Hand’s up if you ever thought fat made you fat?
If you were in a room with others, you’d likely find that most people, whether they do any longer or not, would put their hand up. For many decades, we were told that if you wanted to lose weight, then you should opt for low-fat foods and so the market exploded with fat-free slogans, reduced fat percentages and fat soon became a word to hate.
The irony is that whilst we take lessons from human nutrition and explore whether they can apply to our pets, we seemingly applied the fat argument to our pets too.
If we have a fat pet, we are encouraged to feed low-fat diets.
The scope of obesity warrants its own separate article; there are genetic influences, and nothing is ever as simple as the calorie in, calorie out argument, but for simplicity’s sake, an over-consumption of any of the macronutrients can result in stored fat.
The reality of low-fat diets for our dogs is actually a great detriment to their health, so let us explore the function of fat in more detail, and why it really isn’t something to be feared, especially in your dog.
What are Fats and Oils?
Fats and oils, technically called lipids are made up of collections of molecules called triglycerides. If the collection is liquid at ambient temperature, it is called an oil, if it is solid, it is called a fat.
Lipids can then be further categorised into:
* Simple
* Compound
* Derived
Simple lipids include triglycerides, which are the most common form of fat in the diet. These are the best reserve of food material, and act as an insulator and protector of internal organs.
Compound lipids consist of a lipid and a non-lipid compound; lipoproteins are compound lipids and these are what carry fat around in the blood stream.
Derived lipids are products of simple and compound lipids. These include sterol (sub-group of steroids) compounds such as cholesterol, and fat-soluble vitamins.
Triglycerides are the most important type of fat in the diet. They can be differentiated by the types of fatty acids they contain.
Fatty acids can be saturated, monounsaturated, or polyunsaturated. The difference is in their structure.
Saturated fatty acids contain no double bonds between the carbon atoms and are therefore saturated with hydrogen.
Monounsaturated fatty acids have one double bond (mono=one).
Polyunsaturated fatty acids (PUFAs) have two or more double bonds (poly=many).
The more double bonds a fatty acid has, the more chemically reactive it is.
Fatty acids are also identified by the families to which they belong. These families are known as omega but will only apply to unsaturated fatty acids.
In dogs, the body has a requirement for two distinct EFA families; the Omega-6 and Omega-3 series.
Omega-6
Linoleic Acid (LA)
Arachidonic Acid (AA)
Omega-3
Alpha-Linolenic Acid (ALA)
Eicosapentaenoic Acid (EPA)
Docosahexaenoic Acid (DHA)
The Digestion of Fat
When fat is ingested, it must be digested before it can be absorbed through the intestinal wall. Most of the digestion of fat occurs in the upper part in the small intestine and is accomplished by digestive enzymes known as lipases. The lipases act of fat that has been emulsified with the aid of bile acids.
Bile acids, also known as bile salts, are made from cholesterol in the liver and stored in the gallbladder. They are used by the body to aid in the digestion and absorption of fats, oils and the absorption of fat-soluble vitamins like A, D, E and K.
The lipases work by breaking down the emulsified fat into smaller units. Fat isn’t water-soluble (you’ll notice this when you try to wash olive oil off your worktop without soap), so it needs special carriers that allows it to be transported around the body. And so, it is packaged into particles called chylomicrons.
The fatty acids are now small enough to be passed through the lymphatic system and into the blood stream. As triglycerides are metabolised on their journey, the chylomicron gets smaller, resulting in a remnant which is rich in cholesterol. This is then taken in by the liver and repackaged, available for release at a later stage.
Fatty acids are used throughout the body for a range of functions (but if they are not needed, they can be stored in adipose tissue).
Fat as a carrier of fat-soluble vitamins and nutrients
One of the most important roles played by fat involves the transport of some of the vitamins and fat-soluble phytochemicals such as carotenoids. Without fat, Vitamins A, D, E and K are not efficiently absorbed.
Vitamin A is necessary for normal cell division, immune system function, bone remodelling, the formation of enamel during tooth development, skin health and vision. True vitamin A appears in foods of animal origin; sources include liver, meats, cod liver oil, egg yolks, butter and dairy products from grass-fed cows. Green and yellow fruits and vegetables contain B-carotene, some of which is converted to vitamin A, alongside a diet including adequate fat.
Vitamin D is both a fat-soluble vitamin and a hormone. Vitamin D promotes calcium absorption. In human health, you will have heard it referenced as the sunshine vitamin as it is produced in the skin in response to sunlight (UV) exposure. In studies of hip fractures in humans, there appears to be a seasonal variation; more occur during winter months and fracture patients often have low vitamin D status. When supplemented with Vitamin D and calcium, incidences of fractures often reduce.
Vitamin D is particularly important in bone health and this is no different for dogs.
However, unlike herbivores and omnivores, cats and dogs are unable to synthesize Vitamin D adequately in the skin. There is no seasonal change in concentrations and studies have shown that when fed a diet deficient in Vitamin D, puppies develop rickets which could not be prevented by exposure to UVB light. https://www.sciencedirect.com/science/article/pii/S0016648084711543
There are two forms of Vitamin D. Vitamin D2 is also known as ergocalciferol which occurs in plants and Vitamin D3, known as cholecalciferol, is created in the skin during exposure to UV light and occurs in animals. Cholecalciferol (D3) is of greatest nutritional importance to both cats and dogs and luckily it can be obtained from the consumption of animal products that contain it.
Sources include egg yoks, liver, fatty fish and butter.
