Table of contents
- What is fat?
- Where is fat found?
- What is fat used for?
- Brown fat and white fat
- How is fat used by the body?
- How does fat influence the body?
- How does fat impact on health?
- How much fat should people be consuming each day?
Fat is a word that can have many meanings. It is a type of tissue; it can also refer to a cell type and to a class of molecules. For the purpose of this paper the word fat is a type of molecule. Fats composition can vary and therefore can occur in two states: as a solid at room temperature (called a fat) OR as a liquid at room temperature (called an oil).
As fat is classed as a compound belonging to the lipid group, it means that it is insoluble in water but soluble in organic solvents such as ether, chloroform and acetone. This is often summarised in the phrase “like dissolves like” – as fat is mainly a non-polar compound it will only dissolve in non-polar solvents. This relates to its structure.
Fat molecules consist of a glycerol molecule joined to three chain structures called fatty acids, which in turn is referred to as a triglyceride as shown in Figure 1.
Figure 1: The structure of a triglyceride. Image obtained from Howard Hughes Medical Institute website
Fat, being a non-polar compound, is held together by weak van der Waals interactions. The non-polar solvent molecules are also held together by weak van der Waals forces and the enthalpy change to break this interaction is small. As a result of this the dissolution of the fat in the non-polar solvent (like chloroform) occurs spontaneously.
Conversely, as already discussed, the fat molecule is held together by weak interactions. Polar solvents (like water) are held together by strong dipole-dipole interactions and hydrogen bonds, meaning that it is difficult for the fat molecules to form bonds with polar solvents (Frey & Cassidy, 2001).
This dissolution in non-polar solvents; has important implications for the human body. It means that fat is capable of carrying the fat soluble vitamins (Vitamins A, D, E and K) around the body. These vitamins are necessary for optimal health and have many functions within the body, as outlined in Table 1.
Table 1: Sources and functions of the fat soluble vitamins
All information within this table was adapted from the book Essentials of Human Nutrition by Mann and Truswell.
The fatty acids that make up the triglyceride molecule will determine whether a fat is “saturated” or “unsaturated”. This once again relates to its’ structure. If a fatty acid is saturated this means that no more hydrogen bonds can be added across the molecule and all the bonds are single bonds.
This is shown in Figure 2.
Figure 2: A saturated fatty acid molecule. Image obtained from Howard Hughes Medical Institute website
Unsaturated fatty acids therefore have one or more double bonds between the carbon atoms. This means there is still the ability to add hydrogen atoms across the structure.
These molecules can either be monounsaturated (one carbon-carbon double bond) or polyunsaturated (multiple carbon-carbon double bonds) as pictured in Figure 3.
Figure 3: Mono and polyunsaturated fatty acids. Image obtained from Howard Hughes Medical Institute
Trans fatty acids (Trans fat as many of us know it by) is also a type of fat whereby its structure relates to its function. It is an unsaturated fatty acid that contains a trans double bond between 2 carbon atoms that makes the molecule kinked. Trans fatty acids are rare in nature and mostly formed through a process called hydrogenation. Trans fats are not common in the food supply and for good reason. They offer the double health risk of raising the LDL cholesterol (bad cholesterol) whilst simultaneously lowering the HDL cholesterol (good cholesterol) (Frey & Cassidy, 2001).
The most common fat that people associate with animal foods is the untrimmed fat found on meat like steak or lamb chops. This fat, when left on the meat, can contribute a large portion of a person’s total fat intake for the day. This type of fat is also saturated fat, and one that is best avoided due to its influence on weight and heart health.
Fat is also found within foods and is often referred to as the “good fat” (or mono and polyunsaturated fats). This is the fat found in nuts, seeds and oils that are used when cooking like canola and olive oil.
It is important to remember however that all fat is energy dense. As such, no matter whether it is saturated, poly or monounsaturated, it still has 37kJ/gram and as such should be monitored particularly with respect to weight management.
