Sat, Jun 02, 2012 - Page 9 News List

The diet debacle

US health services are struggling under the weight of an obesity epidemic

By Robert Lustig

Illustration: Lance Liu

Two seemingly benign nutritional maxims are at the root of all dietary evil: “A calorie is a calorie” and “you are what you eat.” Both ideas are now so entrenched in public consciousness that they have become virtually unassailable. As a result, the food industry, aided and abetted by ostensibly well-meaning scientists and politicians, has afflicted humankind with the plague of chronic metabolic disease, which threatens to bankrupt healthcare worldwide.

The US currently spends US$147 billion on obesity-related healthcare annually. Previously, one could have argued that these were affluent countries’ diseases, but the UN announced last year that chronic metabolic disease (including diabetes, heart disease, cancer and dementia) is a bigger threat to the developing world than is infectious disease, including HIV.

These two nutritional maxims give credence to the food industry’s self-serving corollaries: If a calorie is a calorie, then any food can be part of a balanced diet; and, if we are what we eat, then everyone chooses what they eat. Again, both are misleading.

If one’s weight really is a matter of personal responsibility, how can we explain toddler obesity? Indeed, the US has an obesity epidemic in six-month-olds. They do not diet or exercise. Conversely, up to 40 percent of normal-weight people have chronic metabolic disease. Something else is going on.

Consider the following diets: Atkins (all fat and no carbohydrates); traditional Japanese (all carbohydrates and little fat); and Ornish (even less fat and carbohydrates with lots of fiber). All three help to maintain, and in some cases even improve, metabolic health, because the liver has to deal with only one energy source at a time.

That is how human bodies are designed to metabolize food. Our hunter ancestors ate fat, which was transported to the liver and broken down by the lipolytic pathway to deliver fatty acids to the mitochondria (the subcellular structures that burn food to create energy). On the occasion of a big kill, any excess dietary fatty acids were packaged into low-density lipoproteins and transported out of the liver to be stored in peripheral fat tissue. As a result, our forebears’ livers stayed healthy.

Meanwhile, our gatherer ancestors ate carbohydrates (polymers of glucose), which was also transported to the liver, via the glycolytic pathway and broken down for energy. Any excess glucose stimulated the pancreas to release insulin, which transported glucose into peripheral fat tissue and which also caused the liver to store glucose as glycogen (liver starch). So their livers also stayed healthy.

Meanwhile, nature did its part by supplying all naturally occurring foodstuffs with either fat or carbohydrate as the energy source, not both. Even fatty fruits — coconut, olives, avocados — are low in carbohydrate.

Our metabolisms started to malfunction when humans began consuming fat and carbohydrates at the same meal. The liver mitochondria could not keep up with the energy onslaught, and had no choice but to employ a little-used escape valve called “de novo lipogenesis” (“new fat-making”) to turn excess energy substrate into liver fat.

Liver fat mucks up the workings of the liver. It is the root cause of the phenomenon known as “insulin resistance” and the primary process that drives chronic metabolic disease. In other words, neither fat nor carbohydrates are problematic — until they are combined. The food industry does precisely that, mixing more of both into the Western diet for palatability and shelf life, thereby intensifying insulin resistance and chronic metabolic disease.

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