For anyone who wants to get slim or maintain a healthy body weight, reading food labels is widely considered a vital dietary strategy. Supermarkets have thousands of them, those black-and-white Nutrition Facts labels telling shoppers how many calories each portion of a product contains. Many recipes in magazines and diet books also indicate the calories you’ll take in with every serving. But now science is showing that not all calories are created equal and those numbers aren’t always, well, black and white.

The energy value of food is based on the Atwater system, which assigns a set number of calories to a food’s macronutrient components—carbohydrates, fat and protein. Atwater determines that carbohydrates and protein possess 4 kilocalories per gram, while fat has a loftier 9 kcal/g (Painter 2006). That means a food with 8 g of fat, 3 g of protein and 7 g of carbohydrate should (theoretically) deliver 112 kcal to the person who eats it.

Lately, however, scientists have been calling for an overhaul of the century-old Atwater system, contending it has not kept up with modern science and is likely not giving us the full picture of the calorie value of some foods, such as those rich in protein or those consumed in their raw state. Most importantly for people seeking to trim their waistlines, a few dietary tweaks to take advantage of these shortcomings could translate into meaningful weight loss and improvements in health.

Go Nuts

Dieters tend to be very tentative about nuts. While most people understand that nuts are rich in beneficial fats and essential vitamins and minerals, they also know nuts harbor a worrisome number of calories—about 170-195 kcal per ounce (Self Nutrition Data 2012). But emerging research suggests nuts are just one example of a food group where calorie determination has been flawed, meaning they may be an even better midafternoon snack option than previously thought.

A 2012 study published in the American Journal of Clinical Nutrition by scientists at the U.S. Department of Agriculture’s Agricultural Research Service determined that almonds deliver about 129 kcal per 1-ounce serving, 30% fewer than the roughly 167 kcal determined by the Atwater system and what is now shown on nutrition labels (Novotny, Gebauer & Baer 2012). A similar study conducted on pistachios in the British Journal of Nutrition found that the verdant nuts may contain up to 5% fewer calories than previously estimated (Baer, Gebauer & Novotny 2012).

David J. Baer, PhD, a researcher in both these studies, surmises that the strong cell membranes of plant foods like tree nuts may lock in some of their macronutrients (including fat), thereby preventing them and the energy they provide from being fully absorbed through the digestive tract. “The Atwater system is based on assumptions about nutrient digestibility (availability), and those digestibility factors are too high for foods like nuts,” says Baer, a lead researcher with the Agricultural Research Service. So while a handful of walnuts may contain 15 g of fat, which translates into 135 kcal, it’s likely that the human body does not absorb and use all of these fat calories. The same could hold true for other whole-plant foods like seeds, legumes, vegetables and whole grains.

Take-home message. Research suggests that our bodies can absorb more macronutrients (and thus calories) from processed foods when the processing breaks down cell walls within the base foods. This means we may take in more total calories from

  • applesauce than from whole apples,
  • orange juice than from oranges,
  • whole-wheat spaghetti and whole-wheat bread than from whole-wheat berries,
  • chocolate bars than from cocoa nibs,
  • quinoa flour than from quinoa, and
  • soymilk than from soybeans.
  • Baer is conducting a study of nuts processed to different degrees (whole, chopped, butter) to test this hypothesis. In the meantime, it seems prudent to advise weight-conscious people to make the most of the calorie advantage by eating foods in forms as close as possible to their natural state. Besides, foods like nuts, lentils and wheat berries are among the most nutrient-dense options at the store.

    Harness Raw Power

    It’s not necessary for people to completely eschew their ovens when trying to shed some weight, but it might be a good idea to work in more raw foods. A watershed study by Harvard scientists in the Proceedings of the National Academy of Sciences showed that cooking increases the number of calories the body absorbs from food (Carmody, Weintraub & Wrangham 2011). Study authors believe that cooking performs some of the digestive process for us (such as denaturing proteins and gelatinizing starches), meaning that our bodies don’t expend as much energy digesting food; as a result, more calories are available to be burned or stored as fat.

