What are fats, their function, bad and good fats

an avocado cut in half

In this post you will learn what fats are, their function within our body and how these can affect our weight loss journey. With that being said, you will also learn how to calculate the amounts of fats too! Alternatively, you can use our macro-nutrient calculator!

Definition of fats

Fats are organic compounds made up of carbon, hydrogen and oxygen atoms. Fats belong to a group of substances called lipids. Lipids are a group of substances like fats, waxes, fat-soluble vitamins (A, D, E, K), with main function to store energy. These can be solid or liquid.

The most common lipids consist of fatty acids linked to a glycerol molecule. The fatty acids link to a glycerol molecule by a process of condensation reaction which leads to the removal of water molecule so the fatty acids can be linked by an ester bond to form a glyceride. There are three types of glycerides;

  • Monoglyceride – formed when one fatty acid links to a glycerol
  • Diglyceride– formed when two fatty acids link to a glycerol
  • Triglyceride– formed when 3 fatty acids link to a glycerol.

These are all fats, and 95% of the ones we consume are triglycerides and it is in its structural formation that fats are stored in our body.

What is a fatty acid?

Fatty acids are a subgroup of lipids, produced when fats are broken down during digestion, and composed of carboxylic acids which are long hydrocarbon chains varying in length. These chains can be characterised into short-medium and long-medium chains. The type of glyceride that is formed is determined by the types of fatty acids involved; saturated or unsaturated.

A saturated fats and fatty acids

A saturated fatty acid has all it carbon atoms ‘saturated’ in hydrogen atoms and has the maximum amount of hydrogen atoms attached to each carbon atom. They are generally in a straight chain so they can be packed tightly together. Saturated fats are solid at room temperature.

Sources of saturated fats include;

  •  beef
  •  lamb
  •  Pork
  •  Veal
  •  dairy products
  •  Margarine
  •  Butter
  •  some cheeses
  •  cocoa butter
  •  coconut oil
  •  Palm oil.

Unsaturated fats and fatty acids

Unsaturated fatty acids are ones which have not been saturated in hydrogen atoms. Because they are not saturated, they have double bonds. Among unsaturated fats, there are monounsaturated fatty acids which have one double bond and polyunsaturated fatty acids which have multiple double bonds. When double bonds appear in the fatty acid structure, kinks begin to appear, the more double bonds, the more kinks appear within the structure of the fatty acid so it is then difficult to compact the fatty acids close together so melting may be difficult.

These kinks are important for things like membrane fluidity (Norton and Baker, 2019). Membrane fluidity refers to how our body fat is structured. If the body fat membrane fluidity is low, the fat is compacted together and difficult to burn or melt. Some people may refer to this as stubborn fat. Whereas, if the membrane fluidity is well maintained, the kinks mentioned previously allows our body to tap into that fat a little easier for energy supply so burning our body fat would not be as difficult.

What are trans fats?

Furthermore, trans fats, also known as processed fats, are artificially created from polyunsaturated fats through a process of hydrogenation which is simply a process of adding hydrogen atoms. During hydrogenation, a polyunsaturated fat can be converted into a solid saturated fat which is health damaging. These are probably the worst type of fat out there.

What are healthy fats? and what are good fats?

Omega 3 & 6

There are essential fats. Omega 3 (linolenic acid) and Omega 6 (linoleic acid) are polyunsaturated fats, the ‘3’ and ‘6’ refer to the position of the first carbon double bond. These fats are essential for the functioning of cardiovascular, reproductive immune and nervous system, and they help in enabling our bodies to obtain optimal nutrition.

Omega 3 & 6 are also building blocks for a group of compounds called eicosanoids which play an important role in inflammation, immune system and act as messengers in the central nervous systems.

