Fats and oils - are all fats the same?

The main reason that there is a need for fat in the diet is that on a very low fat diet, providing less than about 10% of energy intake, it is difficult to eat a large enough bulk of food to meet energy requirements. Fat provides 37 kJ /gram, compared with 16 kJ /gram from carbohydrate and 17 kJ /gram from protein. In addition, carbohydrates and proteins in foods are associated with a considerable amount of water, while high-fat foods contain much less, and many carbohydrate-rich foods are also sources of non-starch polysaccharides, which add to the bulk of the food without providing a significant amount of energy.

We can add to this a number of other reasons:

• Vitamins A, D, E and K, as well as carotenes and other lipid-soluble compounds are absorbed dissolved in dietary fat, and indeed fatty foods are the main sources of these nutrients.
• Much of the flavour of many foods is contained in the fat.
• Fat lubricates the food in the mouth, and makes chewing and swallowing easier.
• Two families of polyunsaturated fatty acids cannot be synthesised in the body, and are required as precursors for the synthesis of prostaglandins and related compounds.

Conversely, there is a considerable body of evidence that excessively high intakes of fat, around 40% of energy or more, as was typical of western diets towards the end of the 20th century, are associated with a significantly increased risk of developing chronic non-communicable diseases - the so-called diseases of affluence, including:

• atherosclerosis and coronary heart disease
• hypertension and stroke
• obesity and the metabolic syndrome, leading to the development of type II diabetes mellitus

Carbohydrates and proteins in foods are associated with a considerable amount of water, while high-fat foods contain much less. Many carbohydrate-rich foods are also sources of non-starch polysaccharides, which add to the bulk of the food without providing a significant amount of energy. This means that it is easier to consume more of a high-fat food than a low-fat food. Since fat provides 37 kJ /gram, compared with 16 kJ /gram from carbohydrate and 17 kJ /gram from protein, it is easy to see why a high fat diet contributes to the development of obesity.

The consensus that has developed over the last 40 - 50 years is that fat should provide no more than about 30% of energy, and intake of carbohydrates (and especially starches) should be increased from around 43% of energy intake to 55 - 57%.

As can be seen from the pie graphs below, this reduction in fat intake should be at the expense of saturated fats:

This leads us to consider the different types of fat in the diet.

We can divide the physiologically, nutritionally and metabolically important lipids into four groups:

• Triacylglycerols (sometimes known as triglycerides), in which three fatty acids are esterified to the 3-carbon alcohol glycerol. These are the oils and fats in the diet which provide 30 - 45% of average energy intake
• Phospholipids, in which glycerol is esterified to two fatty acids, with a hydrophilic group esterified to carbon-3 by a phosphodiester bond. Phospholipids are major constituents of cell membranes, and also have an important role on lipid digestion and absorption.

• Cholesterol and other sterols, including very small amounts of steroid hormones that are synthesised from cholesterol. Chemically sterols are completely different from triacylglycerols and phospholipids, and are not a source of metabolic fuel. You will consider the importance of dietary cholesterol in a later exercise.
• A variety of other compounds that are soluble in lipid but not water, including vitamins A, D, E and K, as well as carotenes.

Both are triacylglycerol, although they may contain other compounds, including phospholipids, fat-soluble vitamins and sterols. The only real difference is that fats are solid at room temperature, while oils are liquids.

This depends partly on the chain-length of the fatty acids esterified to glycerol, but mainly on the degree of unsaturation of the fatty acids.

Saturated fatty acids have no carbon-carbon double bonds. When they are esterified to glycerol they can pack together closely, and so form a solid that has a relatively high melting point.

Unsaturated fatty acids have one or more carbon-carbon double bonds, and if the double bonds are in the cis-configuration, when they are esterified to glycerol they cannot pack so closely together, so form a solid that has a lower melting point.

Mono-unsaturated fatty acids have only one carbon-carbon double bond; polyunsaturated fatty acids have two or more carbon-carbon double bonds. Note that in almost all polyunsaturated fatty acids the carbon-carbon double bonds are separated by a methylene (-CH2-) bridge, as shown on the right. It is very rare for the double bonds in naturally occurring fatty acids to be conjugated (i.e. alternating single and double bonds)

Vegetable oils, which are liquid at room temperature, and therefore have a low melting point, will be richer in unsaturated fatty acids, while animal fats, which are solid at room temperature, and therefore have a higher melting point, will be richer in saturated fatty acids.

Cis-trans isomerism is the way in which the carbon chain continues on either side of a carbon-carbon double bond.

In the cis-isomer the carbon chain continues on the same side of the double bond.

In the trans-isomer the carbon chain continues on the opposite side of the double bond

Oils rich in cis-unsaturated fatty acids have a lower melting point than more saturated fats because the cis-double bond means the molecule is bulkier and cannot form as tightly packed a solid as can a saturated fatty acid. The trans-isomer can form a solid that is almost as closely packed as the saturated fatty acid, so the fat will have a higher melting point.

