According to Lustig there are two fundamental misconceptions about nutrition and fiet: that a calorie is a calorie, and that you are what you eat. “If a calorie is a calorie, then any food can be part of a balanced diet,” he writes in Project Syndicate, “and, if we are what we eat, then everyone chooses what they eat.” He points out that if obesity and chronic metabolic disease were truly a choice, there wouldn’t be obese toddlers, or an “obesity epidemic amongst six-month olds.” Instead, our modern diet has changed to reflect these misunderstandings about food, and it’s having dire consequences on public health.Lustig pointed out two correlaries in evolutionary biology that are relevant to this argument. First, our hunting ancestors had diets high in fat, but low in carbohydrates. Lustig points out that livers could function efficiently with a single energy source (like fat) and so processed it efficiently. Likewise, our gathering ancestors ate diets high in carbohydrates, but low in fat, and their livers processed the energy sources efficiently. Thus the liver stayed healthy, and the byproduct, whether it be low density peripheral fat (the kind that we all already have) stores or as liver starch. The problem, according to Lustig, is when we began consuming high quantities of both fats and carbohydrates at every meal. The liver can’t cope and uses what Lustig refers to as an escape valve; making new fat to store the excess energy. These new fats develop insulin resistance within the liver, which is the precursor to nearly every type of chronic metabolic disease.
On top of the fat/carbohydrate combination within our diets, sugar is composed of two different molecules, glucose and fructose. Both of these molecules are metabolized differently in the body, and when ingested in excess, lead to the same build up of liver fat that fat and carbohydrate combinations.
Human psychology has been trained over millennia to respond very strongly to these three types of energy sources. After all, our ancestors were often not sure when the next meal was coming, or how big or small that meal would be, and biologically required a strong response to these types of energy when they were available. Today, food companies understand that physiological response very well, and they design foods to contain just the right balance of fat, carbohydrates, and sugar to entice people to buy and consume them. According to Lustig, this balance also makes food difficult to metabolize, and likely to create liver fats that ultimately lead to chronic metabolic disease, 33 percent of Americans (or 1 in 3) at last count.