This group consists of prostacyclin, thromboxane, leukotrienes and prostaglandins;

  • Prostacyclin and thromboxane is a compound produced in arterial walls which acts as an anticoagulant (reduces body’s ability to clot) and vasodilator (promotes blood dilation).
  •  Leukotriene can cause inflammation but mediates leukocyte accumulation during inflammation which fights infection and viruses.
  • Prostaglandins regulate inflammation, fight infection, helps blood clotting, regulates blood pressure, regulates body temperature and supports smooth muscle contraction

Importance of fats in our body

  • They provide more energy than carbohydrates and protein, with 9 calories per gram compared to 4 calories per gram in carbohydrates and protein
  • The fat under the skin provides insulation and helps prevent people from losing body heat
  • Fat transports vitamins A, D, E, K which can only be transported around the body by fat
  • Fat under the skin keeps the skin hydrated and prevents water evaporation
  • They are a major constituent of cell membranes and walls of their body cell membranes and the walls of the body’s cells that body can’t live without.

Fats and weight management

Appetite and our ability to store fat are all controlled by hormones, like insulin, and changes in the levels of these hormones. Eating this macronutrient will not actually cause fat to accumulate in the body, fat storage is regulated by a number of hormones like insulin. Insulin is released in response to rising blood sugar levels which results from carbohydrates and protein not fat. Reducing fats in your diet will create a caloric deficit since fats are higher in calories, providing you with 9 calories per 1 gram of fat.

Also, as there are different types of fats; saturated and unsaturated, gram per gram the calories are the same. Remember low fat foods have higher levels of sugar in them. It will be the excess calories that lead to weight gain. Replacing saturated fats with polyunsaturated fats will simply provide you with cardio protective properties but exchanging the saturated fats for unsaturated like monounsaturated or polyunsaturated will have no impact on weight loss since they are equal caloric value and there is no evidence that one fat may be more thermogenic than another. The thermogenic process refers to creating heat within the body which can assist in weight loss. The only benefit of replacing saturated with unsaturated is the fact that the fat will be easier to be used for energy. Saying that, we conclude that calories in and calories out are what determines weight loss or weight gain.

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How much fat do I need?

The government recommends 33-35% of your total daily calories to come from fats, of which 11% can come from saturated fats and no more than 2% can come from trans fats. It is best to limit the saturated fats and trans fats as much as possible as these can contribute to heart disease as they raise cholesterol. Some may argue that 33-35% is quite high, but some research has shown that lowering your fat consumption to 20% or less of your total daily caloric intake may cause your testosterone to drop which can be detrimental to maintaining your muscles and strength which are both important for weight loss and prevention of weight regain (Hämäläinen et al, 2018). As mentioned in our protein article, the more muscles you have the more calories you burn at rest. – I like this

How to calculate fat intake?

Let’s take example of a hypothetical man who eats 1500 calories per day.

To calculate total fat, we have decided to use 33%. So, 0.33 x 1500= 495 calories per day.

If we divide 495 by 9 (calories per 1gram of fat) then we get 55grams of fat per day.

To calculate saturated fats, 0.11 x 1500=165 calories, divide this by 9 and you get 18.3grams of saturated fats.

Lastly, to calculate trans fats, 0.02 x 1500= 30 calories. 30/9 =3.3grams of trans fats.

So, this means 55g of fats, of which 18.3g can be saturated and 3.3g can be trans per day.


This macronutrient does not make you fat, you would need to overeat on these in order to gain weight. You could even say that these help with weight loss as they are the building blocks of eicosanoids and some vitamins may also only be transported around our body in fats. These vitamins can play an important role in weight loss. 


American Heart Association Nutrition Committee: Lichtenstein AH, Appel LJ, Brands M, Carnetho M, Daniels S, et al. Diet and lifestyle recommendations revision 2006: a scientific statement from the American Heart Association Nutrition Committee. Circulation 2006;114(1):82-96

Hämäläinen EK, e. (2019). Decrease of serum total and free testosterone during a low-fat high-fibre diet. – PubMed – NCBI. [online] Ncbi.nlm.nih.gov. Available at: https://www.ncbi.nlm.nih.gov/pubmed/?term=6298507 [Accessed 17 Jul. 2019].

US Departments of Health and Human Services and Agriculture. Dietary Guidelines for Americans 2005

WHO, Obesity and Overweight, Fact sheet No 311, updated 2011

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