The murky math of calorie counting

In California and elsewhere, public health advocates are battling the restaurant industry over the passage of regulations requiring chain restaurants to post calorie counts on their menu boards. The restaurant industry’s counter-arguments are crassly self-interested, particularly when they include torturous claims of 1st Amendment violations. Nonetheless, it is worth questioning whether posting calorie counts will in fact prompt people to avoid nutrient-poor, energy-dense foods.

It may seem self-evident that providing consumers with information that enables them to choose a 480-calorie meal over a 620-calorie meal is a benefit. That’s true -- unless the individual decides to “bank” those saved 140 calories for a subsequent indulgence. With low-calorie foods, consumers are known to respond by compensation, reasoning that half the calories means one can eat twice as much. The information also isn’t beneficial if it is spurious -- if those seemingly precise numbers are based on guesswork and sloppy science.

FDA regulations allow any of five distinct methods of calculating calories. These methods produce significantly different numbers, depending on the food. The simplest method is the rule of 4-9-4 (calories/gram) for protein, fat and carbohydrates, respectively. (Calories is used here for kilocalories, following common usage.) But food is more complex than that. The energy value of protein actually varies from 2.4 for some vegetable proteins to 4.4 calories per gram for eggs. Carbohydrates vary almost as much, ranging from 2.7 to 4.2 calories per gram, depending on the foodstuff.

Obviously, the fast- and processed-food industries can cherry-pick their data and their methods to produce a more attractive number. But it’s not as if there is a well-validated method available for use in food labeling. The various systems for counting calories, and the very concept of a calorie, were never meant for individuals to use in selecting foods or gauging the energy balance of their meals. These concepts were developed to assess the adequacy of the food supply of whole populations, where one might be totaling up calories by the hundred of thousands, not comparing a 480-calorie sandwich to a 375-calorie salad.

Even the most rigorous attempts to come up with precise numbers for specific foods will fail because of the glorious complexity and natural variability of whole foods (and of the human beings who eat them). Foods are not stable combinations of discrete compounds, nor is the human body a machine that burns fuels in uniform accordance with physical laws.

For example, any sudden increase in fat intake can interfere with digestion of nutrients because the body will not have sufficient metabolic enzymes to deal with the surplus. Individuals may differ in their ability to digest specific nutrients, although this variation has not been much studied. There is some solid biochemistry to support the romantic notion of terroir: Breed and feed, seed and soil, along with the vagaries of climate and the conditions at harvest, all influence the nutritional and energy content of foods. These differences are multiplied when whole foods are mixed together into multi-ingredient dishes -- and again when they are further mixed together inside the diners.

Bomb calorimetry is the most accurate approach to assessing the caloric values for foods, but also the most expensive and laborious. The U.S. Agriculture Department Nutrient Database relies instead on the manufacturer’s own data for prepared foods. “Key foods,” those consumed at the highest levels, are being reassessed under the National Food and Nutrient Analysis Program, but others will never be confirmed by independent analysis. Some years ago, investigators at the Obesity Research Center at St. Luke’s/Roosevelt Hospital Center did one of the few careful calorimetry-based analyses (published in the Journal of the American Medical Assn. in 1993) of the energy content of 40 “health” and “diet” foods, including national and local brands. They found that national brands gave reasonably accurate calorie counts; however, locally prepared foods underestimated calorie content by an average of 80%, and regionally distributed foods did so by 25%.

Numbers are easily misread or misinterpreted, a facet of consumer psychology that is regularly exploited by marketers and retailers in setting prices. If your $3.99 Subway Spicy Italian is listed at 480 calories rather than 500, you may perceive that number as four-something, and you may miss the fact that you have to multiply that by two in order to get the calorie count for the foot-long sub, which costs only $1.75 more. Ditto for your $4.99 Subway Melt with 380 calories. The over-reliance on numbers and labels in selecting foods is part of a larger issue that prescriptive nutritional advice, whether accurate or not, coaches people to regulate their eating by external tokens rather than by the internal and sensory cues that have served that purpose over millions of years of evolution.

Carol Hart, PhD, is the author of “Good Food Tastes Good: An Argument for Trusting Your Senses and Ignoring the Nutritionists.”