4.2 Macronutrients

Topic Progress:

We now need to consider the three main food groups (macronutrients) comprised of carbohydrates, protein and fats that supply us with energy, amino acids and other vital nutrients. We will go into more detail about vitamins and minerals (i.e. micronutrients) in Part 2.


Carbohydrate is an umbrella term for many different classes of compounds. Carbohydrates are classed on structure rather than function as:

  1. “Simple” sugars, e.g. fructose and glucose, due to simple monomer sugar molecules.
  2. “Complex” (i.e. polysaccharides) due to the polymeric chains of greater than 10 sugar monomer units, e.g. fibres and starch.
  3. Oligosaccharide chains comprised of 2-10 sugar monomer units, e.g. prebiotic carbohydrate such as fructooligosaccharides (FOS).

In terms of function, simple carbohydrates provide easily accessible energy, whilst polysaccharides require digestion to provide fuel. Some simple sugars like FOS are important and functionally active within the body, whilst some of the more complex carbohydrates, such as different types of fibre, are relatively metabolically inert. We will discuss this in terms of health later in this module.

Carbohydrates are primarily required for energy (ATP) production via the two cellular pathways we have previously described (anaerobic and aerobic respiration, as well as structural elements and forming functional moieties in living cells. For example, cartilage has a high carbohydrate component in its structure and the substances carried on the surface of certain blood cells that identify the blood groups are of a carbohydrate character.

Carbohydrates are often given a bad rap in the nutrition world, with metabolic syndrome, obesity and even cancers being linked to excessive carbohydrate (sugar) consumption. However, the physiological effects of a carbohydrate-rich meal, as well as its digestibility, depend on the composition and type of carbohydrate.

Simple Sugars

The main simple sugars you will come across in foods include:

Simple Sugar Source & Function
Glucose and fructose Monomers formed when sucrose breaks down, e.g. during digestion, and also occur freely in many natural foods such as vegetables and fruit. Fructose is the sweetest of the simple sugars found in high concentrations in honey, fruits and some vegetables. It’s often described as fruit sugar; high fructose corn syrup (HFCS = glucose combined with fructose) or fruit juice concentrates on food labels and is linked to significant increases in blood sugar levels and insulin release when consumed. We will discuss this in relation to health in Part 2 of this course.
Sucrose A common sugar obtained from beet or sugar cane found in different forms in foods, e.g. granulated, icing, castor, demerara, liquid sugar etc.
Lactose Milk sugar (a major component of milk) that is broken down in the intestine to glucose and a sugar called galactose.
Mannose Less common sugar that occurs in natural foods.
Trehalose and raffinose Special sugars that are characteristic of particular foods; trehalose found in certain plant and animal sources including shrimp; raffinose occurs in plants and whole grains including Brussel sprouts, cabbage and broccoli. The functional significance of these sugars will be discussed later in this module.

You will notice that all these sugars have the suffix   “-ose”. This can help you to easily identify sugars listed on food labels even if you’ve not heard of the type of sugar before!

Polysaccharides (complex carbohydrates)

The polysaccharides are more complex substances that are built from simple sugars. In many cases polysaccharides can be broken down in the body to yield those same component sugars. Significant polysaccharides include:

Complex Sugar Source & Function
Starch Found in plants and exists in various forms but is always comprised entirely from glucose. Used by plants as an energy store whilst living. You will probably be familiar with the expression ‘starchy foods’, which includes potatoes and bread. In the intestines, starch is broken down entirely to glucose and absorbed into the blood.
Glycogen Like starch as it’s comprised entirely from glucose, but is an energy store exclusively for animals. The human body forms glycogen from any excess glucose and lays it down as a temporary carbohydrate (energy) store in our liver and muscles. Therefore, when we eat liver or meat we consume glycogen, which is digested to glucose again in the intestines.
Cellulose A plant polysaccharide built up entirely from glucose, but it is quite different from any other plant polysaccharide as our bodies cannot digest it (although cows and horses can). Cellulose is a major component of plant fibre and passes through our digestive system unchanged.