It was noted above that phospholipids have a major role in the structure of cell membranes.

Phospholipids have a long hydrophobic tail from the two fatty acids esterified to carbons 1 and 2 of glycerol, and a very hydrophilic group formed from the phosphate group and whatever is esterified to it.

This means that the hydrophobic tails can interact with each other, and the hydrophilic head groups can interact with water. Phospholipids are thus very efficient emulsifying agents, permitting lipid droplets to remain suspended in an aqueous medium.

Cell membranes consist of a double layer of phospholipid, with the hydrophobic tails inside and the hydrophilic head groups forming the inner and outer surfaces of the membrane.

Various other lipids, including cholesterol and vitamin E, are dissolved in the hydrophobic core of the membrane, and a variety of proteins are embedded in the membrane, some at one face and some at the other. Two groups of proteins span the membrane:

• transmembrane receptor proteins that bind a hormone or other compound on the outside of the cell, leading to a response inside the cell (you will consider this in more detail in a later exercise).
• transport proteins, which provide a pore through the membrane permitting transport of materials either by facilitated diffusion or by active transport (See the exercise on Poisoned by unripe ackee fruit for more on active transport).

The diagram on the right shows a cis-unsaturated fatty acid compared with a saturated fatty acid. The unsaturated fatty acid will permit more space for lipids such as cholesterol and vitamin E to fit in the inner part of the membrane.

It also permits greater fluidity of the cell membrane, which is important for normal membrane function.

Cell membranes are not static, but continually in motion:

• Cells take up relatively large particles by phagocytosis - engulphing them by putting out pseudopodia, then sealing off the cell membrane so as to form a separate intracellular vesicle containing the engulphed material. You will be familiar with this in macrophages, which take up bacteria, but the same process is involved in receptor-mediated uptake of, e.g. plasma lipoproteins.
• The reverse process occurs in cells that secrete peptide hormones such as insulin and glucagon, as well as neurotransmitters, which are stored in intracellular vesicles. In response to a stimulus to secrete the hormone or neurotransmitter these vesicles migrate to the cell surface, and fuse with the cell membrane, so expelling the contents of the vesicle into the extracellular fluid.
• In the resting state, the glucose transport protein in adipose tissue and muscle is in intracellular vesicles. In response to insulin these vesicles migrate to the cell surface and fuse with the cell membrane, so exposing active glucose transporters at the cell surface.

We know that one of the most important factors in atherosclerosis and coronary heart disease is the serum concentration of cholesterol. The graph on the right shows the mortality rate over 6 years in a large group of people plotted against their serum cholesterol concentration at the start of the study. At a plasma concentration of 7.5 mmol /L the risk of death from coronary heart disease is four times greater than at a plasma concentration of 5 mmol /L or lower.

The graphs below show the effect on serum cholesterol of experimental diets in which the saturated and polyunsaturated fatty acids in the diet were varied at the expense of mono-unsaturated fatty acids, with a constant total percentage of energy coming from fat.

Over the range of intake that might be expected in normal diets, serum cholesterol rose proportionally to 2 x the intake of saturated fatty acids. By contrast, serum cholesterol fell proportionally to 1 x the intake of polyunsaturated fatty acids. You will investigate the mechanisms involved in these effects on serum cholesterol in a later exercise.

There are three parts to this nomenclature:

• C18 shows that the fatty acid has 18 carbon atoms
  • C18:0 shows that the fatty acid has no carbon-carbon double bonds (i.e. it is saturated)
  • C18:1 shows that the fatty acid has one carbon-carbon double bond (i.e. it is mono-unsaturated)
  • C18:2 shows that the fatty acid has two carbon-carbon double bonds (i.e. it is polyunsaturated)
  • C18:3 shows that the fatty acid has three carbon-carbon double bonds (i.e. it is polyunsaturated)
    • C18:1 n-9 shows that the first double bond is at carbon 9 from the terminal methyl group
    • C18:2 n-6 shows that the first double bond is at carbon 6 from the terminal methyl group
    • C18:3 n-3 shows that the first double bond is at carbon 3 from the terminal methyl group

Note that this is the opposite of the systematic chemical way of numbering carbon atoms.

Correctly, carbon atoms are numbered from the carbon atom containing the functional group for which the compound is named. In a fatty acid this would be the carbon of the carboxylic acid group.

Physiologically, what is important is the position of the first carbon-carbon double bond counting from the terminal methyl group of the fatty acid. This is because human tissues have desaturases that can introduce carbon-carbon double bonds between an existing double bond and the carboxyl group of a fatty acid, but not between an existing double bond and the methyl group. Human tissues also have a desaturase that can introduce a carbon-carbon double bond at carbon-9 of the saturated fatty acid, stearic acid (C18:0).

