, a starch is simply a long chain of sugar molecules strung together, so they both end up as the same thing once they enter your blood stream (the only difference is that you need digestive enzymes to break up a starch). Most carbohydrates break down to approximately half glucose and half fructose (let’s ignore fiber and some of the other less common monosaccharides, for now). Glucose is then directly used by your cells for energy, whereas fructose must first be converted into glucose or fat by your liver.
Before we go further, let me say that carbohydrates, specifically glucose, are critical for life. Glucose is energy and your cells need energy to live. BUT, they don’t need anywhere near the amounts typically consumed in the modern Western diet (often in excess of 500g per day!). In fact, the high sugar consumption in our diet is the direct cause of the increase in cardiovascular disease, diabetes and obesity seen in the last three decades (since the move toward low-fat, high-starch diets, which we now know are terrible for our health).
How can sugar be both essential and cause disease? Like so many things, it comes down to dose. Think of the analogy of alcohol: a glass of red wine a day can help prevent cardiovascular disease, stroke and even Alzheimer’s disease; whereas a 24oz bottle of scotch a day will destroy your liver and then kill you. You might think this is a drastic example, but it’s actually a very apt analogy since ethanol is processed by your liver in a very similar way to fructose. In fact, high fructose intake causes fatty liver disease, which is the precursor to cirrhosis.
The problem comes when a person consumes more sugar than they need for immediate energy usage or large doses of fructose. When this happens, a cascade of hormones are stimulated so that the extra sugar can be converted into glycogen or fat for storage. First, the amount of sugar in your blood rises. This sugar reacts with other components of your blood to produce Advanced Glycation End products, which cause oxidative damage to your tissues and increase inflammation in your body. High blood sugar can be life threatening and cause permanent brain damage, so your body has a mechanism to deal with this: the hormone insulin. Insulin helps shuttle glucose into your cells (and when glucose is really high, insulin helps get glucose to into your liver cells to be converted into fat for storage). When your blood sugar levels are chronically high, your cells can become insulin resistant, leading to more insulin production. This is what eventually leads to type II diabetes. Chronically high insulin levels have also been shown to dramatically increase the risk of some cancers. Some other hormones are stimulated as well, like leptin, a hunger hormone… yes, eating sugar makes you hungrier.
Your liver is responsible for converting excess sugar into glycogen or fat for storage. But, toxic byproducts (like triglycerides, which are linked to increased risk for heart disease) are produced as part of the conversion of either glucose or fructose into glycogen or fat for storage. If you consume relatively low carbohydrates, the amount of toxic byproducts is low enough that your body can deal with it easily. When you consume high levels of carbohydrates, the toxic byproducts build up and contribute to oxidative stress, inflammation, tissue damage and disease.
Most of the cells in your body can easily use fat or ketone bodies (produced by your liver when it breaks down stored body fat) for energy. The only cells that absolutely need glucose are neural cells. Although an exact lower limit to glucose consumption is still being debated, there is evidence that a person can survive on as little as 15g of carbohydrates per day.
This doesn’t mean that you have to do a low-carb diet. When you choose low- and moderate-glycemic load foods (that’s all fruits and vegetables, by the way), blood sugar levels are very well regulated even when carbohydrate intake is very high (upwards of 300g per day). This is about avoiding added sugars and high glycemic load foods (like grains, refined and processed foods, and junk). Clinical trials actually show that avoiding high glycemic load foods is far more important than exactly how many grams of carbohydrates you consume. The American Heart Association now recommends that we consume only five percent of our calories as added sugar (this doesn’t include the sugars in whole fruits and vegetables, for example).
One of the tenants of paleolithic nutrition is to remove all added and refined sugars from our diets and get all of our carbohydrates from whole fruit and vegetable sources. I would even add that we should focus on consuming large portions of non-starchy vegetables (with more moderate intake of starchy veggies and fruit), which is the ideal situation for liver, kidney and pancreas health.