Do You Absorb All the Calories You Eat?

Food labels give calories as precise numbers, and apps repeat them to the digit. In reality, the energy your body takes from a food varies with how it is built, prepared and chewed. A well-known experiment with almonds made the point clearly. Here is the science behind it and what it means for everyday eating. This is general education, not medical advice.

Where calorie numbers come from

Most labels use the Atwater system, developed in the late 1800s. It assigns a standard number of calories per gram of protein, carbohydrate and fat, and adds them up. It is a clever, practical shortcut. It also assumes that the body absorbs every food in the same way, which is not always true.

The almond experiment

In 2012, researchers led by Rachel Novotny at the University of Hawaii and David Baer at the U.S. Department of Agriculture published a study in the American Journal of Clinical Nutrition. Eighteen healthy adults ate controlled diets for 18 days, including different amounts of almonds. The team collected urine and stool to measure how much energy was not absorbed.

• The label number was too high. Using the standard factors, a 28-gram serving of almonds comes to about 170 calories. The measured value was about 129 calories.
• About a quarter less than predicted. The measured value was roughly 24 percent below the standard estimate. The authors put it the other way round: the standard factors overestimated the almonds’ measured energy by about 32 percent.
• Another nut, a similar story. Follow-up work from the same group found that walnuts delivered about 146 calories per ounce, roughly 21 percent less than the standard factors predict.

Why would that happen?

• Cell walls. Nuts hold fat inside tough plant cells. If the walls stay intact, some of the fat passes through unabsorbed. Chewing and grinding break more of them.
• Cooking and processing. Heat, blending and milling make nutrients easier to absorb. A smoothie or nut butter tends to deliver more of its energy than whole nuts.
• Fiber and structure. Whole foods with fiber slow digestion and can change how much is absorbed.
• Digesting costs energy. Some of the calories go to the work of digestion, as we explain in the thermic effect of food.

What it means in practice

• Labels are estimates. Differences of 10 to 20 percent are not unusual, in either direction. Our guide to calorie-tracking apps covers that.
• Do not over-trust a single number. What matters is the pattern over days and weeks.
• Whole and processed are different. See our ultra-processed foods guide.
• Do not skip nuts. They are energy dense, but also filling and nutritious, and a small handful is a sensible portion.

Limits of the research

• It was a small study in healthy adults, mostly of almonds.
• Different foods behave differently. Findings for nuts do not automatically apply to everything.
• Individuals vary, including their gut bacteria and digestion speed.

The takeaway

A calorie on a label is a useful guide, not a measurement of what reaches your body. Foods that are whole, chewed well and minimally processed tend to give up their energy more slowly and less completely. Knowing that can make you a little more relaxed about the exact numbers.

Sources

• Novotny JA, Gebauer SK, Baer DJ. Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets. American Journal of Clinical Nutrition, 2012;96(2):296-301.
• Baer DJ, Gebauer SK, Novotny JA. Walnuts consumed by healthy adults provide less available energy than predicted by the Atwater factors. Journal of Nutrition, 2016;146(1):9-13.
• USDA Agricultural Research Service, news summary of the almond energy findings.