You will sometimes see fatty acids named as omega-3, omega-6 and omega-9 instead of n-3, n-6 and n-9. This harks back to the older way of numbering carbon atoms in a compound, in which the carbon atom to which the functional group for which the compound is named is called the alpha-carbon. (In a fatty acid this would be carbon-2, since carbon-1 is the functional group of the fatty acid). Successive carbon atoms are then labelled with letters of the Greek alphabet, in order. Carbon-3, next to carbon-2, would be the beta-carbon, etc. The last letter of the Greek alphabet is omega, so counting from the end of the carbon chain, the last carbon atom is the omega-carbon (regardless of how many carbon atoms there are in the chain).

You will see in a later exercise that there are also enzymes in human tissues that can elongate fatty acids by adding two-carbon units at a time to the carboxyl group.

If both n-3 and n-6 polyunsaturated fatty acids are required as precursors of other compounds, then obviously there must be a source of them in the diet, since human tissues do not have enzymes that can introduce carbon-carbon double bonds between an existing double bond and the terminal methyl group.

If you look at the structures on the right, then in terms of their function in membrane fluidity you would certainly expect trans-unsaturated fatty acids to behave more like saturated fatty acids than cis-unsaturated fatty acids.

Significant amounts of trans-unsaturated fatty acids are formed when unsaturated oils are hydrogenated to reduce some of the carbon-carbon double bonds to single bonds, so as to produce solid fats that can be used in food manufacture, and as margarines, etc, from liquid oils. The process involves reacting the oil with hydrogen gas in the presence of a nickel catalyst; the catalyst weakens carbon-carbon double bonds and while some are reduced to single bonds, others undergo isomerisation from the cis-configuration to the trans-configuration.

In 1993, Willett and coworkers analysed data from 85,095 women in the Nurses’ Health Study, estimating intake of trans-fatty acids from diet questionnaires, and showed that over 8 years of follow-up there was a significantly increased risk of cardiovascular disease with increased consumption of foods rich in trans-fatty acids, even when other risk factors were taken into account. (Willett WC, et al. Intake of trans-fatty acids and risk of coronary heart disease among women. Lancet 341: 581-5, 1993).

Click here for the HealthWatch position paper on trans-fatty acids

Key points from this exercise:

• There is a need for fat in the diet because when fat provides less than about 10% of energy intake it is difficult to consume a great enough bulk of food to meet energy requirements.
• Vitamins A, D, E and K, and carotenes, are absorbed dissolved in dietary fat.
• Two families of polyunsaturated fatty acids (n-3 and n-6) cannot be synthesised in the body, and are required for synthesis of prostaglandins and related compounds, so are dietary essentials.
• Intakes of fat above about 40% of energy intake are associated with increased risk of chronic non-communicable diseases, and it is considered desirable to reduce average intakes of fat to about 30% of energy intake. Thus should be at the expense of saturated fat, with a corresponding increase in carbohydrate intake.
• Triacylglycerols consist of three fatty acids esterified to glycerol, and provide 30 - 45% of average energy intake.
• Phospholipids consist of two fatty acid esterified to glycerol, with a hydrophilic group esterified to carbon-3 by a phosphodiester bond. Phospholipids are major constituents of cell membranes, and also have an important role on lipid digestion and absorption.
• The main difference between oils, which are liquid at room temperature, and fats, which are solid at room temperature, is the degree of unsaturation of the fatty acids esterified to glycerol.
• Saturated fatty acids have no carbon-carbon double binds; mono-unsaturated fatty acids have one carbon-carbon double bond; polyunsaturated fatty acids have two or more carbon-carbon double bonds.
• Most of the naturally occurring unsaturated fatty acids are in the cis-configuration; trans-fatty acids arise as a result of isomerisation during catalytic hydrogenation of oils to produce solid fats for food manufacture.
• Cis-polyunsaturated fatty acids in membrane phospholipids permit greater fluidity of the membrane than do saturated fatty acids. Trans-fatty acids behave more like saturated fatty acids, and it is considered desirable that intake should not exceed 1% of energy intake.
• There are desaturases in human tissues that can introduce carbon-carbon double bonds between an existing double bond and the carboxyl group, but not between an existing double bond and the terminal methyl group. Therefore numbering of carbons in unsaturated fatty acids is from the methyl group, not, as would be chemically correct, from the carboxyl group.
• The shorthand nomenclature for fatty acids shows C followed by the number of carbon atoms, then a colon and the number of carbon-carbon double bonds. For unsaturated fatty acids this is followed by n- (or omega-) and the carbon atom at which the double bond starts, counting from the terminal methyl group.
• Compared with mono-unsaturated fatty acids:
  • saturated fatty acids increase serum cholesterol proportionally to 2 x the intake
  • polyunsaturated fatty acids decrease serum cholesterol proportionally to 1 x the intake