Energy from Food
Life is a product of energy and all energy comes from the sun. Humans can’t photosynthesise, so we must get our energy from plants who can photosynthesise and store energy, as well as from animals that eat plants. This food, as we know it, is chemical energy. Fats, protein, carbohydrate and alcohol are energy derivatives of food known as macronutrients. Humans need to digest, absorb and transform chemical energy into other forms of energy that we need (e.g. electrical energy for nerve impulses, mechanical energy for movement, etc.). A joule or Calorie is a measure of energy for both food and physical activity (1 Calorie (kcal) = 4.184 kilojoules)). There is some insufficiency in the human metabolic system and not all the energy from food can be used by the body (e.g. protein losses in urea). Atwater factors gives an estimate of how much energy per gram of food we can derive and metabolise (Table 1).
There is a common understanding that if we consume more energy then we expend, we will gain weight. However, this theory assumes that the human body treats every single Calorie (or kilojoule) the same, irrespective of whether they came from carbohydrate, protein, fat and alcohol. This theory also assumes that there are no other factors that affect how one processes, stores and burns energy from food. What we do know is that factors including genetics, insulin resistance, medications, hormones, macronutrient ratios, food quality and so much more can impact the way we use energy. To understand more about how our bodies process the foods we consume and how this affects our health, we must move away from viewing food solely as energy, and instead, focus our explorations on individual macro- and micronutrients.
What Foods Contain Carbohydrate?
This article will focus primarily on dietary carbohydrate. Carbohydrates are abundant in our food supply. Every core food group in the Australian Guide to Healthy Eating (Figure 1) includes carbohydrate-containing foods. Foods in the ‘breads and cereals’ group (orange section) such as bread, pizza, pasta, rice, muffins, cereals, grains and crackers, tend to be the most well-known for their carbohydrate content. Though this food group may be the most carbohydrate-dense (i.e., high % of carbohydrate per 100 g food weight), that doesn’t mean that carbohydrates don’t exist elsewhere. Vegetables (dark green section) also contain more carbohydrate than they do fat or protein, but they also contain lots of water so per 100 g food weight they aren’t usually as high in carbohydrates as breads and cereals. Starchy vegetables such as potato, sweet potato, corn, taro, and yam contain more carbohydrates than non-starchy vegetables like spinach, kale, pumpkin, zucchini and capsicum. All fruits (light green section) contain a unique type of carbohydrate called fructose. Fruits with a higher fibre content, such as berries, tend to have less carbohydrate (in the form of fructose) than fruits like mangos, apples and bananas. Dairy foods (purple section) contain carbohydrate in the form of lactose (glucose and galactose) and include milk, yoghurt, cheese and dairy alternatives (i.e., soy milk, rice milk). The ‘meat and protein foods’ group (blue section) tends to be the least known for containing carbohydrate. That is because this food group includes foods such as steak, fish, and seafood, which have a high amount of protein and fat with very negligible carbohydrate (per 100 g of food weight). However, there are lots of protein-food sources that contain carbohydrate. For example, legumes (beans, lentils, peanuts, etc.), tofu (and other vegetarian ‘meat’), nuts and seeds.
Types of Carbohydrate
Just as carbohydrate, protein, fat and alcohol don’t act the same inside your body, different types of carbohydrates have different effects in your body as well. There are three main classifications of carbohydrate: monosaccharides, disaccharides and polysaccharides. Monosaccharides are “simple carbohydrates” and include glucose, fructose and galactose. Two monosaccharides combine to form disaccharides, and include lactose, maltose and sucrose. Polysaccharides are “complex carbohydrates” and include starch, fibre and glycogen. However, there is no need to get too bogged down by all of these words because at the end of the day, your body has to break all carbohydrate you eat down into its monosaccharide components anyway – with most carbohydrate you eat digested into glucose.
It is important to note that out of all the macronutrients, carbohydrate has the greatest impact on our blood glucose levels (i.e., blood sugars). It’s also important to recognise that our body can use this glucose for immediate energy or put it away for a rainy day, as stored energy (fat or glycogen). However, the way that our bodies process the different types of dietary carbohydrate to get to glucose is also proving to be somewhat important in the understanding of our health, with potential benefits extending far beyond blood glucose control (Figure 2). For simplicity, let’s split carbohydrate up into two more common terms, starch and sugar.
Starches are the energy reserves of plants and are therefore found in plant-food products including pasta, banana, pumpkin, baked beans, rice, cake, biscuits, etc. (Oh, you didn’t think cake was a plant-food product? It is!). Starches can be further classified as slowly-digested starch or rapidly-digested starch, with the main difference being the speed at which glucose is released into the bloodstream (Table 2). Another type of starch that is gaining more traction, with increasing focus on the importance of the gut microbiome, is resistant starch. This type of starch escapes digestion in the small intestine and accounts for 3-10% of the total starch in a food. Metabolic effects of resistant starch are still heavily debated, yet strong associations have been made between resistant starch and good gut-health (e.g., colonic fermentation, lower pH in colon, increased free fatty acids, increased faecal bulk and increased bifidobacteria) . In the 2011-12 Australian Health Survey, the leading foods contributing to total starch were breads, mixed dishes where cereal is the major ingredient (e.g., pasta dish), flours and other cereal grains (e.g., rice), ready-to-eat breakfast cereals and potatoes .
