In the world of health and nutrition, the mantra “calories in, calories out” has been a long-standing rule [1]. At first glance, it makes sense: consume more calories than you burn, and you gain weight; burn more than you consume, and you lose weight. While this idea holds some truth, it paints an incomplete picture of how our bodies interact with the food we eat. The reality is that not all calories are equal [2,3]. For example, 100 calories from vegetables or lean protein will have vastly different effects on blood sugar, hunger, and metabolism compared to 100 calories from sugary snacks or processed foods. In this article we will discuss why the quality of the calories you consume matters just as much as the quantity.

A calorie is a measure of energy, but the food you get those calories from contains more than just fuel. Nutrient-dense foods, such as spinach, broccoli, lentils, apples, quinoa, and chicken breast, pack a powerful punch of vitamins, minerals, fibre, and antioxidants. These nutrients are essential for many bodily functions, ranging from brain performance to immune support. In contrast, many processed foods— pastries, soda, and fast food—provide “empty calories”. While they supply energy, they lack vital nutrients. Consuming too many of these low-quality foods can lead to nutrient deficiencies, even if you’re getting the right amount of total calories.

Different types of food affect your metabolism in various ways. For example, your body burns more calories digesting proteins than it does digesting carbohydrates or fats [4]. This phenomenon is known as the “thermic effect of food”. Foods high in protein (like fish, poultry, and legumes) require more energy to digest, making them more “metabolically expensive”. Similarly, complex carbs (unprocessed foods like whole grains and vegetables) digest slowly and have a high thermic effect due to their high fibre content. For example, complex carbohydrates in brown rice or sweet potatoes take longer to digest because more enzymes are required to break them down into individual sugar molecules. This process uses extra energy, meaning some of the calories consumed are burned during digestion. As the sugars are released more gradually, these foods provide a consistent source of energy and help you feel fuller for longer, which can regulate appetite and reduce the chances of overeating throughout the day. In contrast, highly processed foods like white bread, pastries, and sugary drinks have a low thermic effect because they are quickly absorbed without much digestive effort. This rapid breakdown of carbohydrates leads to sudden spikes and drops in blood sugar levels [5], leaving you feeling hungry shortly after eating. As a result, it often increases the tendency to overeat. 

Food influences more than just your energy intake; it also affects the hormones that regulate hunger, fat storage, and metabolism. For instance, a diet high in refined sugars and processed foods triggers a spike in insulin, a hormone that regulates blood sugar levels and fat storage. Chronic high insulin levels can promote fat storage and lead to insulin resistance, increasing the risk of developing type 2 diabetes [6]. Conversely, consuming nutrient-dense foods, particularly those rich in fibre and healthy fats, helps maintain more stable blood sugar levels and promotes the release of hormones like leptin. Leptin signals fullness, reduces appetite, and additionally has shown to reduce fat mass and stimulate the immune system [7]. This hormonal balance can make a significant difference in weight management and overall health.

Recent research has highlighted the critical role of the gut microbiome in digestion, metabolism, and overall health [8]. The gut microbiome refers to the trillions of bacteria that live in your digestive tract (for more information about the microbiome you can read our other articles about probiotics and “good bacteria“). A healthy microbiome is supported by fibre-rich, whole foods, which promote the growth of beneficial bacteria. Diets high in processed foods, sugars, and unhealthy fats however, can negatively impact gut health by shifting the bacterial composition in a detrimental manner by altering the ratio of “good” to “bad” bacteria [9]. Additionally, the high salt, sugar, and/or fat content of processed foods can also lead to damages in the intestinal barrier and even alterations in neurotransmitter metabolism in both the gut and the brain [10]. Hence, an unhealthy gut microbiome is associated with increased inflammation, poor digestion, and even weight gain [11,12].  

By feeding your body high-quality, fibre-rich foods, you provide the healthy bacteria in your gut with a food source, allowing them to reproduce better. Additionally, these bacteria are able to convert dietary fibres into short-chain fatty acids (SCFAs). Research has proven these SCFAs to be vital for the host’s metabolism including central nervous system function, and are also known to create a more mucus rich and acidic gut environment, inhibiting harmful bacteria and promoting the growth of beneficial bacteria [13].

Finally, it’s important to consider the long-term impact of food choices. While focusing purely on calorie intake might help to manage body weight in the short term, it does not take into account the overall health effects of the diet. ​​The processing of food often involves methods such as milling, refining, and cooking, which can degrade or remove fibre as well as affecting the quality of the proteins [14]. For instance, whole grains are rich in fibre, but when grains are processed into white flour, much of the wheat bran and wheat germ—which contain the fibre and much of the nutritional quality—is removed. Additionally, cooking and heating can also denature proteins, which can reduce the food’s nutritional value and also reduces the need for your body to work for digestion and therefore lower the thermic effect. Processed, low-quality foods increase the risk of chronic conditions like heart disease, type 2 diabetes, and certain cancers [15,16]. On the flip side, a diet rich in whole, unprocessed foods—those that provide not only calories but also a wealth of nutrients—has been consistently associated with a longer life and reduced risk of chronic diseases. 

A calorie from a doughnut is simply not the same as a calorie from a bowl of quinoa—your body will react differently to each.