In my Facebook group, Eat to Live your Best Life, we share healthy eating tips, recipes and nutrition and health information. (If you’re on Facebook, search for it and send me a join request – I’d love to have you on board!) The post which has sparked the most discussion so far was all about dietary fats. There is an enormous amount of confusion and misinformation around the subject of fats, probably more than on any other subject in nutrition. So I thought I’d try to bust some of the myths around dietary fat.
Why do we need fat?
Every cell in our body is encased by a membrane that is made up of fats. The composition of fats in our diet influences whether this membrane is stiff (saturated fats) or flexible (unsaturated fats). We need enough stiffness for structural integrity but enough flexibility to enable information to travel in and out of cells easily.
Many hormones, including male and female sex hormones, stress hormones and vitamin D, are based on cholesterol, so sufficient cholesterol is required for optimum hormone function. The brain also requires fats and cholesterol. We need essential fatty acids (discussed later) to control inflammation. And importantly, we cannot absorb the fat soluble vitamins (vitamins A, D, E and K) if we don’t have sufficient fat in our diet.
Fats don’t make you fat
It is such a shame that in the English language we have the same word for both lipids in the diet (fat) as adipocytes (fat cells) in the body. No wonder that there is a common assumption that eating fat makes you fat! This assumption has been strengthened over the years by those who say that weight loss or gain is simply a matter of calories in minus calories out. Fat has more calories per gramme than any other macronutrient (9 calories for 1g of fat as opposed to 4 calories for 1g of carbohydrates or protein), so surely it follows that eating fat is more likely to make us gain weight than eating carbohydrates? I often find from talking to clients that they are scared of eating nuts or avocados because of the number of calories they contain. This sounds like a reasonable concern, so let’s explore it further.
Whilst we do need to consider the overall energy intake from food, it is not the only factor at play when it comes to weight loss or gain. Eating fat results in a very different response within the body than eating carbohydrates. All carbohydrates are ultimately broken down into glucose, and this glucose find its way into the bloodstream. This makes the pancreas release the hormone insulin. Insulin’s role is to move glucose out of the bloodstream and into the cells, where it can be used to generate energy. Insulin also prevents a process called lipolysis. Lipolysis means “breaking down fat” – insulin stops us from using stored fat as a fuel source so that we use the glucose first. Insulin also promotes the storage of any excess glucose from carbohydrates in fat cells in the body. And this is often stored around our middle, the theory being that by storing it near the liver, the liver can easily convert it into an energy source if we ever run low of glucose. The problem is that when we eat too many carbohydrates too frequently we don’t run low on glucose, so the stored fat never gets used as fuel.
I hope you can see now that if we eat a lot of carbohydrates, particularly refined carbohydrates such as cakes, biscuits and refined grains like white bread and white pasta, a lot of insulin is produced. This stops us from burning fat and also promotes storage of fat, often around the middle which is the least healthy place to store it(1).
The other thing that happens when we eat refined carbohydrates is that the release of insulin causes our blood sugar to drop too much. This is called reactive hypoglycaemia. The effect of this is to make us reach for something to give us a quick energy burst….and so the cycle starts again. We can easily get sucked into a pattern of snacking on high carbohydrate foods which make our blood sugar spike and then fall.
Eating fat, however, has a negligible effect on our blood sugar. This means there is no insulin release with its knock-on effects of fat storage, inhibition of lipolysis and tendency towards snacking. Fat helps keep our blood sugar stable. It is also very satiating, so although it is true that it is more energy dense than carbohydrates, it keeps us fuller for longer. Recent studies(2) have shown that low carbohydrate high fat (LCHF) diets can lead to weight loss as much as high carbohydrate low fat (HCLF) diets can. The difference is that because of satiety and blood sugar stability, HCLF diets may be easier to stick to. And we know that losing weight and keeping it off should be about sustainable dietary changes that become your lifestyle as opposed to quick fixes.
Our ancestors ate fat
Another consideration is what we have evolved to eat. Our ancestors thrived in all corners of the globe, and primitive diets varied enormously, from the high animal fat diet of the Inuit to diets in the tropics richer in fruits. However, all primitive diets that have been studied contained fats. If you are interested in ancestral diets, then the seminal book “Nutrition and Physical Degeneration” by Weston A Price is well worth a read. Price was a dentist who travelled the world in the 1930s to try to understand why the dental health of peoples declined as they moved from eating their traditional diet to a more processed, Western diet. Price found that the percentage of energy obtained from fat in ancestral diets ranged from 30% to 80%, but that all traditional diets included some animal fat. There were no “low fat” traditional diets in the sense we think of them today, and fat was often prized.
Fat and cardiovascular disease
Much of the demonization of fats which has occurred over the last 40 years is due to a study published in the 1970s called the Seven Countries Study. The scientist behind the study, Ancel Keys, found that there was an association between dietary intake of saturated fat and heart disease. In recent years this study has been criticised, with critics stating that the association could equally have been between sugar intake and heart disease. Nevertheless, the Seven Countries Study was highly influential at the time and is the reason why current dietary guidelines (such as the Eatwell Plate in the UK) advise limiting dietary fat, particularly saturated fat.
However, it is worth noting that the Food and Agricultural Organisation of the United Nations, when they met in 2010 to discuss the evidence around fat consumption, decided that the evidence is inconclusive on health effects of differing percentages of fat in the diet. In fact they could not agree on an upper limit. The current recommended limit of 35% is historical(3). “Full agreement among the experts regarding the upper value of acceptable macronutrient distribution range (AMDR) for %E fat was not achieved; thus maintaining the current recommendation for a maximum intake value of 30-35%E fat was considered prudent. Further studies and a systematic review of all available evidence are needed to provide better evidence on which to base a recommendation on AMDR for %E fat that are applicable globally.”
