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Not all calories are equal.. Why?

Updated: Dec 24, 2023

'A calorie is a calorie, is a calorie' this is really true only in laboratory settings but not in our body.

What is a calorie?

The Calorie you see on a food package is actually a kilocalorie, or 1,000 calories. A Calorie (kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius.

How is caloric value of food calculated?

One way of calculating calorie of food is by placing it in a sealed container surrounded by water and the food is completely burnt and the resulting rise in water temperature is measured. The apparatus is known as bomb calorimeter. But this method do not measure the energy lost through heat, as well as passing urine and feces.

Thanks to Wilbur Olin Atwater, an American chemist. Through his experiments he created 'Atwater system'. Atwater took the loss of energy through heat, urine and feces into consideration and gave caloric value for different components of food. As per Atwater system, 1 gram of Carbohydrate and 1 gram of protein give 4 calories each and 1 gram of fat gives 9 calories. He also found that alcohol has 7 calories per gram.

Wow! finally it seems like we know the exact energy we get from different foods. But this is not true. Even Atwater system does not take into account the digestive process that happens inside our body but assumes complete conversion of nutrients into energy. But that doesn't happen inside us. Different components of food undergo different digestive process inside our body. For example, fiber even though a carbohydrate is not digested and utilized by our body completely. Hence 1 gram of fiber does not give 4 calories as other carbohydrates.

​Summary: Calorie refers to the amount of energy that we get from food. In a laboratory food can be completely burnt and the amount of energy released can be measured. But our body do not burn the food with fire to get energy. Different food we eat undergo different digestive process and hence the amount of energy we get from food is not exactly same as the amount of energy they release in laboratory settings.

Our digestive process is complex - A sweet example - Glucose vs Fructose vs Sucrose:

If you are trying to loose weight the immediate advice you get is to cut back on sugars. You may wonder weather type of sugar matters. The answer is absolutely YES!

Sucrose, fructose and glucose are three common types of sugars. Even though all these fall under the category of carbohydrates, they undergo different digestive process and have different effect on our metabolism.

Sucrose is the scientific name of table sugar. Sucrose is a disaccharide consisting of one glucose molecule and one fructose molecule, or 50% glucose and 50% fructose. Sucrose is broken down into glucose and fructose by enzymes secreted in our mouth and small intestine. While glucose is directly absorbed into blood stream through the lining of small intestine and excessive glucose is stored as glycogen, fructose on the other hand can only be metabolized by liver. The liver converts fructose into glucose and it also converts excessive fructose into fat. Now if one gram of carbohydrate gives 4 calories do you think the 1 gram of excessive fructose that was converted into fat by liver will give 4 calories instead of 9?

Not only in terms of calories, glucose and fructose affect our metabolism in different ways. These are few to mention:

Ghrelin is a hunger hormone. Studies show that fructose increase the release of Ghrelin than glucose hence causes more hunger leading to increased calorie intake.

Studies show that fructose do not stimulate satiety centers in our brain leading to reduced feeling of fullness.

Studies show that excessive fructose can lead to insulin resistance, increased abdominal fat, increased triglycerides and sugar levels in blood.

Summary: Our digestive process is complex and different for different food we eat. Though both glucose and fructose are carbohydrates they are metabolized differently in our body. Excessive consumption of fructose can negatively impact health. Excessive consumption of fructose found in processed foods can lead to weight gain, increased insulin resistance, increased triglycerides, LDL and sugar levels in blood. Its important to understand that fructose your body gets through eating fruits and vegetables is harmless.

Thermogenic effect of food:

Thermogenic effect of food (TEF), also known as specific dynamic action (SDA) or Dietary induced thermogenesis (DIT), refers to the amount of energy required to digest, absorb and metabolize the nutrients we get from food. In other words our body uses some energy to produce energy from food. Usually food which are hard to digest are assumed to boast TEF. Even though the term 'Negative calorie foods' which refers to foods those require more calories to digest and metabolize them than the calories they provide to our body is a myth, TEF is real. Negative calorie foods do not exist. But its known that different components of food affects TEF differently.

