Fats are the energy dense nutrients that provide 9 Calories of energy per gram, compared to 4 Calories per gram from both carbohydrates and proteins. Fat in the body would last a long time during a famine. This is present in the form of triglycerides, cholesterol, and phospholipid in the body. Fat or lipids come from three sources- from diet, stored in adipose tissue of the body (abundant), and synthesized in the liver.1
When our body does not need a lot of energy quickly- while sleeping, going about the house, or going for leisure walks, this is when fat is used for energy. It is the fuel for the body for low to moderate intensity activity and is the fuel in prolonged low intensity exercise.2
The waxy cholesterol is the type of fat produced mainly by liver which are needed to make plasma membrane, are the raw materials of many hormones (steroid hormones- the sex hormones and the corticosteroids), makes Vitamin D, cells in nervous system, and synthesize bile salts. Liver produces 80% cholesterol. Other types of fat are needed for transport of vitamins A, D, E, K and minerals, form brain and nervous system and is the largest fuel depot in the body.3,4,13
Fatty acid is the unit of fat. There are three types of fatty acids:
- Saturated fatty acids: Saturated fatty acids have no double bond in the hydrocarbon chain. Overconsumption of these fats can lead to higher cholesterol levels and an increased risk of heart disease. Usually solid or semi solid in room temperature.5,14
They can be found to raise LDL or “bad” cholesterol levels and increases the risk of heart disease (Leonard 2008). They can be found in butter, coconut, cream, beef, lamb, mutton, palm oil, whole milk, cheese, yogurt, dark chocolate.6,15
- Unsaturated fatty acids: These are good fats that may lower cholesterol levels and reduce the risk of heart disease. They are divided into monounsaturated fatty acid and polyunsaturated fatty acid.
Monounsaturated fatty acid can be found in avocados, almonds, Brazil nuts, cashews, canola oil, egg yolk, hazelnut, olives, peanut, pecan, pistachios, sunflower and safflower oil, chicken, and duck fat.15
Polyunsaturated fatty acids are essential fatty acids like omega-3, and omega-6, which can be found in chia seeds, cod liver oil, corn oil, fish oil, flaxseeds, mayonnaise, sesame seeds, soybean oil, sunflower seed butter, walnuts, soft margarines, fatty fish.6,15
Omega-3 fatty acids keep cell membranes fluid which enhances transmission of neurochemicals in brain cells and insulin sensitivity in muscle cells. Important Omega-3 fatty acids are alpha-linolenic acids (ALA), docohexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Important Omega-6 fatty acids are- linoleic acid (LA), gamma-linolenic acid and arachidonic acid.16,17,19
Linolenic and alpha linoleic acid are the essential fatty acids that are sourced from diet and the rest are made in the body. The properties of Omega-3 and Omega-6 are of utmost importance as they both affect eicosanoids like prostaglandins, thromboxanes, leukotrienes, and lipoxins. These are the chemical mediators balancing the inflammatory processes in the body.7
Omega 3- anti-inflammatory | Omega 6 pro-inflammatory | Target Ratio: Raise Omega-3 |
Dilates blood vessels | Constricts blood vessels | Consume food having more Omega-3 |
Lowers inflammation | Increases inflammation | Avoid vegetable oil high in Omega-6 |
Prevent blood coagulation | Causes blood clotting | Take Omega-3 supplements |
Decreases pain | Increases pain | |
Dilates airway | Constricts airway |
- Trans fat: Hydrogenation of oils creates trans fats, which are found in some processed foods. They increase food shelf lives. They are known to increase the risk of heart disease and are considered harmful. They may lower good cholesterol HDL and raise LDL, suppress excretion of bile acid, increase cholesterol production, lower essential fatty acid.20 They are found in hard margarines, shortening, cakes, cookies, crackers, croissants, doughnuts, pastries, muffins, and other packaged foods.5,6
Fig- 1: Types of fatty acid.5
Dietary fat comes in triglyceride form that has one glycerol attached to three fatty acids which can be any of the three types of fatty acids. It takes a long time to digest fat. Not only is fat a more efficient fuel as it yields more than double the calories of carbohydrates and protein per unit, but it does also not store extra water molecules. One molecule of glycogen requires 3 molecules of water when it gets stored. Unlike carbohydrates, fat is abundantly available in the body. Fat is stored as body fat in the entire structure of the body in subcutaneous or visceral fat form. The body also converts excess carbohydrates, fat, and protein into body fat. Fat burning requires oxygen and during exercise it takes longer time to metabolize.2
Fat Digestion and Circulation:
All kinds of fats- triglycerides, cholesterol, phospholipids are not water soluble so they cannot travel in blood by themselves. They need to bind to proteins and form lipo-protein to remain in circulation.3
First, bile emulsifies dietary triglycerides and breaks it into small fat droplets in the small intestine. Pancreatic lipase breaks down triglycerides in these small droplets to fatty acids and glycerol, this fatty acid then enters intestinal cells and is repackaged into chylomicrons as triglycerides again. Chylomicron is the largest lipo-protein that enters the lymphatic system and then enters the blood circulation via thoracic duct. Once in the circulation chylomicrons are again broken down into fatty acids and glycerol by lipoprotein lipase so that they can pass through the cell membrane of any cells. Once in the cell they can be used as fuel to provide energy or can be converted by lipogenesis into triglycerides for storage in skeletal muscle, liver, and adipose tissue.