Animals (and humans!) use fat as an energy store and for insulation. It is found in the fat stores on the body known as adipose tissue. The energy store component is an important function of fat, particularly when the body is unable to obtain enough energy. This fat is broken down and subsequently used as fuel for the body until the next eating occasion. As a macronutrient fat, as discussed above, is very energy dense thereby providing a large amount of energy per gram eaten. Fat’s role in insulation helps to keep the body warm and protects the internal organs.
There are two different kinds of fat, both of which have significant health implications for humans. White and brown adipose tissue are similar with regard to a number of highly specialised biochemical functions, such as the synthesis and storage of triglycerides (lipogenesis) and the breakdown of fats to release fatty acids (lipolysis). However the function of these two tissues is the complete opposite: white fat stores energy and brown fat dissipates energy (Wronska & Kmiec, 2012).
White fat (or white adipose tissue) is composed primarily of tightly packed, large spherical adipocytes supported by a richly vascularised loose connective tissue. Adipocytes’ size varies in relation to the cell lipid content, ranging from about 30-130 µm in diameter. The volume of an adipocyte is a determinant of cell’s functionality, with larger adipocytes generally exhibiting larger metabolic activity. In mature adipocytes, a large lipid droplet fills almost entire cell volume, being bounded only a lipid monolayer enhanced with a range of structural proteins.
A dense network of capillaries in adipose tissue provides adequate delivery of substrates and oxygen and ensures sufficient routes for the release of hormones and cytokines which act in an endocrine function (Wronska & Kmiec, 2012).
There are two types of white adipose tissue: visceral fat, localised within the abdominal cavity and subcutaneous fat in the hypodermis. Visceral fat correlates with increased risk of insulin resistance and cardiovascular diseases.
Brown fat is highly vascular and is intensely innervated by sympathetic nerves. Brown adipocytes are smaller and contain less lipid. However, the unique feature is their expression of uncoupling protein in the inner mitochondrial membrane, which functions to uncouple mitochondrial respiration. In mitochondrial of cells, other than brown adipocytes, fuel oxidation is tightly couple to conversion of ADP to ATP. Oxidation of fuels via the electron transfer chain results in extrusion of protons from mitochondria, creating a significant proton electrochemical gradient. Protons re-enter mitochondria via ATP synthase, the released energy driving conversion of ADP to ATP. If ATP is unavailable, however, protons are unable to re-enter via ATP synthase, increasing the proton gradient and limiting further electron transfer and duel oxidation. In contrast, mitochondria of brown adipocytes possess UCP, which dissipates the proton gradient, thereby uncoupling fuel oxidation from the availability of ADP. Thus, the physiologic consequence of UCP activity is unrestrained oxidation of fuels with the sole by-product being the generation of heat. (Lowell & Flier, 1997)
When the body needs to use stored energy, lipases break down lipids into glycerol and fatty acids through a series of reactions known as lipolysis as shown in the following diagram:
Most fatty acids must be transported from the cytosol into the mitochondrial matrix, where they are slowly disassembled as 2-carbon units are chopped off. This happens one unit at a time in a process called β-oxidation (step 3 in the above diagram) (Silverthorn, 2004).
In most cells, the 2-carbon units from fatty acids are converted directly into acetyl CoA, whose 2-carbon units feed directly into the citric acid cycle (step 4 above). Because acetyl CoA molecules can be produced from a single fatty acid, lipids contain 37kJ (9Cal) of stored energy per gram.
Lipid synthesis is difficult to generalise, due to their diversity however most lipids are synthesis by enzymes in the smooth endoplasmic reticulum and in the cytosol (Silverthorn, 2004) as shown in the following diagram:
The glycerol component of triglycerides can be made from glucose or from glycolysis intermediate (step 1). Fatty acids are made from acetyl CoA when the 2-carbon acyl groups are linked together to carbon chains by a cytosolic enzyme called fatty acid synthetise (step 2). This process also requires hydrogens and high energy electrons from NADPH (Silverthorn, 2004).