    Digestion is a very metabolically costly bodily operation, so the Atwater system likely overestimates the number of calories we obtain from raw foods. The number gleaned from raw broccoli or raw fish could very well be less than the number garnered from the same portion of boiled broccoli or fish sticks. Similarly, the more al dente the pasta, the fewer of its calories will be absorbed, as indicated by less of a spike in blood sugar. Case in point: The same researchers in the aforementioned study determined that when the diets of snakes in a laboratory were switched from raw beef to cooked meat, the energy they expended on digestion, absorption and assimilation dropped by 13% (Boback et al. 2007).

    Further, larger quantities of raw food require more laborious chewing, which expends additional energy and encourages satiety. And since we digest cooked food more quickly than raw, there may be less time for intestinal bacteria to have their share when food is cooked, leaving more calories to be absorbed.

    In his book Catching Fire: How Cooking Made Us Human (Basic Books 2009), Harvard anthropologist Richard Wrangham convincingly argues that cooking with fire allowed our ancestors to develop bigger brains and reproduce at greater rates by increasing the energy density of food, making it easier to digest and freeing up more calories for the body to use. While the benefits of cooking helped ancestral humans to meet their energy needs in an environment of scarcity and to build robust societies, in today’s society—where we spend most of our days in front of a computer snacking on heavily processed cooked food instead of foraging or hunting for dinner—the same benefits may contribute to caloric excess and weight gain.

    Take-home message. Perhaps our growing collective girths could be partly attributed to the reality that nutritionally suspect cooked foods like frozen meals and canned soups have largely nudged out raw foods from the Standard American Diet. Dieters should be encouraged to make their digestive tracts and jaw muscles work a little harder by incorporating more raw foodstuffs into daily menus. Tossing a handful of raw sunflower seeds or almonds into breakfast oatmeal and serving a multicolored raw salad at every dinner meal can help accomplish this. Snacking on raw baby carrots and using large leafy greens instead of bread for lunch sandwiches are other good options. Thinly shaved zucchini is a wonderful substitute for cooked pasta. And the blender can be used to whip up cold raw soups like gazpacho and garnish them with microgreens.

    Turn Up the Heat

    The troubles with the Atwater system spring from the fact that it does not account for the thermic effect of feeding (TEF). “Think of the thermic effect of feeding as the energy cost of chewing, digesting, absorbing, transporting and storing the food you eat,” says John Berardi, PhD, exercise physiologist and cofounder of Precision Nutrition. He adds that the thermic effect of feeding can account for 5%-15% of the total energy we expend each day. “The actual number depends on what a person eats and how much a person eats, as well as other influences, such as genetics.”

    In other words, if you take in 600 calories at lunch, you may net only 510-570 of them after digestion takes its cut. So it seems reasonable to assume that those who can bolster their TEF through dietary manipulation might be in a better position to avoid a Buddha belly.

    One such way is to take in more protein. The Atwater system does not address the fact that protein has a higher TEF than carbs or fat. Berardi says the TEF of protein ranges from 20% to 35%, meaning that up to 35% of the calories it provides are burned up during digestion and processing. “In contrast, it costs us only about 5%-15% of the energy consumed from carbohydrates or fats to digest and process them,” notes Berardi.

    Studies indicate that overweight people experience an even lower TEF response to dietary fat (Swaminathan et al. 1985). So even though carbs and protein have the same calories per gram, the human body likely stores fewer calories from protein. Furthermore, protein contains nitrogen, which must be stripped off and eliminated by the liver. “This extra metabolic step, as well as other biochemical differences between the macronutrients, could be why the body requires more energy to handle protein,” says Berardi.

    Berardi says the high TEF of protein could be a major reason why studies show high-protein diets—up to 40% of total calories—are an effective weight loss strategy: Replacing some carbohydrates and fats with protein is more energetically costly for the body (Aldrich et al. 2013; Tang et al. 2013; Wycherley et al. 2012). Grazers take note: Regularly eating food throughout the day also appears to elevate TEF compared with going long periods without food intake (Farshchi, Taylor & Macdonald 2004).

    Take-home message. To keep your metabolism revved up, look for ways to include quality sources of protein during meals and snacks. Greek yogurt makes for a protein-packed snack. Try swapping sugary boxed cereal for a veggie-studded omelet at breakfast. Blitz whey protein, which has a particularly high TEF, according to an American Journal of Clinical Nutrition study (Acheson et al. 2011) on postworkout shakes. For most people, a healthy daily protein calorie range would be between 25% and 35% of total calorie intake.