What could be better than free calories – calories you take in that never get a chance to be stored as fat? Sounds pretty sweet, doesn’t it? Well, there is such a thing as free calories – sort of. The truth is you only absorb a certain portion of the calories from foods you take in and how many you actually absorb depends on the macronutrient content of the food you’re eating.
As you probably know, there are three macronutrients – protein, carbohydrates, and fat. Each provides a certain number of calories. When you break down protein and carbohydrates, for each gram you take, your body gets around 4 kilocalories of energy, assuming you absorb all of the protein and carbs you consume.
In contrast, when you break down fat, it supplies your body with 9 kilocalories of fuel per gram, assuming perfect absorption. Fat is more calorically dense than protein and carbs. However, before assuming you should avoid fat due to its higher calorie content, you also have to consider the hormonal effects each macronutrient has on your body. So, it’s more complicated than it appears.
Determining the theoretical number of calories in a certain quantity of food is as easy as multiplying by the number of grams of each macronutrient. For example, if one serving of a food contains 10 grams of carbohydrates, 15 grams of protein, and 5 grams of fat, assuming perfect absorption, the number of kilocalories you take in would be:
Carbs 10 grams x 4 kilocalories/grams 40 kilocalories
Protein 15 grams x 4 kilocalories/grams 60 kilocalories
Fat 5 grams x 9 kilocalories/grams 45 kilocalories
Grand total: 145 kilocalories
In practice, it’s not so simple, partially because of the issue of absorption. No matter how efficient your digestive tract is, you don’t absorb every calorie from the food you eat and the amount you do absorb varies depending on the type of macronutrient.
Of the three macronutrients, carbohydrates are the most completely absorbed and protein the least. Research shows 98% of the calories in carbs are taken in and used by your body, 95% of the calories in fat, and only 92% of the calories from protein makes it past your digestive tract. So, you’re shaving off a few calories simply because digestion and absorption aren’t perfect.
With plant-based foods, the absorption issue is even more unreliable. Many vegetables and fruits have hardy cell walls that are tough to break down. If your digestive tract can’t crack open the cell walls, the inside of the plant cells, where all the calories are, can pass through your digestive tract without being absorbed. You see this mostly with raw foods. Cooking helps to break down stubborn plant cell walls so more of the calories inside are absorbed. That’s why eating more raw foods could theoretically be a strategy for weight loss.
Carbohydrates That Aren’t Absorbed
You’ve gotta love foods high in fiber and resistant starch. When you heat and cool down a potato and allow it to set in the refrigerator for at least 24 hours, some of the potato’s starch, a third or more of the potato’s absorbable starch, is converted to resistant starch, the type your body can’t break down and use. So a potato exposed to cold no longer has the calculated number of calories a portion has been converted to indigestible starch. However, bacteria in your gut can break down resistant starch to form short-chain acids that help keep the lining of your colon healthy. Free calories and a healthier colon? Those are good reasons to eat foods high in resistant starch.
Foods that are high in resistant starch include green bananas, barley, brown rice, beans, whole grains, and seeds. Bonus points: resistant starch also appears to improve blood sugar control and helps attract healthy gut bacteria. One downside is resistant starch can also cause gas and bloating if you introduce too much into your diet too quickly.
Nuts are another example of a food that’s incompletely absorbed. Research shows when you eat almonds or pistachios, and probably other nuts, a portion of the calories never make it out of your digestive tract and are excreted. For example, if you ate 23 almonds, the calories content would theoretically be 170 calories, but when you take absorption into consideration, the calorie count drops to 129 calories. A real bonus if you’re a nut lover!
Other Factors That Impact Absorption of Calories
Did you know the length of your intestines impacts how many calories you absorb? If you have a longer intestinal tract than someone else, other factors being equal, you’ll likely absorb more calories. Then there’s the issue of digestive enzymes. Your pancreas and small intestines produce a variety of enzymes that break down carbohydrates and fats. If you produce fewer of certain enzymes, you absorb fewer of these macronutrients. Of course, you may also experience digestive issues like gas and bloating as well. So there are some drawbacks to inefficient macronutrient absorption.
What type of food is absorbed the fastest? Processed food – because the food molecules are already broken down into smaller components that can be quickly and easily absorbed. One study showed people who ate a whole food snack consumed 10% fewer calories relative to those who ate a similar amount of processed food snack. When you cook processed food, it increases the absorption rate even more.
In addition, digestion and absorption can be affected by the types of bacteria you have in your gut. Research shows gut bacteria influence absorption of minerals and may also impact macronutrient absorption. Interestingly, people who are lean have populations of gut bacteria that differ from those who are obese.
Finally, the number of calories you take in with a meal is also affected by the health of your digestive tract. If you have an inflamed intestinal tract due to celiac disease or another inflammatory bowel disease like Crohn’s or ulcerative colitis, you’ll absorb fewer macronutrients and micronutrients.
The Bottom Line
The calculated number of calories in a food or meal assumes you’re able to digest and absorb all of the macronutrients, which is seldom the case. You likely take in fewer calories when you eat whole foods, foods high in fiber and resistant starch, and foods that are uncooked and haven’t been exposed to heat. Now you have another reason to add more whole, unprocessed foods to your diet.
PR Newswire. “New study shows almonds have 20% fewer calories than originally thought”
Scientific American. “The Hidden Truths about Calories”
Nutr Clin Pract. 2012 Apr; 27(2): 201-214. Published online 2012 Feb 24. doi: 10.1177/0884533611436116.
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