Plant fibre is made up largely of cellulose, but also contains many other complex plant structural oligo- and polysaccharides. Cereal fibres are generally insoluble, whereas fruit, vegetables and nuts contain a higher proportion of soluble fibre. We do not digest fibre; rather it passes through the digestive tract relatively unchanged until it reaches the colon, where colonic bacteria ferments some fibres producing metabolic by products including short chain fatty acids (SCFAs) as we discussed in module 3. This is one of the main reasons why fibres are so vital for healthy digestive and immune function.

Fibre Source & Function
Oligosaccharides, including inulin and FOS Provide fuel for the colonic microbiota. These symbiotic bacteria then support our health in a variety of ways by producing SCFAs that are used by the gut to support growth and turnover of colonic epithelial cells, as well as influence functional aspects of our immune and central nervous system. These types of carbohydrates are commonly referred to as prebiotics and are functionally very important part of our diets and are found in sources like garlic, asparagus, leeks, Jerusalem artichoke and chicory.
Insoluble fibre (“bulking agents”) Absorbs water and swells in the digestive tract providing faecal bulk to the intestinal contents to add softness to the stools, decreasing transit time and facilitating regular elimination.
Soluble fibre (e.g. pectin) Has a water-holding capacity keeping intestinal contents and stools moist, as well as functional roles in delaying gastric emptying increasing satiety value of a meal and facilitating intestinal elimination of cholesterol.

The provision of bulk by consuming enough fibre in the diet is very important to allow the proper action of the intestinal wall as it works physically upon the food, mixing it, and advancing it along in the intestinal system. Without this dietary bulk we can become constipated. Partially digested food may then take longer than is healthy to travel through the digestive tract, leading to the propensity towards putrefactive fermentation of protein rich foods and other digestive health issues that can affect the whole body, as described in module 3, section 3.3. This is a great example of foods that despite their overall properties being healthful, their transit process can result in them becoming dysfunctional to our health when there is a clinical imbalance in systems such as the digestive tract.

Increasing dietary fibre is therefore a key to managing digestive health and all the positive related implications. However, the suggested minimum 30g per day for UK adults[1] remains well below the daily intake of approximately 120g a day by one of our near hunter gatherer relatives the Hadza, from Tanzania. In fact, despite many years of discussion on fibre intake the average UK dietary daily intake is only around 17-20g. Increasing plant matter is one way to increase fibre and sometimes common dietary supplement sources are suggested including wheat bran, psyllium husks and oat fibre, and prescribed “bulking agents”. However, many individuals experience reactions to wheat (gluten) and/or high levels of fibre being added to the diet in one go. We don’t want to tip the balance the other way!

In terms of functional nutrition, adjusting fibre intake is essential to support digestive and systemic healthy but fibre intake from natural dietary sources is recommended being introduced gradually, coupled with drinking plenty of water (1.5 – 2L daily for adults), to avoid constipation or diarrhoea. There are beneficial gluten-free food fibres including rice bran, apple pectin and apple fibre, which can be easily added into a diet, such as through the addition of daily stewed apples. Please read this article on the benefits of stewed apples to health including immune and gut function.

Is This the Perfect Functional Meal for Mucosal Tolerance?

We’ll explore gut health and the functional impact of dietary fibres, and how to get the best therapeutic effects with minimal adjustment related side effects of dietary fibres later in Part 2 when we look at different therapeutic diet.


Fat, like carbohydrates, form important tissue energy reserves; both for ourselves and for the food organisms we eat. Fats are laid down in large amounts in the seeds of plants like nuts and beans, and in the fatty tissues of animals. However, despite similar molecular components, fats are totally different from carbohydrates in that they do not dissolve in water, dissolving only in organic solvents like chloroform. This big difference is because their oxygen content, unlike carbohydrates, is very small, making them repel water (i.e. hydrophobic).

