Triglycerides, Lipogenesis, Ketosis and Digestion

Question 1

Triglycerides

Answer 1

Triglycerides (TGs) are the major form of dietary fat, and the form in which fat is stored in the body. They circulate in the blood when released for energy.
• TGs are lipid molecules made up of one unit of glycerol and three fatty acids.
• The three fatty acids can differ in length (number of carbon atoms) and degree of saturation (number of hydrogen molecules attached)
• High levels of triglycerides in the blood have been linked to atherosclerosis, and hence heart disease and stroke.

Question 2

Synthesis and Storage of Triglycerides

Answer 2

The body synthesises triglycerides whenever caloric intake exceeds energy requirements.
• Excess dietary energy is converted to triglycerides via lipogenesis, and excess ingested fat is taken up by adipose tissue.
• Lipogenesis takes place in adipose tissue and the liver.
• Adipocytes are supplied by an extensive network of blood vessels.
• They acquire TGs from circulating lipoproteins, chylomicrons and very low density lipoproteins.

Question 3

Lipogenesis

Answer 3

Lipogenesis is the process through which acetyl-CoA is converted to triglycerides for storage in fat.
• Fatty acids are synthesised when there is an excess of carbohydrates.
• Acetyl CoA, created from glucose during glycolysis, as well as from fats and amino acids, is built up with the addition of 2 carbon units to form palmitic acid (C16).
• Three fatty acids are bound to glycerol and stored as triglycerides
• The sites of fatty acid synthesis are the liver, adipocytes, kidneys and lactating mammary glands.

Question 4

Lipolysis

Answer 4

When dietary energy is limited, the fatty acids from triglycerides are mobilised from adipocytes into circulation.
• Triglycerides are hydrolysed by lipase into fatty acids and glycerol for use in the body

• Lipolysis is stimulated by:

• Adrenaline, noradrenaline.
• Adrenocorticotropic hormone (ACTH).
• Glucagon and growth hormone.
• Thyroid stimulating hormone (TSH) and thyroxine.
  • Insulin antagonises the lipolytic effects of these hormones. As a result, insulin resistance (e.g. Type 2 diabetes) = central adiposity.

Question 5

Fatty Acid Catabolism

Answer 5

Fatty acids can be broken down to produce energy.
• Fatty acids cross the cell membrane, traverse the cytosol and reach the mitochondria.
• Carnitine facilitates the transport of fatty acids across the mitochondrial membrane.
• The fatty acids undergo beta oxidation and are broken down into 2 carbon blocks as acetyl-CoA, which is oxidised via the Krebs cycle to CO2 and H2O.
• Energy is then generated using the electron transport chain.

Question 6

Ketosis

Answer 6

When carbohydrate levels are low, fat becomes the primary fuel for energy production. Ketone synthesis becomes necessary because the brain cannot metabolise fatty acids.
• Ketones are made when glucose is in short supply. This occurs overnight, and during dieting or fasting.
• By a process known as ketogenesis, acetyl-CoA is converted to the ketones acetoacetate or β-hydroxybutyrate (β-OHB).
• Acetoacetate can undergo decarboxylation to another ketone acetone
• Acetone build up gives a characteristic sweet smell to the breath.

Question 7

Ketogenic Diet

Answer 7

Low carbohydrate diets initiate a fundamental shift in the body’s primary fuel source from glucose to fat.
• This allows energy needs to be met by utilising fat (fatty acids or by ketone bodies)
• For most adults, this happens when carbohydrates are restricted to less than around 40 g a day
• Ketosis is linked with health benefits including weight loss, and the management of epilepsy, Parkinson’s and Alzheimer’s disease.
• Nutritional ketosis is different from ketoacidosis an unstable and dangerous condition that occurs when there is insufficient pancreatic insulin response to regulate serum β OHB.

Question 8

Lipid Digestion

Answer 8

Triglycerides form the bulk of fat in a Western diet.
• The digestion of triglycerides is aided by gastric lipase in the stomach and pancreatic lipase in the duodenum, which act to separate the glycerol and fatty acids.
• This process is facilitated greatly by the emulsifying action of bile, which increases the surface area of fat
• The resulting two free fatty acids and monoglyceride are transported into enterocytes, where they are
rebuilt in the cell, packaged into chylomicrons and transported via the lymphatic system to the bloodstream.
• The fatty acids can be used or stored in adipose tissue.

Question 9

Optimising Lipid Digestion

Answer 9

• Chew adequately and avoid drinking with meals.
• Increase bile production by optimising stomach acid levels via zinc and B6 rich foods, bitter foods (e.g. chicory, rocket ); stress
• Choleretics (increase bile production) and cholagogues (increase bile flow); e.g. dandelion, artichoke and turmeric.
• Ensure good hydration to support bile flow.
• Increase glycine and taurine, which are components of bile. Good sources include legumes, sea vegetables, spinach and eggs.
• Olive oil can stimulate bile secretion.

Question 10

Lipid Recommendations

Answer 10

Type of fat:
Amount (% of energy):
Amount in grams (female, 2000 calories / day):
Total fat
20- 35%
44‒78g
Saturated fat
<10%
10 - 22g
Polyunsaturated fat
6 - 11%
13 - 24g
Omega 3
0.5 - 2%
1.1 - 4.4g

Question 11

Lipid Recommendations: Government Guidelines

Answer 11

The UK Government’s Scientific Advisory Committee on Nutrition (SACN) Review (2019) recommended that saturated fats should not exceed >10% of energy.
• The Eatwell Guide advises:
- Choose low fat dairy options.
- Choose unsaturated oils and
spreads, and eat in small amounts.
• There is no emphasis on healthy fats such as oily fish, avocado, nuts, seeds, extra virgin olive oil, egg yolk and grass fed meat.
• Low fat dairy options often contain high levels of sugar. Unsaturated oils may be highly refined and contain trans fats.

In past decades, dietary guidelines have advocated reducing the intake of total fat and dietary fat:
• Low fat diets led to fat in foods being replaced with refined carbohydrates and sugar.
• Without adequate energy from fat, people struggle to be sufficiently satiated. This has resulted in the consumption of ultra processed foods
• The PURE study in 2017 found that high carbohydrate diets led to the highest mortality rates . People consuming more fat, 35% of daily energy, were less likely to die than those consuming 10% daily energy.

Question 12

Healthy Dietary Fats

Answer 12

The importance of fat as a macronutrient in the diet has been understated and demonised.
• Eating fats from natural, unrefined foods should be the priority. Fat in the diet should be a mixture of saturated, monounsaturated and polyunsaturated fats, but absent of trans fats.
• Fat soluble antioxidants, e.g. vitamin E, are important when including fats in the diet. Foods rich in vitamin E include sunflower seeds, almonds and wheat germ .
• Focus on the quality of the fat and combine with foods naturally rich in antioxidants.

Benefits of including good amounts of healthy fats in the diet:
• Greater satiety value.
• Sources of essential fatty acids.
• Sources of choline (needed to synthesise phosphatidylcholine).
• Sources of essential fat soluble vitamins and phytonutrients.
• Greater flavour enhancement in cooked food.

Question 13

Healthy dietary fats’ food sources

Answer 13

• Fruit avocado, olives.
• Seeds chia, flax, pumpkin, hemp, seed butters.
• Seed oils flax oil, chia oil, hemp oil, sunflower oil, olive oil Ensure oils are cold pressed.
• Nuts almonds, cashews, walnuts, Brazil nuts, nut butters.
• Other coconut oil, grass fed meat.
• Oily fish salmon, mackerel, anchovies, sardines, herring.