The density of a lipo-protein is dependent on the protein-lipid ratio. Depending on the protein content, lipoproteins are named low-density, very-low-density or high-density lipoproteins where the protein parts are less, very less, and high consecutively. Broadly, the fate of triglycerides produced in the liver and other cells are as following:
The newly processed triglycerides in the liver are transported as very-low-density-lipoprotein (VLDL- 92% lipid and 8% protein) to the adipose tissue. Low-density lipoproteins (LDL- 80% lipid and 20% protein) carry cholesterol made by liver to all cells of the body to make cell membrane, hormones, vitamins etc. While HDL (HDL- 50% lipid and 50% protein) carries the cholesterol from cells and other lipoproteins (LDL or VLDL) back to the liver where they will be excreted in the bile or recycled to make cholesterol to be sent out to tissues that need it. While traveling, LDL’s can drop off cholesterol in our blood vessels which can build plaques leading to atherosclerosis which is why it is known as bad cholesterol. HDL has the property to carry these dropped cholesterol back to the liver which is why HDL is called the good cholesterol.21,22
Fat Metabolism:
- Fat Transport and Lipogenesis
- Fat Mobilization and Lipolysis
- Fatty Acid Synthesis
- Beta Oxidation
- Ketone Formation
- Cholesterol Synthesis and Catabolism
To lose weight and build muscle it is important to limit carbohydrate intake and only a 10-30% fat intake. Also, food rich in omega-3 may help burn fat by enhancing the body’s response to leptin. Leptin inhibits appetite. So, when leptin is stimulated, it reduces the activity of neuropeptide Y which is a hunger trigger. Food like coconut oil enriched in medium chain triglycerides suppress appetite and help lose weight.6
Beta oxidation breaks down fatty acids to Acetyl CoA. It is a spiral process where the fatty acids convert to Acetyl CoAs. Fatty Acids are long chain carbon-hydrogen bonds and Acetyl CoAs are only two carbon molecules, so most fatty acids end up producing a lot of Acetyl CoAs. These Acetyl CoAs enter the Krebs Cycle and produce ATPs just like from pyruvate (the end product of glucose). Approximately one 16 carbon fatty acid can produce 106 ATP. One Triglyceride molecule can end up generating 337 ATPs (the 3 fatty acids and the glycerol together) compared to the 36 ATPs from one glucose. However, beta-oxidation is efficient but slow since it requires a lot of oxygen. If excessive Acetyl CoA is formed and Krebs Cycle is overloaded, then the Acetyl CoAs are diverted to form ketone bodies which is an alternative source of fuel for cellular activities.1
Ketone Bodies, Ketosis, and Ketone Metabolism:
When diets consist almost entirely of fat, the body enters a state of ketosis. In the absence of carbohydrates, in starvation and fasting state the body uses fatty acids as fuels from the adipose tissue. The fatty acid breaks down to Acetyl CoAs from beta oxidation and enters the Krebs cycle, and surplus of Acetyl CoA converts to ketone bodies which oxidizes to provide energy.9
Ketone bodies are water soluble, and organs thought to be dependent on glucose like the brain, muscle and red blood cells can use ketones as a source of energy when glucose supply is limited. If ketone bodies are produced faster than it can be utilized, it breaks down into CO2 and acetone where the latter is exhaled through breath. The acetone in breath gives the breath a smell which is an indication of ketosis. However, in diabetics, the excess CO2 can acidify food leading to diabetic ketoacidosis, a dangerous condition.1
In ketone synthesis, excess Acetyl CoA is converted to β-hydroxybutyrate, the primary ketone body in the blood. To produce energy for the brain, this β-hydroxybutyrate is oxidized, and in two subsequent steps produces two molecules of Acetyl CoA (Fig-2) which finally enter the Krebs Cycle to produce energy for the brain.1
Body Fuels:
The human body burns a mix of both glucose and fat for energy, but these are driven by what nutrition we take, fasting state and exercising. The body’s preferred fuel source is carbohydrate as it is easily available from food and is the fastest to breakdown into glucose by glycolysis to produce ATP that provides energy for all cellular activities. Even in a state of rest and sleep, the body uses stored glycogen in the liver which can last for atleast 12-24 hours. In cases of high intensity workouts, the body has enough glucose stored as glycogen in muscle and liver to supply for an hour before it depletes; but after glycogen depletion the body feels exhausted and will require to slow down while it switches to fat burning mode.
