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Prof Jennie Brand-Miller and Dr Alan Barclay answer the most common questions we are asked about dietary fats. 

WHAT ARE FATS? They are an essential nutrient like protein and carbohydrate playing numerous roles in our bodies including:

  • Forming the structure of our body cells 
  • Helping make bile and sex hormones 
  • Insulating us from cold 
  • Surrounding and protecting vital organs like our kidneys 
  • Providing us with fat-soluble vitamins (A, D, E and K), and 
  • Storing energy. 

They are also energy dense – 37kJ per gram (9 calories), compared with 17kJ per gram (4 calories) for protein and carbs.

Range of fats and oils
Most of the fats we eat are triglycerides – fat molecules composed of three fatty acids joined to a glycerol (sugar alcohol) backbone. During digestion our bodies break the fatty acids down into free fatty acids which the cells in our intestine absorb and release into the bloodstream.

Fats may lower insulin requirements when we first consume them because they delay the rate that foods are emptied from the stomach into the small intestine, and are absorbed into the blood. However, an increasing body of evidence suggests that they raise insulin requirements 3–5 hours after a meal, most likely due to increased insulin resistance.

WHAT ARE THE DIFFERENT FATS IN OUR DIET? Saturated fat, trans fat, mono-unsaturated fat, and polyunsaturated fat are the main fats in the foods we eat. Collectively, mono-unsaturated fat, and polyunsaturated fat are called unsaturated fats because of their chemical structure. Most fats and oils, however, are a combination of saturated, poly- and mono-unsaturated fats.

WHY ARE SOME FATS CALLED GOOD FATS? Unsaturated fats are the ones with a health halo. These are the fats found in plant foods including nuts and seeds, avocadoes and olives, and oils and spreads made from plants and seeds. They can help lower LDL (bad) cholesterol and raise HDL (good) cholesterol reducing our risk of cardiovascular disease (e.g., heart disease and stroke). It’s important to eat some good fats every day.

Polyunsaturated fats are particularly important because they provide us with the essential omega 3 and omega 6 fatty acids which our bodies can’t produce. Best sources are oily fish (e.g. salmon and mackerel), seeds (flax seeds, chia seeds, sunflower seeds) and nuts (especially walnuts and Brazil nuts). For mono-unsaturated fats, enjoy avocadoes, almonds cashews and peanuts or use oils made from them along with olive oil of course.

WHY ARE SOME FATS CALLED BAD FATS? Saturated fats have earned the label “bad fats” because too much of them will raise our LDL (bad) cholesterol which can block blood vessels and increase our risk of cardiovascular disease (heart disease and stroke). Trans fats not only increase LDL cholesterol, they also lower HDL (good) cholesterol so their overall effect is even worse for our health.

  • Foods naturally high in saturated fats include animal foods like fatty meats (sausages, bacon, salami, Devon, etc), butter, ghee, cream, hard cheese, and full cream milk. 
  • Foods high in trans fats include some margarines (e.g., cooking and some cheap table varieties), and cooking fats for deep-frying (particular those used in fast food restaurants) and shortening for baking (pies, pastries, cakes, biscuits/cookies, buns, etc). 

WHAT ABOUT CHOLESTEROL? This is a special kind of fat that’s only found in animal foods such as meat and dairy foods. It’s actually the saturated fat and trans fats in these foods that cause LDL cholesterol levels to rise, which is why for many years, dietary advice to help people lower their blood cholesterol has focused on the fats in our diets. There is now growing evidence that the kind of available carbohydrate (sugars, maltodextrins and starches) that we eat has an effect on our blood cholesterol levels too.

However, when it comes to lowering blood cholesterol levels, some carbs are helpful. For example, oats, legumes, fruits and vegetables, contain certain kinds of fibre (e.g., beta-glucans) that may help lower blood cholesterol levels by binding it in our guts.

HOW MUCH FAT SHOULD I EAT? It all comes back to balance. Because the different kinds of fats are found in such a broad range of foods, it’s essential to eat a variety of foods, avoid trans fats as much as possible and for every gram of saturated fat you consume, eat 2 grams of unsaturated.

You only have to look around the world to see that there are very different dietary patterns with very different fat intakes that are associated with good health and long life. For example, traditional Mediterranean and Japanese diets, which are both linked with a long and healthy life, couldn’t be more different. The Mediterranean diet is relatively high in fats and tends to be rather moderate in carbs. The Japanese diet, like most Asian diets, is high in carbs and low in fats. What they have in common and what seems to matter most is that they are based on good, wholesome foods and ingredients. Mostly plants. If you need advice on what to eat, see a registered/accredited dietitian.

HOW MUCH FAT SHOULD CHILDREN EAT? Children need fat for energy, growth, brain development and a healthy immune system. For babies, breastmilk and infant formula are high in fat to sustain the rapid growth during the first year of life. Once weaned, children start obtaining fat from other foods and between 2 and 3 years of age, growth slows down and fat becomes less important in the diet. As children begin consuming a more varied diet, the type of fat in their diet should become a higher consideration. A reasonable amount of fat for a child is around 30 per cent of their daily kilojoule intake. For 4–8-year-olds this equates to 50–60 grams, for older kids around 60–80 grams of fat per day.

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