Sugars can be classed as ‘naturally occurring’, ‘refined’ or ‘added’ (can be natural or refined) (Table 3). But don’t be fooled by these somewhat deceiving titles because there is not one kind of sugar that is necessarily better than another. Sugar is carbohydrate, and again, most carbohydrate is broken down into simple glucose molecules in your body and it is analytically impossible for your body to tell apart synthetic glucose from natural glucose. Next time you notice that a food label promotes, “no refined sugars” or even “no added sugars”, this does not guarantee that the product is free from all sugars! Any benefit of consuming one type of sugar over another comes more from the choice of the food as a whole. For example, for someone who is trying to include more fibre in their diet, choosing an apple with naturally occurring sugar is more beneficial than choosing lollies (with refined or added sugars).
Excess consumption of sugar (any type) has been linked to obesity, dental caries, insulin resistance and the metabolic syndrome, and greatly increases one’s risk of chronic conditions such as diabetes and cardiovascular disease. As such, the World Health Organization (WHO) has put a hard limit on added sugars to no more than 10% of total energy intake, and no more 5% of total energy intake (i.e., six teaspoons or 25 grams) for added health benefits . In 2011-12 the average adult intake of sugar was 103 grams per day, though much higher than what is recommended, this is a reduction from previous years . However, a paradox exists in that the obesity epidemic continues to rise despite this reduction in added sugar consumption. It appears that the problem may not necessarily lie in the added sugars, but what people are replacing these added sugars with. Population data shows that the food with the strongest link to weight gain is not food that contains sugar (as we commonly think of it it), it is food that contains poor-quality, starchy carbohydrate – potato chips (Figure 3) ! Once again, with consideration that most carbohydrate breaks down to the same thing in your body at the end of the day – glucose – this may not be so surprising. This data also suggests that replacing carbohydrate with carbohydrate (different types) may not be the best approach, at least from a weight point-of-view.
The Australian Dietary Guidelines recommend Australian’s limit their intake of foods and drinks containing added sugars such as confectionary, sugar-sweetened soft drinks and cordials, fruit drinks, vitamin waters, energy and sports drinks. However, this advice could be counterproductive in tackling the health epidemics it was designed to address as it appears people are replacing added sugars with foods that are proving to be even worse, like potato chips! Maybe the emphasis should shift away from what food and drink we need to avoid and focus more toward the food and drink we need to consume more of. Consuming foods that provide us with a variety of essential micronutrients, nourishing fats and bioavailable protein will naturally displace poor quality choices that are damaging to our health.
This article is designed to improve your understanding of dietary carbohydrate to assist you in your own health journey. Making recommendations based on one macronutrient is not effective, and dietary education is far more useful for a wide audience who all have their own unique health goals. For personally tailored advice, please consult an Accredited Practicing Dietitian (APD). For the purposes of a summary, the take-home points of this article are:
Fats, protein, carbohydrate and alcohol are energy derivatives of food known as macronutrients.
Factors including genetics, insulin resistance, medications, hormones, macronutrient ratios, food quality and much more impact the way we use energy.
Every core food group in includes carbohydrate-containing foods.
Your body breaks most of the carbohydrate you eat down into glucose.
Carbohydrate (sugar and starch) has the greatest impact on our blood glucose levels.
Strong associations have been made between resistant starch and good gut-health.
“No refined sugars” or even “No added sugars” on a food label does not guarantee the product is free from sugar.
The Australian population is replacing added sugars with other carbohydrate foods, like potato chips, and the obesity epidemic continues to rise.
Choosing foods that provide us with a variety of essential micronutrients, nourishing fats and bioavailable protein will naturally displace poor quality choices that are damaging to our health.
By Jessica Turton
Accredited Practising Dietitian
This article provides general information from the current scientific evidence base and clinical judgement of the author. It is designed for educational purposes only and should not be substituted for medical advice. The author recommends you seek personally tailored support from a qualified healthcare practitioner before undertaking any major lifestyle change.
 Aller EEJG, Abete I, Astrup A, Martinez JA, van Baak MA (2011) Starches, Sugars and Obesity. Nutrients 3: 341-369
 Fong M (2016) Carbohydrates. In: Nutritional and Food Science
 Australian Bureau of Statistics (2014) Australian Health Survey: Nutrition First Results - Foods and Nutrients, 2011-12 Available from http://www.ausstats.abs.gov.au/ausstats/subscriber.nsf/0/4683FD7315DFDFDBCA257D080014F9E0/$File/australian health survey nutrition first results - food and nutrients, 2011-12.pdf
 World Health Organization (2015) Sugars intake for adults and children: Guideline. Available from http://www.who.int/nutrition/publications/guidelines/sugars_intake/en/
 Mozaffarian D, Hao T, Rimm EB, Willett WC, Hu FB (2011) Changes in Diet and Lifestyle and Long-Term Weight Gain in Women and Men. New England Journal of Medicine 364: 2392-2404