Over the years we've been told that the cure for losing weight is simple: eat fewer calories and exercise more. But have we been somewhat misled? The implicit suggestion is that there are no bad calories. But the evidence is very clear that not all calories are created equal.
In the science world, a calorie is defined as the energy needed to raise the temperature of 1 gram of water through 1°C. In nutrition, it's a way of describing how much energy your body could get from eating or drinking something. The macronutrients are fat, carbohydrate and protein. Fat contains 9 calories per gram, carbohydrate contains 4 calories per gram and protein contains 4 calories per gram. A measure of energy is a measure of energy, there is no disputing that, and calorie is simply a calorie. However, it would be inaccurate to suggest that all calories are equal.
The most important thing in this argument is to not overlook the biochemistry of what happens to the foods we eat. Food is the tasty carrier bag containing nutrients needed to keep us well. It provides us with large macronutrients like carbohydrate, protein and fat as well as smaller micronutrients such as vitamins and minerals. A calorie of fat is digested and metabolised differently to a calorie of carbohydrate which is again different to a calorie of protein.
Even if we break these macronutrients down further, different types of fats are digested and metabolised differently to each other. An example is Medium chain fatty acids (MCFA) and Longer chain fatty acids (LCFA). Fats are carbon chains classified by how long they are and how they are joined together. MCFA are shorter in length than LCFA. The two behave very differently in the body. MCFAs are more easily absorbed and can be absorbed directly into the circulation heading straight to the liver. Long LCFAs need to be broken down into smaller pieces and packaged up before they're transported around the body. They then go the longer way to the liver. What research is showing us is that this difference matters.
Additionally, different types of carbohydrates are digested and metabolised differently, such as glucose and fructose. Glucose is used as an energy source by all cells of our body. Glucose is what the brain nearly always uses as its energy source. The other cells in the body often use glucose but sometimes use fat. Fructose is naturally present in fruits and vegetables. It can be either found by itself or linked to glucose to form sucrose (table sugar). In contrast to glucose, fructose cannot be directly metabolised in most cells. Instead, it undergoes a first step in processing in our liver. Therefore, they may be both carbohydrates, but what happens when they enter the body is different. What research is showing us is that this difference matters.
Despite the discussion of singular nutrients, it's rare that food is eaten in this way. Usually a food contains a mixture of carbohydrates, proteins and fats. Food is complex. An example of this is the difference between a can of fizzy drink and lentils. The fizzy drink offers up an easy-to-digest portion of carbohydrate. This carbohydrate is easier for the body to digest when compared to the same amount of carbohydrate coming from lentils. The reason for this is that not a lot stands in the way of that carbohydrate entering the blood stream. On the other hand, lentils contain more than just carbohydrate. For example, they offer lots of fibre and some protein.
Fibre is important as it slows down the release of food from your stomach and into your intestines. This slows the release of the carbohydrate into the body, drip feeding the carbohydrate it contains into the body in a very manageable manner. The carbohydrate from the fizzy drink will hit the blood stream at speed. Often when people exercise, or in young people, the body easily manages this. However, as people age, with decreased activity levels or even with a genetic predisposition to diabetes, the person's body can struggle with this influx. The struggle is when damage can occur to the body which can lead to disease. Therefore, each gram or carbohydrate is treated differently depending on the food source and depending on the body that receives it.
Fibre not only acts as a dam for carbohydrate, it has other roles too. Fibre is the sum of dietary fibre and functional fibre. It refers to non-digestible carbohydrates and lignin that are found in plants. Functional fibre has beneficial physiological effects in humans. Fermentable fibres, often referred to as prebiotics, are food for the health-promoting good bacteria. The extent of fermentation varies from person to person as things like transit time through the gut and gut microbial community make a difference. Therefore, each morsel of fibre acts differently in the body and each morsel acts differently in different bodies.
Lentils also provide plant protein. Protein is an interesting macronutrient as it takes a lot of work for the body to break it down and use it - calories are used up using it. This is known as the thermic effects of food or diet-induced thermogenesis (DIT). Generally, protein has a bigger effect than carbohydrate which has a bigger effect than fat. Most of us have a mixed diet which results in a DIT of 5 to 15pc. However, changing where your calories come from can change how many calories a body uses when breaking down and using the daily amount of food. This means that the calorie of one macronutrient is not equal to the calorie of another macronutrient.
Again, as before, it goes beyond the amount of calories you eat from each macronutrient group. In fact, there are differences within each macronutrient group. For instance, a systematic review provided evidence that indicated that the consumption of polyunsaturated fatty acids causes a greater DIT than saturated fatty acids. Also, the consumption of MCFAs compared to LCFAs has been shown to increase DIT. It may also matter how many times we eat.
A systematic review suggested that consuming the same meal as a single meal compared to multiple small meals or snacks was also associated with a significantly higher DIT. Therefore, what percentage of our calories coming from each macronutrient, and the percentage of each of our calories coming from each subgroup of macronutrients, matters when it comes to how many calories our body burns digesting and metabolising the food we eat.
Finally, compare the same fizzy drink to a handful of nuts that contain the same amount of calories. Research has shown us that not all the calories are absorbed. Some pass through our gut and into the toilet without being digested, which depends in part of how much the nut was chewed and how processed it was before it was eaten.
In some ways you could over simplify these scientific nuggets and suggest that if you want your body to work harder for the nutrients, and if more often than not, you want more nutrients available, you would veer to a diet more rich in wholefoods and away from foods that are processed. In fact, many studies have shown this to result in weight loss and in better health outcomes.
Our body is far too complicated to suggest that if one person eats one calorie, that it will be the same in the body of someone else who eats that same calorie. Additionally, food is far too complex to say that any calorie is the same as any other calorie. Many things are at play here from how much we chew our food to our gut bacteria. None of these factors make massive differences but they do add up over the typical 80 years of life. It is time we stopped comparing apples to pears.
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