Total daily energy expenditure

basal metabolic rate

Non-exercise activity thermogenesis

Exercise activity thermogenesis

Thermic effect of food

(TDEE) =

(BMR) +

(NEAT) +

(EAT) +



60 - 70%

15 - 20 %



We all know that to loose weight our total energy expenditure (total calories we burn) per day must be more than the calories we eat. Hence its important to understand the component of Total daily energy expenditure (TDEE). Basal (Resting) metabolic rate, i.e. the energy our body needs to perform essential life sustaining functions like breathing, circulation, nutrient processing etc., forms the major component of TDEE and accounts for 60- 70%. The energy we spend by doing activities of daily living and exercise accounts for approximately 20% and the energy we spend to digest, absorb and metabolize food accounts for 10-15 % approximately. So through this its clear that we can increase our daily energy expenditure by increasing thermic effect of food.

Different components of food have different TEF value. TEF value of Carbohydrates is estimated to be around 6-8%, fat is around 3-5% and protein is around 20-30%. This means if you eat 100 calories of protein you burn 20-30 calories by digesting them. Protein intake also increases satiety. No wonder studies consistently proved that high protein diet aids weight loss. Whole grains and fiber rich foods also requires more energy to digest than processed foods.

​Summary: Thermogenic effect of food (TEF) is the amount of energy required to digest, absorb and metabolize the nutrients we get from food. Though negative calorie foods do not exist, we can boost our energy expenditure by eating foods those are high in TEF value. For this you need not get obsessed with numbers or search for a super food. All you need to do is replace processed foods with whole grains and include considerable amount of protein in your diet. Among carbohydrates, fat and protein, protein has the highest TEF value of 20-30%

Different foods affect our Hormones differently - Processed vs Unprocessed food:

Food we eat affects our hormones in different ways. The most spoken hormone in weight loss industry is Insulin. Foods with high glycemic index trigger greater insulin release than fiber and protein rich foods. Excessive insulin in blood can trigger storage of glucose as fat leading to weight gain.

Foods also affects satiety hormone leptin and hunger hormone Ghrelin differently. Ultra processed food that lacks fiber can trigger greater insulin and also high fructose content in them can release more Ghrelin leading to increased appetite. On contrast fiber and protein rich foods helps in release of satiety hormone leptin that gives a feeling of fullness after eating.

​Summary: Processed foods made of refined carbs and high fructose can further increase appetite leading to increased calorie intake while foods rich in fiber and protein release satiety hormone (leptin) that makes us to feel full. Its easy to consume 500 calories from ultra processed foods like ice cream but its difficult to consume 500 calories of eggs or vegetables.

Gut microbiome - Last but not least:

The gut microbiome refers to trillions of bacteria, fungi, and other microorganisms that reside in our digestive tract. Interestingly even these organisms influence our metabolism. This may be because of some organisms can breakdown the undigested foods and extract calories from them than other type of organisms. Recent research suggests that the composition of the gut microbiome can predict an individual’s likelihood of obesity. Though researches are still working to understand how the gut microbiome affects our metabolism, by now we know they play a major role.


Weight loss is not a simple math that works on the principle of calories in and out. Its too complex. Different foods undergo different digestive process inside our body and affects our hormones differently. Hence quality of diet means more than just counting calories. Consuming 500 calories from ultra processed food can negatively affect our health and weight while consuming 500 calories from fiber and protein rich foods can enhance health. So lets stop just counting calories and focus on healthy eating habits and physical activity to stay healthy. Having said this its not necessary to adapt any diet forms especially those promote extreme low carb intake. Carbs are not bad. They form an essential part of balanced diet. But source of carb we eat really maters. The carbs from highly refined sources are unhealthy while complex carbs we get from whole grains, legumes and pulses support our gut health there by improving overall health. Gut microbiome play a vital role in gut health but it doesn't mean that we should take pro-biotic or pre-biotic supplements. All we need to do is to include fermented foods in our diet and have at least 7-8 hours of quality sleep every night.

- Sangeetha Duraiswamy

B.P.T., C.S.P.T., M.I.A.P., M.B.A. (HRM),

M.Sc.(Diet and food serivice management)

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