The problem with the low fat diet trend was that when fat is restricted in the diet, we tend to replace it with refined carbohydrates and sugar. And we have certainly witnessed an increase in incidence levels of chronic diseases such as Type 2 diabetes during the years when a low fat diet has been recommended.
Are there other good reasons to eat fat?
There is a group of fats called essential fatty acids (EFAs) which deserves a special mention. These are fats which the body cannot make itself, so we need to get them from the foods that we eat – that’s why they are called “essential”. There are two types of EFA we need to consider: omega 3 fatty acids and omega 6 fatty acids.
Omega 3 fatty acids are found in their largest concentration in oily fish, such as salmon, mackerel, anchovies, sardines and herring (remember these using the acronym SMASH). Grass-fed meat also contains some omega 3. Vegetarian sources of omega 3 include walnuts, flaxseeds, chia seeds and some algal products. Omega 6 fatty acids are primarily found in nuts, seeds and their oils, as well as in conventionally reared red meat.
EFAs are used in the body to form substances called eicosanoids. These are locally acting hormones. Eicosanoids made from omega 3s tend to be anti-inflammatory, whereas eicosanoids from omega 6s are pro-inflammatory. We need a balance of the two. Unfortunately, today’s Western diet contains a lot of omega 6 (from meat and vegetable seed oils such as sunflower oil used in cooking and in processed foods) and very little omega 3. The ratio of omega 6: omega 3 has been found to correlate with chronic disease risk. The Western diet typically results in a ratio as high as 17:1, whereas much lower ratios of 2-4:1 help to prevent chronic disease(4).
We should all therefore make efforts to reduce our intake of omega 6 (by minimising processed foods and the use of vegetable oils) and increase our intake of omega 3.
Which fats should I eat?
Firstly, I think it’s important for most people not to get too focused on how much fat versus carbohydrate we should eat. What is more powerful as a tool for achieving health (and maybe weight loss too) is to eat a diet consisting of real foods in their whole state. When it comes to fat sources, we would then be consuming foods such as:
- Nuts and seeds, especially walnuts for their omega 3 content – eating a handful of nuts each day has been linked to a variety of health benefits(5)
- Wild oily fish – for omega 3
- Olives and olive oil – rich in monounsaturated fats and a key component of the Mediterranean diet, great cold in salad dressings or drizzled over vegetables, or used for low temperature cooking (see below)
- Avocados – also rich in monounsaturated fat as well as fibre, vitamins and minerals(6)
- Eggs – contrary to popular belief, eating eggs does not raise cholesterol levels
- Grass fed organic meat – eating grass fed is important as the omega 6:omega 3 ratio is lower than in grain fed meat
- Coconut products
Which fats should I avoid?
Not all fats are good for you. Trans fats are made industrially when vegetable oils are hydrogenated at high temperatures, usually in an attempt to give them a longer shelf life or to make oils solid at room temperature (such as in making margarine). The evidence is clear that trans fats are bad for our health(7). Once again, the simplest way to avoid these is to eat a diet low in processed foods.
Vegetable oils such as sunflower oil should also not be used to cook with. Because they are polyunsaturated, they are very delicate and can easily become oxidised when exposed to heat or light. The oxidised components can potentially cause mutations to DNA. Olive oil is safe to cook with at low temperatures (up to 160C), but for higher temperature cooking it is best to use saturated fats. These are stable at room temperature and include coconut oil, butter, ghee and animal fats such as lard.
- Ghigliotti, G., Barisione, C., Garibaldi, S. et al. (2014). ‘Adipose Tissue Immune Response: Novel Triggers and Consequences for Chronic Inflammatory Conditions, Inflammation, 37(4), pp1337-1353. [Online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4077305/ (Accessed 26 February 2018).
- Gardner, C.D., Trepanowski, J.F., Del Gobbo, L.C. et al. (2018). ‘Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial’, JAMA, 319(7), pp667-679. [Online]. Available at https://jamanetwork.com/journals/jama/article-abstract/2673150?redirect=true (Accessed 26 February 2018).
- Food and Agriculture Organisation of the United Nations (2010). ‘Fats and fatty acids in human nutrition: Report of an expert consultation’, FAO Food and nutrition paper, Available at http://foris.fao.org/preview/25553-0ece4cb94ac52f9a25af77ca5cfba7a8c.pdf (Accessed 26 February 2018).
- Simopoulos, A.P. (2008). ‘The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases’, Experimental Biology and Medicine, 233(6), pp674-688. [Online]. Available at http://www.ncbi.nlm.nih.gov/pubmed/18408140 (abstract only) (Accessed 26 February 2018).
- Bao, Y., Han, J., Hu, F.B. et al. (2014). ‘Association of Nut Consumption with Total and Cause-Specific Mortality’, The New England Journal of Medicine, 369(21), pp2001-2011). [Online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3931001/ (Accessed 26 February 2018).
- Dreher, M.L. and Davenport, A.J. (2013). ‘Hass Avocado Composition and Potential Health Effects’, Critical Reviews in Food Science and Nutrition, 53(7), pp738-750. [Online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3664913/ (Accessed 26 February 2018).
- De Souza, R.J., Mente, A., Maroleanu, A. et al. (2015). ‘Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies’, BMJ, 2015 Aug 11;351:h3978. [Online]. Available at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4532752/ (Accessed 26 February 2018).