Fat is an essential nutrient with benefits like providing energy and protecting cells.
But not every type of fat is good for your body. Evidence shows that eating some types of fat is bad for you and may significantly increase inflammation in your body (*).
In this guide, you’ll learn how to identify unhealthy fats and which fats you should include in your diet instead.
There’s more than one type of fat you’ll want to avoid especially on a high-fat diet like keto. These “bad” fats can range from deep-fried foods to margarine.
As a shortcut to making good decisions, you can also keep in mind that most unhealthy fats have one thing in common: they’re highly processed or contain additives that differentiate them from natural, whole food sources of fat.
Below you’ll learn the worst offenders and why they’re capable of causing problems on keto (or any diet).
Artificial Trans Fats
Artificial trans-fats are made by adding a hydrogen atom to unsaturated fat, resulting in a fat that is solid at room temperature. Hydrogenation of oils to make margarine and vegetable shortening generates a significant amount of trans-fats.
Manufacturers put oils through hydrogenation to improve their texture and shelf-life. Trans fats have adverse effects on health, including increasing the risk for cardiovascular disease (*) and other chronic illnesses like type 2 diabetes (*).
Governing bodies on nutrition including the FDA, CDC, and American Heart Association (AHA) recognize trans fats as bad and recommend that we eat as little of them as possible.
Other than hydrogenated fat products sold for cooking, like vegetable shortening, other commercially available sources of trans fats include:
- Some cookies
- Some creamers
- Most microwave popcorn
- Many brands of store-bought dough
- Many deep-fried foods
In acknowledgment of the harm trans fats can cause, the FDA ruled that from June 2018 onward, no food on the market can contain artificial trans fats (*).
However, there’s unfortunately still a loophole: manufacturers can still sell and label foods as trans-fat-free if they contain 0.5 g of trans fat per serving or less.
Foods that list hydrogenated or partially hydrogenated oils often contain trans fats even if they claim to be trans fat-free.
Avoiding them altogether is difficult, but it’s worth your time to pay close attention to nutrition facts and ingredients to minimize your intake of trans fats.
Highly Processed Fats
Fats that undergo extensive amounts of processing can be harmful because they go through several stages of heating at extremely high temperatures.
When fats are heated, they undergo oxidative damage, forming free radicals. Free radicals from processed fats are detrimental to your health because they cause oxidative damage and inflammation in the body (*).
Processed oils also tend to have high levels of omega-6 fatty acids, which can be dangerous if consumed in excess. Studies show that consuming too much omega-6 increases the risk of heart disease and other chronic illnesses (*).
Highly processed fats include:
- Vegetable oil (which usually refers to a blend of canola, corn, or seed oils as opposed to any real “vegetables”)
- Soybean oil
- Safflower oil
- Corn oil
- Canola oil
- Processed cheese
As an example, canola oil goes through a seven-step process that involves solvent extraction and multiple heating steps. While canola and other so-called vegetable oils may sound healthy, they’re among the most processed food ingredients you’ll find.
On the other hand, note that while they may sound like vegetable oils, avocado oil and olive oil are healthy fats (which we’ll cover later in this article), not industrial vegetable or seed oils.
Fats with Added Preservatives or Other Chemicals
Some fats like margarine and vegetable oil also contain synthetic preservatives to prolong their shelf life. As you might imagine, adding various chemicals only makes these already unhealthy fats even worse for your health.
Manufacturers add synthetic antioxidants like artificial BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), and TBHQ (tertiary butyl hydroquinone) to protect the oil from oxidation.
The FDA considers all of these additives safe in limited quantities, but some animal studies link BHA and BHT to cancer, and the United States National Toxicology Program classifies BHA as ”reasonably anticipated to be a human carcinogen” (*).
Processed oils like canola and sunflower may also contain emulsifiers, colors, and flavors with similarly unknown safety effects.
When purchasing oils or fats, be sure to always read the product ingredient list to get an understanding of what it contains. Usually, the fewer ingredients, the better.
Better yet, opt for minimally processed fats without synthetic preservatives like butter, lard, and avocado oil.
Fats Made From GMO Seeds
GMOs or genetically modified organisms are plants or other living organisms whose genetics are artificially manipulated to boost yields or achieve other properties that increase profitability.
There’s a lack of conclusive evidence around the potential harms of GMO foods, and little research has been done on them to determine their long-term effects on health.
Seeds like corn, rapeseed, and soybeans usually go through genetic engineering to make them more resistant to pests and diseases (*). Food manufacturers then use these genetically altered seeds to make vegetable oil. For example, over 90% of the rapeseed or canola grown in the United States is GMO.
Resistance to herbicides like Roundup or glyphosate is another common reason for genetic manipulation. As a result, fats made from GMO seeds are also more likely to contain glyphosate, which studies link with gut health problems and cancer (*).
While the potential health effects of GMOs remain unknown, there’s certainly no benefit to including them in your diet aside from possibly saving a few cents per serving.
When deep-frying, you’re cooking food in oil heated up to 350oF (175oC) or higher. At these temperatures, the oil composition starts to change, generating harmful free radicals (*). In restaurants, oil is sometimes used hundreds of times before it’s switched out.
The free radicals generated in oil during deep-frying have been shown to increase inflammation levels in the body. Many chronic diseases including cardiovascular diseases, Alzheimer’s disease, cancer, dementia, and type 2 diabetes are linked to chronic inflammation (*).
A study on pregnant women found that regular consumption of fried food before pregnancy negatively affected blood sugar control during pregnancy resulting in gestational diabetes (*).