Other fatty substances have important structural or functional roles in the body. One functional role is in the formation of prostaglandins (such as the anti-inflammatory PGE2 we’ve previously discussed), an important part of the immune system. Two vital structural roles of certain fats, as we discussed in module 3, are in forming the outer membranes of cells and cellular organelles and as the myelin sheath insulation that wraps around nerve fibres, without which our nerves could not communicate.

Many of these functional or structural fats contain nitrogen and phosphorous (as well as carbon, hydrogen and oxygen) and fall into the group called phospholipids, which contain only 2 fatty acids rather than 3 found in triglycerides (Figure 4.1). Phospholipids of different types form a majority part of many cell membranes. For example, phospholipids in the brain contain up to 35% Docosohexaenoic Acid (DHA; a type of fatty acid we will describe later in this section), whilst the retinal cell membranes contain up to 60% DHA in their phospholipid fatty acids. As you now know from module 2, the shape, composition and ratio of different fatty acids in different cell membranes, in particular phospholipids, is key to membrane permeability and cell/tissue function.

Lecithin is also a well-known functional phospholipid, forming part of bile, synthesised and released by the liver and gallbladder to aid fat digestion in the digestive tract. Nutritionally, lecithin can help support optimal cell structure and function including that of the liver, cardiovascular system and the mitochondrial cell membrane. It’s naturally found in sources such as soya, sunflower seeds and egg yolks. You’ll explore this functional health-positive nutrient further in Part 2.

Saturated & Unsaturated Fats

Dietary fats are commonly triglycerides, i.e. oneglycerol combining with three fatty acids – in any one triglyceride the three fatty acids that combine may be all the same or may be different (Figure 4.1).

Structure of A Triglyceride Fat
Figure 4.1 Structure of a Triglyceride Fat

All fatty acids are carbon chains and can be divided into two groups based on their structure – saturated and unsaturated depending on the absence (saturated) or presence (unsaturated) of double bonds in their carbon chain structure. This may seem like a bit of a chemistry lesson but the structure of fats is key to their structural and functional (and dysfunctional) properties in the body.

Examples of saturated fatty acids include:

  • Myristic acid
  • Palmitic acid
  • Stearic acid
  • Arachidic acid

Saturated fatty acids are grouped into carbon chain length including short chain fatty acids (SCFAs; 4-6 carbons) discussed in the previous section about fibre, and medium chain fatty acids (8-14 carbons; also called Medium Chain Triglycerides (MCTs), which include myrisitc acid, caprylic acid and lauric acid found in coconut oil.

Examples of unsaturated fatty acids include:

  • Oleic acid
  • Linoleic acid
  • Alpha linolenic acid
  • Arachidonic acid
  • Gamma linolenic acid
  • Eicosopentaenoic acid
  • Docosohexaenoic acid

Oleic acid is an example of a group known as mono-unsaturated, and is the principal fatty acid of olive oil, also sometimes called Omega 9. All the other unsaturated fatty acids listed above are known as polyunsaturated, due to the presence of several double bonds throughout their carbon chains. An easy way to identify food content of different types of fats is that a high proportion of saturated fatty acids are generally solids at room temperature, e.g. butter, while fats with a high proportion of unsaturated fatty acids are generally oils (e.g. olive oil) or soft solids at room temperature.

As outlined in module 3, fats are digested and absorbed in the small intestine where at least a proportion of them are broken down to glycerol and their component fatty acids through the digestive action of lipase enzymes and bile. Fat micelles are then absorbed into the lacteal system, which drains into the blood stream for circulation through the liver and around the body.

None of the saturated fatty acids are essential in our diet but certain polyunsaturated fatty acids, namely essential fatty acids (EFAs) can only be obtained from certain foods. Within this vitally important group of polyunsaturated fatty acids there are two EFA subgroups, known as Omega 6 and Omega 3 fatty acids.

