A Calorie Is Not A Calorie: Why All Calories Are Not The Same

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When I was in the second grade and was told to cut calories, I wanted to know why if the calories were the same, I couldn’t just cut out the squash and beans at dinner for the equivalent caloric amount of ice cream. “Vitamins” were too abstract for me and besides I took a vitamin pill. It wasn’t until low carbohydrate diets became popular that I started learning about different metabolic actions of different types of food, and later about different nutrients, the need for one nutrient to absorb another and biological responses to ingested foods that affect what is used.

The first law of thermodynamics indicates that energy is conserved and thus that a calorie might be a calorie if you equalize calories in and calories out, plus fat storage. However the second law says that no machine is completely efficient. Some of the available energy is lost as heat and in the internal chemical changes and in entropy. In the human body this means that hormones, enzymes(1) , allergic processes, constraints from lack of essential minerals and cofactors all have an impact on the food retained.

It takes more energy to break down some foods than others. For protein to break down to energy it is transformed into glucose. That step takes ATP (energy) and thus a calorie of sugar transforms into energy (or stored fat) more efficiently than protein. 100 calories of sugar will make you fatter than 100 calories of boiled egg. And people on low carbohydrate diets lose more weight, lose it faster and keep it off longer than people on low calorie diets.
The amount of calories lost as heat is approximately 2–3 % for fats, 6–8 % for carbohydrates, and 25–30% for proteins. You would typically lose 140 calories as extra heat on the early phase of an Atkins, South Beach or Protein Power diet just based on the metabolic effects.(1)
The form of food counts: fruit juice is worse than fruit if you want to lose weight. The fructose is unbound and can inactivate the appestat that prevents us from overeating. (2) The glycemic index of flour is higher than that of pasta. Generally the more processed (ground, liquefied) forms of the same food raise your blood sugar faster.(3)
Sugars differ. Fructose doesn’t suppress the hunger hormone, grehlin like glucose. Fructose makes you lay down more fat. 80% of glucose is absorbed immediately sending only 20% to the liver vs. 100% of fructose. That causes the liver to store it as new fat. This also creates VLDL cholesterol (the worst kind.) Glucose makes 1% as VLDL and Fructose has 3 times as much, with the additional waste product uric acid which causes gout and hypertension. 30% of fructose becomes fat compared to 1% of glucose. Table sugar is 50% glucose and 50% fructose, so is poison. Yes, poison. (2)
Fiber protects food from being completely broken down. Our digestive system is not a bomb calorimeter (the tool used to determine how much energy food releases in kilocalories) where everything is burned completely. Fiber sweeps undigested food through the colon and probably is intended to clean it out. So whole grains per ounce tend to give fewer calories than refined grains. (This effect is often negated by, say, slightly heavier slices of whole grain bread.)
Fiber does not raise blood sugar or blood insulin as high because it takes more time in the stomach to release glucose from foods.(2,3)
Fiber sweeps food through the colon, decreasing intestinal transit time so the small intestine has less time to absorb nutrients. Also it specifically reduces the absorption of carbohydrates in the intestines (2)
Fat is not all burned, In fact it often lubricates the colon, decreasing intestinal transit time and lowering the amount of nutrients absorbed. This is especially true when there is insufficient bile produced.
Micronutrients differ. 100 calories of cooked kale has a lot more micronutrients than 100 calories of iceberg lettuce. Minerals are higher in darker greens. There are more flavonoids in blueberries than strawberries. Trace minerals are often part of enzymes necessary to break down or to absorb food..
Some nutrients need cofactors: mineral rich greens are better absorbed with some fat..(4)
Some foods are allergenic or irritating. Wheat contains gluten which has several degrees if irritation: celiac disease, atopic allergy, gluten sensitivity and normal irritation in the colon because it is hard to digest. Depending on the degree of sensitivity, the gluten protein can cause severe diarrhea and weight loss or retention of fluids and weight gain.(5)
Different gut flora dispose us to gain or lose weight. Thin people have more of the bacteria that will give us diarrhea in excess. Fat people have more of the gut flora that break down food- the same flora found in yogurt.(6)