Omega-6 fats are a type of unsaturated fat that’s beneficial in small amounts but, linked with harm in higher intake levels. Eating too much omega-6 increases the risk of inflammatory diseases like liver disease, inflammatory bowel disease (IBD), and cardiovascular disease (*).
High omega-6 consumption also increases your risk of becoming obese or overweight (*).
Studies suggest that we should all try to lower the ratio of omega-6 fats to omega-3 fats in our diet. Potential sources of omega-6 fats in your diet include safflower oil, sunflower oil, and corn oil. The research-recommended ratios for omega-6 to omega-3 fats range one to one (equal amounts) to two to one (no more than twice as much omega-6 relative to omega-3 intake) for optimal health (*).
But according to estimates from some researchers, the average dietary ratio of omega 6 to omega 3 in the United States is around 15-to-1.
This means that you can eat up to twice as much omega-6 as omega-3 every day, but should limit omega-6 intake in relation to omega-3s far more than most people do.
Cholesterol is a type of fat present in all animals. The human body makes its own cholesterol but it can also come from eating animal foods.
The foods richest in cholesterol are animal foods like eggs, meat, and dairy products. However, as long as these foods are part of a healthy balanced diet, their cholesterol content should not be a problem.
The bottom line: recent studies suggest that the truth about dietary cholesterol is that it has no impact on your risk for heart disease (*). It turns out that even the so-called bad cholesterol (LDL cholesterol) may not be an important determiner for heart health (*).
Instead of worrying about cholesterol intake, it’s best to focus on removing unhealthy fats from your diet and replacing them with healthy fats.
In contrast to unhealthy fats, most healthy fats are unprocessed or minimally processed. Here’s what you need to know about good fats to eat, on a low-carb diet or any healthy diet.
If you’ve been following nutrition advice for a while, you might be surprised that saturated fats didn’t make the “bad fats” list. While it’s admittedly still a controversial issue, some recent studies actually show no link between eating healthy saturated fats and risk for heart disease (*).
This type of fat doesn’t contain any double bonds in its structure (unlike mono and polyunsaturated, which do contain this type of molecular bond). For this reason, saturated fats are naturally more stable and less likely to undergo oxidation (*). As a result, they’re often less processed — and as we covered in the previous section on bad fats, high levels of processing and oxidation are both major indicators of unhealthy fats.
In fact, newer evidence even links saturated fats to some health benefits. In the context of a keto diet, the potential health benefits of saturated fats include:
- Raising HDL (good cholesterol) to prevent the buildup of LDL (bad cholesterol) in the arteries (*)
- Maintenance of a healthy gut microbiome (*)
- Support for brain and central nervous system health (*)
- Male hormonal support (*)
Animal foods like beef, dairy, pork, lamb, and chicken are the richest sources of saturated fats. Other good sources of saturated fat include:
- Butter and ghee (preferably grass-fed)
- Heavy cream, cheese, and other full-fat dairy products
- Eggs (preferably pasture-raised; look for deep orange-yellow yolks, which are the sign of a healthy animal and greater nutrient density)
- Coconut oil
- Palm oil (look for a sustainable, rainforest-certified brand; unsustainable palm oil farming destroys rainforests)
- Cocoa butter and dark chocolate
Certain plant oils like coconut and palm also provide a significant amount of saturated fat. About 83% of coconut oil is saturated fat (*).
Monounsaturated fatty acids (MUFAs) are fats that have only one double carbon bond in their molecular structure.
A significant amount of evidence links the intake of monounsaturated fats to health benefits. Thirty-two studies on almost 850,000 people found that consuming MUFAs reduces the risk of cardiovascular disease, stroke, and mortality (*).
Good sources of monounsaturated fats for keto include:
- Olives and olive oil
- Avocado and avocado oil
- Nuts such as peanuts, macadamia nuts, cashews, and almonds, and nut butters
Polyunsaturated fats (PUFAs) are fats with more than one double carbon bond in their molecular structure. The two most important types of PUFAs are omega-3 and omega-6 fatty acids.
However, be sure to keep your omega-6 consumption low, as too much increases your risk for chronic illnesses (*).
The benefits of omega-3 rich PUFAs include:
- Improved heart health (*).
- Better blood sugar control by improving insulin sensitivity (*).
- Decreased risk of autoimmune disorders and other inflammatory diseases (*).
- Improved mental health, including relief from depression and ADHD (*).
- Weight loss. Omega-3 rich foods particularly fish are great for weight loss as they are nutrient dense but fairly low in calories (* , *).
Omega-3 rich foods include:
- Fish like mackerel, salmon, cod, and herring
- Shellfish like oysters, mussels, and crab
- Flax seeds
- Chia seeds
- Omega-3 supplements
The best sources of omega-3 are fish, seafood, and algae. Plant sources are inferior to these sources in terms of both quantity and quality.
When eating foods rich in PUFAs, be sure to prepare them with little to no heat as high temperatures generate inflammatory radicals. One good way to eat PUFAs without heating is by eating them in raw salads.
Always store oils rich in PUFAs in a cool dark dry place as they’re very sensitive to heat, light, and moisture.
Fats to avoid on keto include highly processed or industrial oils like canola oil, sunflower oil, safflower oil, and corn oil, margarine, deep-fried foods, and other highly processed junk foods.
Instead, use healthy sources of fats like avocados, olive oil, meat, chicken, eggs, nuts, seeds, dairy, fish, and high-quality fat supplements like MCT oil powder.
These fats will not only help you reach ketosis but will also help you maintain overall health.
Souza R et al. 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. 2015 August 12