Omega 6 comprises:

  • Linoleic acid (LA)
  • Arachidonic acid (AA)
  • Gamma linolenic acid (GLA)

Omega 3 comprises:

  • Alpha linolenic acid (ALA)
  • Eicosopentaenoic acid (EPA)
  • Docosahexaenoic acid (DHA)

We can only obtain these 2 groups of fatty acids from the diet; the body cannot synthesise them like it can other types of fats. This means that we must have enough of each subgroup in our diet but, with their distinct functional roles, having more of either one of them will never compensate for insufficiency of the other.

Functionally these EFAs are important for wide variety of reasons. For example, the Omega 6 GLA is important for the synthesis of anti-inflammatory PGE2 whereas, as we’ve just discussed, Omega 3 DHA is required for cell membrane structure and function in the nervous system and retina. ALA is the parent Omega 3 EFA and is commonly found plant sources such as flaxseeds and flaxseed oil; it requires enzymatic conversion to the bioactive EPA form (Figure 4.2).

Metabolic Pathways of Omega 3 and Omega 6 Essential Fatty Acids
Figure 4.2 Metabolic Pathways of Omega 3 and Omega 6 Essential Fatty Acids

Omega 6 EFAs occur widely in many foods and are generally plentiful in most UK diets as these fatty acids are especially abundant in most vegetable oils (used in many processed food products, margarines, cooking oils etc.), as well as seeds and nuts. Arachidonic acid (AA) is a precursor to a pro-inflammatory cascade in the body, where it’s converted via the cyclooxygenase 2 (COX-2) enzyme to a range of pro-inflammatory prostaglandins, as discussed in module 3 and outlined in Figure 4.2.

AA is found in high levels in red meat, hence reducing red meat in the diet may be one factor in reducing inflammatory burden in the body. We will explore anti-inflammatory diets further in Part 2 of this nutrihub advanced course. For now, please familiarise yourself with Figure 4.2 showing the conversion of Omega 6 and Omega 3 EFAs in the body. Note that blocking COX enzyme activity is the mechanism of action of commonly used Non-Steroidal Anti-inflammatory Drugs (NSAIDs) like aspirin and ibuprofen, as described in module 3.

The Omega 3 EFAs are not so well distributed in foods and only occur in high concentration in oily fish, like salmon and mackerel, fish oils and in their parent form in whole flaxseeds and flaxseed oil. A moderate amount is present in rapeseed oil, walnut oil, hemp oil, pumpkin seed oil and their respective whole seeds and nuts. Hence, a relative insufficiency of Omega 3 EFAs is a common nutritional problem in our society. We will explore the role of EFAs in health and disease when we investigate different functional nutrition programmes, especially the anti-inflammatory diet in Part 2.

So to recap:

Omega 3 and Omega 6 EFAs are essential because:

  • We cannot synthesise them.
  • They are needed within the body as a raw material from which to make a group of important anti-inflammatory hormones called prostaglandins.
  • They are needed to maintain the integrity of cell membranes.
  • They are involved in many essential functions within the body, from maintaining the integrity of the digestive system to regulating the immune system, and regulating the clotting of blood.

Problems that can arise from EFA deficiency include susceptibility to heart attacks, hormone imbalances (e.g. Premenstrual Syndrome (PMS), and endometriosis) and under functioning or disorders of the immune system resulting in a manifestation of events such as allergies or a poor ability to fight infections. You will learn about supplementing EFAs through sources such as fish and krill oil in Part 2.