The combination of glycerol and fatty acids into triglycerides takes place in the smooth endoplasmic reticulum (step 3). The steps that turn triglycerides into phospholipids also take place in the smooth endoplasmic reticulum.
The traditional role attributed to white adipose tissue is energy storage, with fatty acids being released when fuel is needed (Trayhurn & Beattie, 2001). It is now widely accepted that white adipose tissue is not merely a fuel storage organ, but also a key component of metabolic homeostatic mechanisms (Wronska & Kmiec, 2012).
For example the leptin gene is expressed in all white adipose tissue cells and suggests that the hormone may have an autocrine or pararcrine function in adipose tissue. Leptin affects energy expenditure, and acts as a major signal to the reproductive system. There are a number of factors which acutely influence leptin synthesis in white adipose tissue, and these are superimposed on the endogenous level of production associated with the amount of body fat. Fasting leads to a rapid inhibition of the gene expression, and there is a fall in the level of circulating leptin. These effects are reversed upon re-feeding (Trayhurn & Beattie, 2001).
Another suggested role is that of C-reactive protein (CRP). The levels of this have been shown to rise with BMI and elevated levels of this inflammatory marker have been associated with both obesity and diabetes, falling with weight loss (Trayhurn & Wood, 2004). The possibility has been raised that adipose tissue contributes directly to the circulating pool of CRP; however more research needs to be conducted.
Of course, there are many other proteins secreted by white adipose tissue, such as TNF-α, TGFβ, Adiponectin and IL-6, whose roles are vast in the human body.
Storing excess fat can pre-dispose individuals to many health related problems such as hypertension, dyslipademia, type 2 diabetes and polycystic ovary syndrome (PCOS) in girls. The effects of storing excess fat do just lie in the cardiovascular and endocrine effects, but also in the psychosocial and pulmonary aspects. Things such as poor self-esteem, depression and sleep apnoea all affect individuals and can play important roles in individuals who have excess stored fat.
Ideally, individuals should be aiming for around 20-35% of energy in the diet being derived from fat. Saturated and trans fat together should be limited to no more than 10% of energy. For healthy adults (aged 19+) the following adequate intakes (AI’s) have been set (Australian Government Department of Health and Ageing, 2006):
Total LC n-3 (DHA+EPA+DPA)
This article was written by our dietitian Rhiannon Welsh who is a nutritionist, Accredited Practising Dietitian and Dietitians Association of Australia member.
If you have any questions about how fat in the diet or weight, contact your local doctor who will arrange for you to see a dietitian in Sydney. Contact us today!
Australian Government Department of Health and Ageing, 2006. Nutrient Reference Values for Australia and New Zealand. Canberra
Frey, R. & Cassidy, R., 2001. Nutrients and Soluility. [Online]
Available at: Nutrient Solubility – Department of Chemistry [Accessed 27 June 2013].
Lowell, B. & Flier, J., 1997. Brown Adipose Tissue, B3-Adrenergic Receptors, and Obesity. Annual Review of Medicine, Issue 48, pp. 307-316.
Silverthorn, D. U., 2004. Human Physiology An Integrated Approach. 5th ed. United States: Pearson Education.
Trayhurn, P. & Beattie, J. H., 2001. Physiological role of adipose tissue: white adipose tissue as an endocrine and secretory organ. Proceedings of the Nutrition Society, Issue 60, pp. 329-339.
Trayhurn, P. & Wood, I. S., 2004. Adipokines: Inflammation and the pleiotropic role of white adipose tissue. British Journal Of Nutrition, Issue 92, pp. 347-355.
Truswell, J. & Mann, S., 2007. Essentials of Human Nutrition. 3rd ed. Oxford: Oxford University Press.
Wronska, A. & Kmiec, Z., 2012. Structural and Biochemical Characteristics of Various White Adipose Tissue Depots. Acta Physiologica, Issue 205, pp. 194-208.