Leisha Good Fats, Bad Fats: Why Warmth Matters
When I talk to clients about seed oils, I sometimes see a look come over their face that says it doesn’t really matter, but it matters a great deal, and far more than most people realise. (Yes, we’re talking about fat again, because I just can’t seem to leave this topic alone.)
Here in Ecuador, most people have had seed oils in their food since the day they could eat, and nobody ever sat them down to explain the difference between a good fat and a bad fat, or what either one does once it is inside the body. This article is my attempt to put that as plainly as I can.
Your body is built out of fat. Every cell wall, your brain and the insulation wrapped around every nerve are made from the fat you eat, so if you put the wrong fat in, that is what you end up built from.
Warm fats and cold oils
The simplest way I know to look at it is that a good fat is a warm fat, either grown in the heat like the avocado, the olive and the coconut, or made inside a warm body like the butter from a cow or the tallow off the beef. There is real chemistry behind that, because everything alive makes its fat to suit the temperature it lives at. A cow runs at around 38 degrees and a coconut ripens in tropical sun, so both can afford a firm, stable fat, saturated or monounsaturated, that holds together under heat, light and air.
A seed sitting through a cold northern winter has no such luxury. It needs fat that stays runny in the cold, so it makes polyunsaturated fat, which is the fragile kind that grabs oxygen, breaks down and goes rancid. You can see this in your own kitchen, where butter and coconut oil go solid on a cool morning while soybean and sunflower oil stay liquid even in the fridge, and the one that stays runny is the one that is fragile.
The good fats
Among the tropical fats, coconut oil is the toughest plant fat there is, around ninety percent saturated, and it will take high heat all day without breaking down. Cacao butter is another, and here in Ecuador we are sitting on some of the best cacao in the world. The avocado and the olive are fruits of warm places and both are full of stable monounsaturated fat, and macadamias belong in the same family, being about the lowest in fragile fat of any nut you can buy.
The warm-bodied fats are just as good. Butter and ghee are best from grass-fed cows, with the ghee taking the high heat, while beef and lamb tallow are made for frying and roasting. Duck and goose fat make the best roast potatoes you will ever eat, bone marrow and suet give body to your broths, and cream, whole milk, proper aged cheese and egg yolks are there for every day.
Then there is pork lard, which is what Ecuador cooked with before the bottled oils turned up and which deserves its place back. It is worth knowing that a pig is what it eats, so a pig fed on corn and soy carries more of the fragile kind of fat, and the best lard comes from pigs that lived outdoors.
The bonus with nearly all of these is that the fat-soluble vitamins A, D, E and K2 come riding in on them the way nature intended, whereas the refined oils bring you almost none.
I encourage people to put the animal fats at the centre, because I feel they are some of the most important foods we have. Butter, tallow, lard, egg yolks and liver carry vitamins in a form the body can use straight away: true vitamin A rather than the beta-carotene a plant offers, vitamin D3, and vitamin K2, which is found mainly in animal and fermented foods. They hold up in the pan, they keep you full, and they are the fats our ancestors built their bodies on for as long as there have been people. The plant fats on this list are good, but the animal fats are the foundation.
The one that breaks the rule
Oily fish such as sardines, mackerel, salmon and anchovies, along with cod liver oil, all come from cold water and are full of polyunsaturated fat, yet they are good for you, so it is fair to ask why they don’t fall foul of the rule.
The answer is that it is a different fat doing a different job. Fish carry omega-3, the EPA and DHA that your brain, eyes and heart are built from, and nothing else supplies it as well, whereas seed oils are loaded with omega-6 linoleic acid, which most of us are already getting far too much of. With fish the thing to watch is freshness rather than heat, so eat the whole fish, keep your fish oil cool and sealed, and never fry with it.
The bad fats
The bad fats are the seed oils: soybean, canola (which is rapeseed under a new name), corn, sunflower, safflower, cottonseed, grapeseed and rice bran, along with anything sold simply as “vegetable oil” and the margarine and shortening made from them.
Ask yourself whether you could get oil out of a corn kernel in your own kitchen. Nobody could, which is why nobody ate these oils for almost all of human history, and getting it out takes a factory. The seeds are crushed and heated and the oil is pulled out with hexane, a petroleum solvent. The crude oil is then refined, bleached and deodorised at high heat, because by that point it already smells of what it is, an oil that has started to go off.
What comes out the other end is clear, tasteless and cheap enough to sit on a shelf for months, and it is packed with linoleic acid, the fragile omega-6 that breaks down with heat, light and time. These oils are also very new to the human diet, since cottonseed oil was first sold as a cooking fat in 1911, a leftover from the cotton mills given a second life in the kitchen, and in barely a hundred years refined seed oils have gone from nothing to one of the biggest sources of calories on the planet.