Dietary Fats & Health

One major nutritional problem that our society faces is that fats in general have become subject to over-consumption. In the UK some 40% of dietary energy is derived from fats and oils. Obesity is a significant problem in our society through eating too much food, especially too much “junk” food like chips, burgers and fried breakfasts. However, science now also suggests that it’s the conversion of excessive dietary sugars that is one of the main causes of obesity rather than the over-consumption of fat per se. Fats can be formed in the body from carbohydrates (apart from the essential fatty acids, of course) but carbohydrates cannot be formed from fats. Therefore, it is possible to become obese from having too many carbohydrate foods like bread and potatoes, as well as sugary foods, even if no fat is taken along with them. However, we can still say that over-consumption of fats is not suitable for optimal structure and function of cells, especially as many dietary fats can be damaged by cooking (heating) processes and this creates a highly damaging, toxic, dysfunctional substance for the body to deal with. As we discussed in module 2, polyunsaturated fatty acids are particularly susceptible to damage by heat because of the double bonds in their structure. Frying, or other high-temperature cooking and food processing, especially in the presence of air, produces oxidative damage that converts these fatty acids into altered substances (e.g. trans fats), which our bodies can no longer utilise and may even be toxic to enzyme function and cell membrane structure. For this reason cooking with a little saturated fat, e.g. butter or ghee, is preferred to avoid fat damage from cooking compared to over heating plant oils like olive oil or flaxseed oil.

An increasing number of studies are now demonstrating that damaged trans fatty acids in foods contribute to serious conditions including atherosclerosis through increasing oxidative stress. This is another clear example of how certain foods can create clinical imbalances – not all fats are harmful (quite the opposite for unaltered EFAs) but a careful dietary balance of fats is required to optimise cellular pathways and restore optimal biochemical function. Remember that we can also modify the diet to include nutrients that protect fats (lipids) in the cell membranes from oxidative stress and lipid peroxidation, namely antioxidants like Vitamin E and C, which we will discuss further in Part 2 of this course.

As a final addition to the fat and diet discussion, not all fats are straight carbon chain fatty acids but may also contain a wide variety of groups with ring-like structures, collectively referred to as sterols. We will be looking at the positive health-impact of plant sterols on cholesterol levels in Part 2.


Like carbohydrates and fats, proteins contain carbon, hydrogen and oxygen, but they all also contain nitrogen. Proteins exist in literally thousands of different varieties, so there is little point in trying to list them. We find protein in plant and animal sources including eggs, animal meat and plant seeds. Wheat gluten, wheat gliadin, maize gluten, zein (from maize) are all individual types of plant proteins. Albumin and myosin are also regularly consumed proteins found in egg whites and muscle meat, such as steak or chicken fillets, respectively.

The important nutritional point about proteins is that they are all built up from amino acids, which we first introduced in module 2. Please reread module 2, section 2.2 to remind yourself about protein synthesis and amino acids. Animal protein, and some vegetarian sources including quinoa, contains a mix of all 22 types of amino acids, repeated many times in the chain to make up the complete protein. However, other plant sources of protein may contain just a few of the 22 amino acids, and are termed incomplete protein.

As a reminder to your previous studies, protein is broken down via the action of stomach acid and digestive enzymes in the small intestines to its constituent amino acids. These are then absorbed through the digestive tract into the blood stream to be distributed and used to manufacture our own human proteins that make up the body tissues and enzymes. Human proteins are usually quite distinct from the proteins of other species and from the proteins in food due to their own characteristic arrangements, or sequences, of amino acids within the previously described amino acid chains.

Of the 22 amino acids, eight are essential in our diet, i.e. there is no way we can make them for ourselves in the body. They are known as the essential amino acids and include:

  • Lysine
  • Leucine
  • Isoleucine
  • Phenylalanine
  • Methionine
  • Valine
  • Threonine
  • Tryptophan

A few other amino acids are termed conditionally essential meaning that there are circumstances in which the amino acid is essential in the diet and other circumstances in which it is not. For example, cysteine can be made from methionine if there is an excess of the latter in the body, otherwise cysteine becomes essential though we can also inter-convert cysteine and cystine; tyrosine can be made in the body but only if there is plenty of spare phenylalanine.

