We are told to use polyunsaturated oils instead of saturated fats. But research tells us that consuming polyunsaturated fats leads to cell aging, inflammation, obesity, diabetes, hypothyroidism, and hormonal imbalances. This post tells you the truth about polyunsaturated fats — and why the oils sitting in most kitchen cupboards may be quietly damaging your health.
Polyunsaturated fats are liquid oils such as corn, canola (rapeseed), soy, safflower, sunflower, flaxseed (linseed), sesame, and peanut oil. Saturated fats, in contrast, are found in coconut oil and butter. Health authorities and the media tell us that eating polyunsaturated fats in place of saturated fats protects against heart disease. However, according to several researchers — including Dr. Ray Peat — eating saturated fats has no negative effect on heart and arterial health. It is the polyunsaturated fats that are unhealthy and damaging.
According to many scientists and nutritionists, including Dr. Ray Peat, the consumption of polyunsaturated fats is linked to obesity, cell aging, inflammation, and hormonal imbalances including hypothyroidism and insulin resistance (diabetes).
The Cholesterol Myth
In the 1960s and 1970s, people were eating a diet high in saturated fats — including butter and coconut oil — and they were generally lean and healthy. Then misleading research linked saturated fats to heart disease, which changed dietary recommendations toward polyunsaturated fats.
It is true that eating saturated fats raises cholesterol, but there is no solid scientific evidence connecting cholesterol to heart disease. According to research, heart disease and arteriosclerosis are caused by inflammation. Cholesterol is present in injured arteries as a protective molecule — it is not the cause of arteriosclerosis.
The notion that cholesterol causes heart disease has been repeated so often that people accept it as fact. This cholesterol myth has led to dietary guidelines that promote polyunsaturated fats as "heart-healthy" while demonising saturated fats. Prepared foods that once contained saturated fats now contain polyunsaturated fats — meaning the modern Western diet is saturated with these unstable oils, with devastating health consequences for the population.
In my courses, I cover cholesterol and heart disease in depth, examining the actual evidence and what it means for your thyroid and metabolic health.
Polyunsaturated Fats Are Chemically Unstable
The structural difference between unsaturated and saturated fats gives them very different biological properties.
In saturated fats, the carbon atoms are connected by single bonds — which are very stable and non-reactive. In unsaturated fats, some carbon atoms are connected by double bonds, which introduces an open and unstable structure into the fat molecule. Monounsaturated fats have one double bond, while polyunsaturated fats contain more than two double bonds. These multiple double bonds make polyunsaturated fats highly unstable and prone to chemical reactions [1].
Polyunsaturated Fats and Oxidative Rancidity
Because of their multiple double bonds, polyunsaturated fats are prone to oxidative rancidity [1]. This process involves the formation of free radicals from the unsaturated fat molecules, which then react with oxygen — forming a chain reaction that ends with the production of foul-smelling compounds such as ketones and aldehydes [1].
You may have noticed that liquid vegetable oils often develop an unpleasant smell over time. This is because they go rancid quickly. They are not suitable for regular human consumption. Even if the oil is not visibly rancid at the time of eating, oxidative rancidity occurs easily inside the warm, oxygen-rich environment of the human body. When polyunsaturated fats are consumed, they spontaneously oxidise, spreading free radical chain reactions inside the cells. The more unsaturated the fat, the more vulnerable it is to this process [1].
Before the health industry began promoting polyunsaturated oils as food, they were used industrially — in paint. Polyunsaturated oils work beautifully for paint. They are far less suitable for your body.
How Polyunsaturated Oils Harm the Body — and the Thyroid in Particular
The free radical processes triggered by oxidative rancidity harm cells in several ways: they inhibit the cells' ability to produce energy, and they contribute to cell aging and chronic inflammation [2].
Your thyroid gland is especially vulnerable to this damage. Polyunsaturated fats very powerfully inhibit thyroid function, contributing to weight gain, fatigue, and all the other symptoms of low thyroid function. They also promote systemic inflammation, suppress immune function, damage the digestive system, and drive hormonal imbalances including hypothyroidism and insulin resistance.
I cover all of this in detail in my book How to Boost Metabolism with Food and my Metabolic Nutrition Mastery course — including how to recognise the signs of PUFA-related thyroid suppression and what to do about it.
What Fats Should You Eat?
Given the damage caused by polyunsaturated fatty acids, it is very important to limit their intake as much as possible. This is not straightforward, because polyunsaturated fats are present in almost all foods to varying degrees — including many foods considered "natural" and "healthy."
In my courses, you will learn how to identify polyunsaturated oils, which foods are naturally high in them, and how to build a diet centred on genuinely thyroid-supportive fats. You will also find guidance on how to safely reduce your body's accumulated PUFA stores — a process that takes time and requires a specific approach.
If you would like personalised support implementing these dietary changes, I am available for one-to-one nutritional and metabolic counselling sessions. These sessions are tailored to your individual situation, health history, and goals.
Why Saturated Fats Like Butter and Coconut Oil Support Your Thyroid
The narrative that saturated fats are dangerous has been thoroughly debunked by independent researchers, yet it persists in mainstream dietary advice. The truth is that butter and coconut oil are not just harmless — they are actively supportive of thyroid function and metabolic health.