What the farmers learned
There is an old story from American farming that says a great deal about these fats. In the 1940s coconut oil was cheap and plentiful, so farmers tried feeding it to their animals to fatten them up, and it did the opposite. As the biologist Ray Peat told it, the animals became lean, active and hungry, eating more and moving more, which is the last thing a farmer selling meat by the pound wants to see.
So the farm boards and the bankers went looking for something that worked. According to the same account, drugs that slowed the thyroid were used for a few years in the late 1940s to fatten livestock on less feed, until soy and corn turned out to do the job more cheaply.
Then corn took over everything. In the 1970s the US Secretary of Agriculture, Earl Butz, rebuilt farm policy around cheap corn and paid farmers to grow as much of it as they could, which left a mountain of subsidised corn and a whole system built to use it up. That system was the feedlot. Cattle that had grazed on grass for thousands of years were moved into pens and fattened on corn, and the grading system that rewards marbled beef favoured the corn-fed animal over the grass-fed one. The same cheap corn went on to become the corn oil and corn syrup in the processed food on our own plates.
The lesson is worth sitting with. Farmers knew exactly which feed made an animal lean and which made it fat, and they chose the one that fattened. We have been eating out of that same system for fifty years, so it should not surprise anyone that we have ended up much the way the cattle did.
Here in Ecuador
Most of the bottled oil on the shelves here is refined palm oil, palm blended with soya, or sunflower oil. Sunflower oil is one of the most popular cooking oils in the country, and the ordinary kind is also one of the most fragile, being very high in linoleic acid. Palm is a tropical fat and more stable than soybean, so by my rule it ought to be fine, but not once it has been through the factory. Refined, bleached and deodorised palm has had the same treatment as any seed oil, and the blends still carry the soya.
It turns up everywhere, from the fryer at the street stall, often running on the same oil for days, to the plantain chips, crisps, biscuits and packet snacks, the supermarket bread and pastries, the mayonnaise and bottled sauces, and the margarine and vegetable shortening. That is why it pays to read the label. If you see vegetable oil, soybean, sunflower, canola or corn oil, refined palm oil or palm olein, you know what you are holding, and if it only says vegetable oil, it is safest to assume the worst.
The good news is that Ecuador already had the answer, because the old kitchen ran on pork lard, butter, coconut on the coast, avocado and cacao, and the grandmothers had it right all along.
What it does inside you
Most of the damage comes down to oxidation, which is fragile fat breaking down, first in the pan and then inside you. In the pan, heating a polyunsaturated oil starts to break it apart into harmful by-products, aldehydes among them, and reusing it the way a street fryer does all day makes it worse with every round. Coconut oil, tallow and ghee hold up far better, but old, rancid oil heated again and again is no good for anybody.
From there it moves into you, because you are built from what you eat and the fat you eat ends up in your cell walls and your body fat. In the United States, the linoleic acid stored in people’s body fat has more than doubled in the last fifty years, and it doesn’t leave in a hurry, since stored fat turns over slowly across a couple of years.
That fragile fat needs guarding, and the guard is your antioxidants, so the more of it you carry, the more vitamin E and the rest your body spends keeping it from going rancid inside you. Meanwhile the oil itself brings next to no vitamin A, D or K, and when it pushes the butter, the egg yolks and the liver off the plate, those vitamins go with them. That is the trap, with less protection coming in and more of it being spent.
None of this happens in a week; it is the slow work of years of the same oil, meal after meal, quietly building you out of the wrong material.
It catches up with you
This is what makes seed oils so easy to dismiss. You can eat them for twenty or thirty years and feel perfectly well, which is exactly why that look on a client’s face says it doesn’t matter. The body is very good at compensating, so it quietly spends its reserves, patches the damage and carries on, and for a long time there is nothing to show for it. Then the reserves run low and the damage starts to outpace the repair, and that is when it catches up and the dis-ease begins to show.
Often the first place it shows is blood sugar. Years of processed food, with the oil and the sugar arriving together, put the liver under strain and leave the cells less and less responsive to insulin, until the blood sugar creeps up and the diagnosis is type 2 diabetes. By the time someone hears that word, the process has usually been running quietly for a decade or more.
The nerves are next, or sometimes alongside. Nerves built from fragile, oxidised fat and short of their protective vitamins are already under strain, and high blood sugar adds damage of its own. In my view, much of the tingling, burning, numbness and pain in the hands and feet that people put down to getting older has its roots in years of the wrong fat and too much sugar.