All the other amino acids can be made within the body provided there is some spare nitrogen to make them from. These are termed non-essential amino acids. In practice, those amino acids that are non-essential can be produced provided there is a surplus of other amino acids, either essential or inessential, from which to make them. This is achieved by biochemical processes to break down the surplus amino acids, splitting off the nitrogen and then re-incorporating this nitrogen into the amino acids that the body needs.

Functional actions of individual amino acids include anti-inflammatory and antioxidant roles of sulphur amino acids including cysteine (cysteine is part of the tripeptide unit glutathione – often nicknamed “the body’s master antioxidant) and amino acids involved in Phase 2 conjugation pathways in liver detoxification processes. Please refer back to module 3, section 3.1 to remind yourself about liver detoxification. L-glutamine is utilised by replicating cells, including intestinal epithelial cells and fibroblasts. In fact intestinal epithelial cells turnover every 3-15 days so L-glutamine is a very important functional amino acid for digestive health. Glutamine can therefore be use as a directionally useful amino acid sometimes included in gut support supplements. We will discuss amino acids supplementation, either as powder combinations or individual amino acids in Part 2.

The best quality proteins are those in which the percentage of each of the essential amino acids approaches most closely to the percentages in our own bodies. This quality factor is termed the protein biological value. When proteins are available in amino acid ratios closely approaching the balance in our own bodies, then they can be used with maximum efficiency for the synthesis of our own tissue proteins. From this point of view, egg protein shows the most favourable biological value of all. Most animal-derived proteins also have a very favourable biological value.

Plant proteins have highly variable biological values with many plant proteins lacking in one or more of the essential amino acids. They are therefore often referred to as second class (or incomplete) protein. However, this effect can be minimised by mixing different plant protein sources together in a meal that are known to contain a mix of all of the 22 amino acids. Examples of good complementary proteins, in terms of amino acids, are soya and sesame proteins, which complement each other well. These two can be combined to give a mixture having 90% of the value of egg protein and as good as that of beef. Maize protein and bean protein is another combination, which is better than either separately.

Since we must have a protein supply with which to first build and then to repair and replace the tissue proteins of our own body, it follows that a certain level of protein in the diet is quite essential. Some authorities put this from as low as 35g per day up to 65g per day for adults. As we shall see, the requirements of individuals vary depending upon the quality of the protein they eat and how well they digest it, but probably in average circumstances, about 45g per day is enough for many adults. Body weight, gender and activity levels is also a factor since the larger body, and more active people, requires more amino acids for energy, maintenance and repair.

Some dieticians and practitioners think that the more protein there is in the diet the better, especially where sport and muscle building is concerned. However, this is not realistically the case as takes a lot of the body’s energy just to digest protein therefore reducing the available energy for other purposes, like detoxification and ATP producing cellular processes. Excess dietary protein may, over time, overwhelm the digestive system, allowing undigested protein to pass through the intestines. It may transition very slowly through the lower intestines and putrefy, leading to potential toxin formation within the colon, which may be absorbed into the blood (i.e. putrefactive fermentation, as described in module 3). Excess protein in the body is converted by the liver into urea, ready for excretion by the kidney. Excess urea (uric acid) can contribute to build up of internal toxic material, which can form kidney stones. The synthesis of urea also takes up energy, so the excessive intake of protein makes energy demands upon the body, not only for digestion but also for elimination of the waste products of its breakdown.

So to conclude this section; we must be aware of both the structural and functional impact that the three types of macronutrients have on the body and not just view them as calories solely necessary for energy production. This section has given you some background knowledge on the macronutrient groups, which we will discuss further within the functional nutrition framework in Part 2.

If you want more information on fibre and health then please read the optional articles and references

  1. British Nutrition Foundation Fibre factsheet
  2. Schnorr (2013) Gut microbiome of the Hadza hunter-gatherers. Nature Comms 5: 3654 Full paper