Coconut oil is composed predominantly of medium-chain fatty acids (MCFAs), primarily lauric acid. Unlike long-chain polyunsaturated fats, these medium-chain fats are absorbed directly into the portal circulation and delivered rapidly to the liver, where they are converted into energy rather than stored as fat. Crucially, coconut oil does not suppress thyroid function. In fact, it appears to actively support the thyroid by improving the efficiency of T4-to-T3 conversion — the key step that makes thyroid hormone biologically active in your cells.
Dr. Ray Peat, whose research underpins much of the pro-metabolic nutritional framework (which I teach in my book How to Boost Metabolism with Food), consistently highlighted coconut oil as a thyroid-supportive fat. His work showed that the medium-chain saturated fats in coconut oil increase cellular energy production, raise basal metabolic rate, and help to counteract the metabolic suppression caused by accumulated polyunsaturated fats in the body's tissues.
Butter is rich in short- and medium-chain saturated fatty acids and contains a profile of fat-soluble nutrients that are essential for hormonal and metabolic health: vitamins A, D, and K2, as well as butyrate — a short-chain fat that feeds the intestinal lining, reduces inflammation, and supports the liver. Vitamin A in particular is essential for thyroid hormone receptor sensitivity. Without adequate vitamin A, your cells cannot respond properly to thyroid hormone even if your thyroid is producing it normally. Butter from grass-fed cows is especially rich in these nutrients.
Both butter and coconut oil are completely stable at cooking temperatures. Unlike polyunsaturated oils, they do not oxidise or form free radicals when heated — making them the only genuinely safe fats to cook with. Their stability inside the body is equally important: they do not accumulate in cell membranes in ways that impair cellular signalling or energy production.
If you are working on recovering your thyroid and metabolic function, replacing polyunsaturated oils with butter and coconut oil in your daily cooking is one of the most impactful dietary changes you can make. You can learn exactly how to implement this shift — and everything else to consider alongside it — in my book How to Boost Metabolism with Food and my Metabolic Nutrition Mastery course.
Frequently Asked Questions
Are polyunsaturated fats really bad for you?
Yes — according to a growing body of independent research, polyunsaturated fats (PUFAs) are chemically unstable and oxidise readily inside the body, generating free radicals that damage cells, suppress immune function, promote inflammation, and inhibit thyroid enzyme activity. Health authorities promoted them as heart-healthy based on flawed research from the 1960s and 70s. More recent and methodologically sound research does not support that claim, and substantial evidence now links high PUFA consumption to obesity, hormonal disruption, and accelerated cellular ageing.
Why is coconut oil good for the thyroid?
Coconut oil is composed mainly of medium-chain saturated fatty acids — particularly lauric acid — which are metabolically neutral in the sense that they do not suppress thyroid enzyme activity. Unlike polyunsaturated oils, which directly inhibit the conversion of inactive T4 into active T3, coconut oil supports this conversion process and has been observed to raise basal metabolic rate. It is also completely stable when heated, meaning it does not oxidise or form damaging compounds during cooking. I explain how to build a thyroid-friendly diet around the right fats in my book How to Boost Metabolism with Food and my Metabolic Nutrition Mastery course.
Can I eat butter if I have hypothyroidism?
Yes — butter is one of the most thyroid-friendly fats available. It provides fat-soluble vitamins A, D, and K2 that are essential for thyroid hormone production and receptor sensitivity, as well as butyrate which supports the gut lining and liver function — both critical for thyroid health. Grass-fed butter is particularly nutrient-dense. Unlike margarine and refined vegetable oils, butter does not interfere with thyroid enzyme activity. I cover the full list of thyroid-friendly and thyroid-suppressing foods in my book How to Boost Metabolism with Food.
How long does it take to clear polyunsaturated fats from the body?
Polyunsaturated fats are stored in cell membranes and body fat and are released slowly over time as tissues turn over. Depending on how much has accumulated — which reflects years of dietary intake — a meaningful reduction in stored PUFAs typically takes anywhere from one to four years of consistent dietary change. This is why Dr. Peat emphasised patience and a specific nutritional strategy during the transition period. My courses and counselling sessions cover this process in detail.
What oils are safe to cook with?
The safest cooking fats are those that are stable at heat — meaning they do not oxidise and form free radicals when exposed to cooking temperatures. Butter, ghee, coconut oil, and beef tallow are all excellent choices. Extra virgin olive oil is relatively stable due to its predominantly monounsaturated fat content, but it should not be used for high-heat cooking. Sunflower, rapeseed, canola, soy, corn, and other vegetable oils are all high in polyunsaturated fats and should be avoided entirely, especially when heated.
Does coconut oil raise cholesterol — and is that a problem?
Coconut oil can raise total cholesterol levels, including LDL. However, as discussed in this article, the link between cholesterol and heart disease is not supported by solid scientific evidence. Cholesterol is a vital molecule — it is the precursor to all steroid hormones, including thyroid hormone, oestrogen, progesterone, and cortisol. Low cholesterol is associated with higher overall mortality in multiple large-scale studies. The more meaningful question is whether a fat causes oxidative damage and inflammation — and coconut oil, being highly saturated and stable, does not.
