Macronutrients are nutrients that are required in large quantities as part of our diet.
The three macronutrients required by humans are:
Carbohydrates
Proteins
Fats
Energy is provided by each macronutrient in the form of calories (kcal). The approximate amount of calories each macronutrient provides per gram (g) is as follows:
Carbohydrates (1g) = 4 kcal
Proteins (1g) = 4 kcal
Fats (1g) = 9 kcal
Carbohydrates
A carbohydrate is a biomolecule that is made of carbon, oxygen and hydrogen atoms. The hydrogen and oxygen atoms have a ratio of 2:1; much the same as water, albeit with a few exceptions, which may explain the significant loss of water weight on low carb diets such as the ketogenic diet.
Technically speaking, carbohydrates are the hydrates of carbon (hence the name).
The term carbohydrate is synonymous with saccharide - a group that includes cellulose, sugar & starch. The saccharides are split into four main groups:
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
Monosaccharides and disaccharides are commonly referred to as 'sugars' and are usually recognizable by names with the suffix "-ose." E.g., glucose. Oligosaccharides and polysaccharides are typically polymers of simple sugars like monosaccharides, the amount of which is between 3-10 monosaccharides for oligosaccharides and more than 10 for polysaccharides.
Polysaccharides are what make up your glycogen stores in your muscles and liver.
Proteins
Proteins are macromolecules (aka large bio-molecules) made up of one or more long chains of amino acid residues. A protein is made up of at least one long polypeptide (a linear chain of amino acids residues). Short polypeptides containing less than 20-30 residues are considered as peptides (or oligopeptides) and not as proteins.
Once a protein is formed, it will exist for a certain period of time (ranging from minutes to years, but for most proteins in human cells it is 1-2 days) before becoming degraded and ultimately recycled through a process known as protein turnover.
Many proteins act as enzymes that catalyze biochemical reactions. As such, they are incredibly important to metabolism.
In humans, proteins are fundamental in the diet to provide the essential amino acids that cannot be otherwise synthesized from within the body itself.
Protein is commonly known for its role in the growth and repair of our bodies and those looking to build muscle mass often favour higher amounts of protein in their diets for this reason.
Fats
Fats, also known as triglycerides, are all esters of the alcohol glycerol and fatty acid chains. Fats in the wider sense are commonly synonymous, and placed under the broad umbrella of lipids (as not all lipids are triglycerides). However, in the stricter sense, fats are lipids that are solid at room temperature, whereas oils are lipids that are liquid at room temperature.
Fats undertake structural and metabolic functions, and as such, they are a necessary part of the human diet. This is due to the fact that some essential fatty acids are not synthesized by the human body, so consumption is important.
Fat-soluble vitamins such as A, D, E, and K can only be digested, absorbed and transported in conjunction with fats. Fats play a fundamental role in promoting healthy cell function, protecting organs against shock, maintaining body temperature, and maintaining healthy hair and skin.
Fatty acids tend to be described based on length:
SCFA = Short Chain Fatty Acids
MCFA = Medium Chain Fatty Acids
LCFA = Long Chain Fatty Acids
VLCFA = Very Long Chain Fatty Acids
Fats and oils are categorized dependent on their molecular structure - in particular, the number and bonding of carbon atoms. Saturated fats have no double bonds between the carbons in the chain, whereas unsaturated fats have one or more double bonds between the carbon atoms (those with multiple double carbon bonds are referred to as polyunsaturated fats). Unsaturated fats can be split into cis fats and trans fats; the latter of which is rare in nature.
Studies favor cis unsaturated fats over saturated fats in regards to cardiovascular health.
"Lifestyle advice to all those at risk of cardiovascular disease and to lower risk population groups should continue to include permanent reduction of dietary saturated fat and partial replacement by unsaturated fats"
- Hooper et al 2015
However, trans fat, a form of unsaturated fat, has been found to increase the risk of cardiovascular disease. It is advised generally to replace trans and saturated fats with monounsaturated and polyunsaturated fats in your diet.
Despite Hooper et al's findings in 2015, an article published in the British Medical Journal in 2016 found that dietary saturated fat was of no consequence to the health of one's heart.