Fat is in abundance in the body, and it provides slower and sustained energy but takes longer and many steps of metabolism to ultimately feed into the Krebs cycle to produce ATP. As seen above, in carbohydrate restriction diet, fatty acid produces ketones which are an alternative fuel. Much emphasis is given on metabolic flexibility nowadays which is attaining a physical state where the body can more easily switch between glucose and fatty acids for fuel. This can ensure the body always remains energized during short supply of carbohydrate, fasting or while doing regular activities. A healthy regime of losing weight targets at burning more fat and preserving lean muscles. When the body can adopt a metabolic state where fat can be used readily for energy, it can improve insulin sensitivity, decrease inflammation, reduce visceral fat which will reduce the chances of getting diabetes, cancer, or cardiovascular disease.
To prioritize more fat burning a carbohydrate restrictive diet is advised to provide body with less glucose which will lessen the insulin levels and ultimately rev up the fat burning process. Sleep will play a pivotal role to balance ghrelin, leptin, growth hormone to name a few that will mobilize fat loss more efficiently in a low carb diet. Additionally, moderate to intense exercise or even low to moderate intensity exercise with weight training will burn fats more consistently while increasing the overall metabolism as well.10
Intermittent Fasting:
Intermittent fasting has gained momentum in the recent time to utilize fat storage in the body, particularly for people suffering from obesity to reduce visceral fat mass, and other metabolic syndromes like type 2 diabetes, insulin resistance, and fatty liver. It is described as an eating pattern in which very few calories or absolutely no food is consumed for a longer period of time which can range from 12 hours to several days on a recurring basis. This is described as a metabolic switch that represents an evolutionarily conserved trigger point that shifts the whole idea of lipid or cholesterol synthesis and fat storage to fatty acid oxidation, ketone body formation which, on top of burning fats will also preserve muscle mass. The metabolic switch occurs between 12 to 36 hours of initiating the fast and depends upon the liver glycogen at the beginning of fast and the individual’s energy expenditure. The fasting state metabolic switch to burning fat follows the same principle mentioned above of beta-oxidation fatty acid transformation to Acetyl CoA and the oxidation of ketones to form Acetyl CoA. Once Acetyl CoA is formed from both the processes, it enters the Krebs Cycle to generate energy for brain and muscle cells.
Studies on early human evolution of hunter-gatherer societies suggest humans evolved in conditions where they intermittently experienced extended time-periods with little or no food.11
Carbohydrate restricted diet, low carb diet, fat adaptation diet, keto diet, intermittent fasting- these are all diets that promote fat burning to help reduce obesity, metabolic syndromes, insulin resistance, and prevent type 2 diabetes etc. Ketosis in the initial phases can cause “keto-flu” where one can experience- headache, fatigue, nausea, bad breath, muscle cramp, weight loss. It dissipates in a few days before one starts to feel positive again.12 It must be remembered that to stay in a continuous state of ketosis or fat burning, it requires extreme resolution, dedication, commitment to be able to sustain the dietary restrictions. Maintaining keto diet for a period of time and then going back to having regular carbohydrate consumption may be more harmful and there is no research evident at the moment to state what happens to overall health in such metabolic switch. Will there be more weight gain? Will it harm the organs more? Will there be mental health issues?
It is advisable to follow up with expert nutritionists and medical professionals before commencing on shifting to metabolic flips. The underlying physical conditions need to be of the highest consideration. However, a diet enriched with good protein (to maintain healthy muscles, bones and organs), fiber rich carbohydrates (sustained release of glucose to avoid sugar spice and the fiber content to feed the gut microbiome), good fats (optimum maintenance of nervous system and skin health) along with regular exercise to maintain lean muscles, promote good sleep, increase metabolism, and regulate energy balance would be key to good health.
Author of this article
Dr. Maliha Mannan Ahmed, MBBS (BMC), MBA (ULAB), Masters in Healthcare Leadership (Brown University, USA) and Level 1 Certification on Precision Nutrition.
The Executive Editor of The Coronal.
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