Then come the circulation and the heart. Oxidised fat in the blood is one of the things that drives the build-up of plaque in the artery walls, and as the vessels stiffen and narrow, the blood flow to the legs, the feet, the brain and the heart muscle falls away. Cold hands and feet, wounds that are slow to heal, high blood pressure, chest pain and, in the end, a heart attack or a stroke are the same story showing up in different places.
The hopeful part is that the body wants to repair itself, and the earlier you change the fat, the more of the damage it can put right.
It goes deeper than the cell wall
The real story plays out in every single cell. Each one is wrapped in a thin double layer of fat, and the tiny gates in that layer that let a nerve fire only work properly when the fat around them is the right fat. When a cell wall built from seed oil oxidises, it throws off harsh breakdown products that damage those gates and the proteins around them. In my view, this is a large part of why a nervous system built on seed oil ends up on edge and overreacting to everything around it.
The body does have defences. The insulation around your nerves carries special fats that are built to take the hit first and spare the rest of the cell, and vitamin E and the body’s own antioxidants stand alongside them. Feed the system a steady stream of rancid oil, though, and you use those defences up faster than you can replace them. The power plants inside every cell are also built on a fragile fat that oxidises easily under that kind of stress, and when it does, the cell makes less energy and a nerve struggles to hold its charge.
Then there is what gets stored. Many of the chemicals in processed food and the modern world dissolve in fat, and when the liver cannot clear them fast enough, the body locks them away in fat tissue to keep them out of the blood. A struggling liver turns a fat problem into a storage problem. That is why getting your fats and your metabolism right matters so much, so that the old fat, and whatever is locked inside it, can be burned and cleared rather than left to sit.
The sugar that rides with it
Seed oils almost never come on their own, because they come in processed food, and processed food comes soaked in sugar. Give the body more sugar than it can burn and the liver literally manufactures fat out of it, so you store more, and over time you build a fatty liver, which is the very organ you need for processing fat and clearing toxins.
Sugar does its own damage on top of that. It sticks to proteins in a process called glycation, which sets off oxidative damage of its own, so the sugar and the oil end up working together, with the oil supplying the fragile fat and the sugar adding fuel to the fire that oxidises it. That is why swapping the bottle in your kitchen only gets you halfway if the packets and the soft drinks stay in the cupboard.
The gut nobody ferments any more
Whole countries have stopped eating anything fermented, and the steady supply of varied, beneficial microbes that used to come with sauerkraut, kimchi, kefir and other live foods has simply dried up. What replaced it is something much closer to a monoculture, with one factory yeast in the bread where there used to be a wild spread of cultures. A narrowed microbiome stops doing its job of holding the balance, making vitamins such as K2 and some of the B vitamins, and keeping the bad bugs in check. On the hopeful side, a Stanford study found that eating more fermented food measurably widened the variety of people’s gut microbes within ten weeks.
This matters for everything above, because the fat-soluble vitamins only get into you if the gut, the bile and the digestive enzymes are doing their work. It also feeds on itself: a damaged microbiome means worse absorption, worse absorption lets fewer nutrients in, and fewer nutrients slow the gut’s healing, so round it goes until you break the loop. A gut lining damaged enough to fail on fat is usually failing on B12, folate, magnesium and zinc as well, which is how one bad fat ends up pulling the whole lot down together.
Why some people eat seed oils and seem fine
The obvious objection is that seed oils are everywhere, so why isn’t everyone ill? Look closer and the pattern holds, because the oil does its worst damage when there is nothing in place to protect you.
Vegans and vegetarians are a good example. On the fat argument alone they ought to be doing worse, yet many of them do well, and what saves them is the antioxidant load from whole plants, the polyphenols, vitamin C and vitamin E that stop polyunsaturated fat from going rancid, along with lower iron stores, since iron is one of the metals that drives oxidation.
Thailand is another. Almost everything there is cooked in seed oil, yet the Thais eat fish from the rivers and lakes constantly and many still farm in the sun all day, so the fish and the sunlight hand them the omega-3 and fat-soluble vitamins that the oil lacks. Take away the fish and the sun while keeping the same oil, and I would expect the damage to show. That is the heart of all this: rancid seed oil hurts you most when there is no protection, and when you put the protection back, the body copes.
Hot countries and cold countries
The geography backs the warm-fat rule up, and it is well documented. Large population studies have found that people who eat chilli and spicy food regularly have lower death rates, both overall and from heart disease, and since capsaicin is known to fight oxidation and inflammation, the spice in a hot-country kitchen is part of the protection.