Vitamin E was first identified for its role in reproduction. Rats fed a devoid diet, could not reproduce. Hence the name tocopherol, Greek tokos meaning birth.
However, Vitamin E functions as an important antioxidant within cells, protecting lipids, particularly the polyunsaturated fatty acids in cell membranes, against oxidative damage caused by free radicals that may be generated during metabolic processes.
This leads to a higher vitamin E requirement in a diet containing PUFAs. It has been recommended the dietary ratio of alpha tocopherol:PUFA (mg/g) of 0.6:1 is maintained as a minimum to protect against PUFA peroxidation. Rancid fats are particularly destructive of vitamin E, so these should be avoided in diet.
Sources of vitamin E include green leafy vegetables, liver, egg yolks, nuts and seeds.
Vitamin K comprises a group of compounds called the quinones. Vitamin K1 occurs naturally in green leafy plants and vitamin K2 is synthesised by bacteria in the large intestine. Vitamin K3 is the most common form of synthetic vitamin K and has a vitamin activity two to three times higher than that of natural vitamin K1. Vitamin K is required for normal blood clotting and is also involved in the regulation of calcium phosphates in growing bone. The synthesis of vitamin K by intestinal bacteria of dogs contributes significantly to the requirements of vitamin K for the species and interestingly, coprophagy is thought to increase vitamin K absorption to.
Food sources of vitamin K include liver, leafy green vegetables, cabbage-vegetables and green tea leaves.
The following are also fat-soluble nutrients, and so, in cases of low fat-diets, their transport and utilisation are significantly impaired:
- Carotenoids including carotene, lutein, lycopene and zeaxanthin.
- Carnitine,
- Coenzyme Q10,
- Inositol,
- Lipoic acid,
Fat as an energy source
Dogs are far more adept at using fat as energy than us humans. It is thought that albumin (a protein made by the liver) binds more free fatty acids in dogs than in any other species. This results in a higher concentration of fatty acids in the blood, and therefore the delivery of fatty acids to muscles is higher.
When we consider energy systems, we usually think of carbohydrates. But carbohydrates and fat are both chains of carbons. Fats are just much longer chains. For that reason, they take more digestion, but this slow digestion allows for the gradual release of energy. Stored fat can also be used as reserve too!
And as we know, dogs have no dietary requirement for carbohydrates. They can sufficiently utilise fat for energy and protein, if necessary, through a process of gluconeogenesis.
In terms of energy, fat provides more calories per g than both protein and carbohydrates.
Protein 4kcal per g
Carbohydrate 4kcal per g
Fat 9kcal per g
But fat also plays a role in many other body functions.
Fat and Skin Health
In a series of studies in the early 1900s, rats were fed a diet completely devoid of fat. These rats developed visible skin abnormalities, increased water loss across the skin and other body-wide issues. However, when PUFAs were introduced into the diet, these defects were reversed.
The skin, especially the epidermis, is organised into layers with a distinctive lipid composition. Linoleic acid (LA) is the most abundant PUFA present in the epidermis.
The presence of LA directly correlates with barrier function of the skin. The main sources of LA are vegetable oils, nuts, seeds, meats and eggs.
https://www.sciencedirect.com/science/article/pii/S0005273617300986
Fat and Eye Health
DHA is found in very high concentrations in the cell membranes of the retina; it is essential in the normal development and function. Studies are supportive of the role of omega-3 supplementation in certain inflammatory eye disease. https://www.karger.com/Article/Fulltext/455818
Fat and Brain Health
The brain consists largely of fat. DHA is taken up by the brain over other fatty acids and studies have shown that high levels of DHA are associated with better performance in cognitive learning tasks. Not only that but low levels of PUFA are regularly associated with:
- Depression,
- Anxiety,
- Reduced resilience to stress,
Levels of Fat in the Modern Dog
Unfortunately, levels of PUFA are often depleted in many commercial dog foods after oxidative damage resulting from prolonged storage or if antioxidants aren’t including in sufficient amounts.
Not only that, but in cases of inflammatory bowel conditions, liver issues or exocrine pancreatic insufficiency, there are often issues with fat absorption and metabolism.
Fatty Acid Deficiency
Dogs will generally present with a dull, dry coat, and often scaly skin. There may be hair loss and greasy skin particularly in and around the ears and toes. There may be a poor functioning immune system or other health issues. There may be cognitive failing or symptoms of deficiency in fat-soluble vitamins and nutrients.
If we look to the evolution of the dog, wolf ancestors would self-select around 45% fat in their diet. But what is particularly interesting is that the omega content is vastly different. Wolf diets would contain a ratio of omega 6:3 of 2:1, whereas the modern dog would eat on average a ratio of anywhere from 8:1 to 17:1.
The modern dog could certainly warrant an increase in omega-3 fat.
Sources of Omega-3:
ALA – green leafy vegetables, flax seed and chia seed
EPA and DHA – oily fish, krill oil and algae oil
Fat perhaps isn’t the foe we thought it was, and beneficial fats are in fact essential to both ours and our dog’s health. They are deemed essential fatty acids after all. An excess of any of the macronutrients can contribute to obesity, so we’re back to the goldilocks analogy. We need just enough of these nutrients for optimal health. But that just enough can change from human to human, and dog to dog, depending on their health status and lifestyle.
If you are concerned about the health of your dog it is essential to seek the advice of a qualified professional.
Written by Lisa Hannaby - Bsc. Psych. Hons, MSc Human Nutrition