Head the other way, away from the equator, and the picture gets worse. Multiple sclerosis rises steadily with distance from the equator, so the colder, darker countries carry far more of it, and the same gradient turns up in other autoimmune conditions. The leading explanation is less sun, which means less vitamin D and less protection. In the cold the body also holds on to its fat while making very little vitamin D of its own, so the fragile fat sits there with nothing guarding it. People in hot countries, by contrast, eat the spice, work in the sun and keep their protection topped up.
The Inuit are the proof from the other direction. They live in about the coldest place on earth, yet their traditional diet of fish and marine fat, loaded with omega-3 and vitamin D and with no seed oil anywhere near it, was what first pointed scientists towards the protective effect of omega-3. The cold did not hurt them because the protection carried them, and it shows that the problem is never one thing on its own but the fat you store, minus the sun, the spice, the fish and the fat-soluble vitamins.
The evidence from the real world
Some of this has been put to proper experiments. In a large randomised trial, people taking vitamin D for about five years had around a fifth fewer new cases of autoimmune disease, which is a clear case of restoring the protection and seeing fewer people get sick. In another, a Mediterranean diet built on olive oil, a stable fat, cut heart attacks and strokes by roughly a third compared with a lower-fat diet.
History tells the same story. Populations that leave a traditional fish or whole-food diet for Western processed food pick up the Western diseases within a generation, as researchers saw with the Pima in Arizona, the Tokelau islanders who moved to New Zealand and Japanese families who settled in Hawaii, which points to the food rather than the genes. The Amish, who eat little processed food, have strikingly low rates of allergy and asthma, and the research points to their farm life and the rich mix of microbes they grow up around, which ties straight back to the gut.
You can even test some of this yourself with bread. Most supermarket bread has seed oil baked into it, and some of what gets blamed on gluten may really be the oils and the factory yeast, so if you react to supermarket bread but do fine on a traditional sourdough made with olive oil, it may never have been the wheat at all.
What the critics say, and where I stand
I would rather meet the objections head-on than pretend they aren’t there, so here is where things stand.
There are parts of this that nobody argues with: saturated and monounsaturated fats are chemically more stable than polyunsaturated ones, oil reused for frying throws off harmful by-products, ultra-processed food (which is where most seed oil ends up) is tied to poorer health, and the olive-oil diets of the Mediterranean have proper trial evidence behind them.
Other parts are still being fought over. Plenty of nutrition researchers point to studies where swapping saturated fat for linoleic acid didn’t raise heart disease and in some cases lowered it, while others say those studies never accounted for oxidation or for years of storage in the body. That argument isn’t settled, and I won’t pretend it is.
You don’t need to win it, though, to act on the part everyone agrees on, which is that whole, stable fats that have barely been touched beat factory oils and the processed food they come in. That much stands on plain nutrition, whatever you make of the rest.
How to come back
This is a long game rather than a weekend fix, but you start the day you change the fat.
Begin with the bottle in your kitchen, getting the vegetable oil out and putting coconut oil, pork lard, ghee or tallow in its place for frying, with extra-virgin olive oil or avocado oil for gentle cooking and salads. After that, start reading the labels on the bread, the snacks and the sauces, because that is where most of it hides, and cut back on fried food from outside the house, since you have no idea what oil the stall is using or how many days it has been in the pan.
Eat the fats that carry the vitamins, which means egg yolks, butter, liver and oily fish, and get sardines or mackerel in two or three times a week for the omega-3. It is worth checking your supplements too, because a lot of vitamin D, E and K capsules are dissolved in soybean or sunflower oil, so you can end up swallowing the vitamin and the problem in the same capsule. Look for olive oil or MCT oil as the carrier, or simply take them with a meal that has real fat in it.
None of it lands if the gut is wrecked, so fixing absorption comes before almost anything else. That means clearing any parasites, getting the bile and digestive enzymes working, healing the lining and bringing back fermented foods such as sauerkraut, kimchi and kefir. MCT oil is also worth keeping in the cupboard as a clean, stable fat that gives you fast energy before a run.
After that it is a matter of giving it time, because the fat stored in your body turns over slowly and you should think in months and years rather than days. For anyone living with nerve pain, this is where the hope lies. Nerves can recover far more than most people are told, and when the right fats and the A, D, E and K go back in and the protection around the nerves rebuilds, it is possible for the burning, tingling and sensitivity to ease, and for some people the pain comes right down. Every meal cooked in a warm fat is a step in the right direction.
In the end it comes back to cooking the way your grandmother did, with fats that were made in warmth.
P.S. I seem to be writing a lot about fat lately. It started with one question and turned into a full research adventure, and I keep finding more to share. If you’re curious, you can read Are We Treating Fat Like Dirt? and my earlier piece, Good & Bad Fats.
