Not all fats are created equal.
Some of them are simply used for energy, while others have powerful health effects.
CLA (short for "Conjugated Linoleic Acid") is a fatty acid that belongs to the latter group (1).
It is found naturally in beef and dairy, and has been shown to cause fat loss in many studies (2).
CLA is actually one of the most popular weight loss supplements in the world, and some believe that it can have other health benefits as well (3).
This article takes a detailed look at CLA and its effects on your weight and overall health.
Let me explain exactly what Conjugated Linoleic Acid is...
Linoleic acid is the most common Omega-6 fatty acid, found in large amounts in vegetable oils but also in smaller amounts in various other foods.
The word conjugated has to do with the arrangement of the double bonds in the fatty acid molecule.
There are actually 28 different forms of CLA, but two of the most important ones are "c9, t11" and "t10, c12" (4).
On the image, you see regular linoleic acid on the top, with the two most important forms of conjugated linoleic acid below (5):
CLA actually contains both cis (c) and trans (t) double bonds, and the numbers (like t10, c12, etc.) have to do with the placement of these bonds on the fatty acid chain.
The difference between the CLA forms is that the double bonds (seen as a double line on the image) are arranged differently, but it's important to keep in mind that something as miniscule as this can make a world of difference to our cells.
So... basically, CLA is a type of polyunsaturated fatty acid, with both cis and trans double bonds. In other words, CLA is technically a trans fat, but it is a natural type of trans fat that is found in many healthy foods (6).
Bottom Line: There are 28 different forms of CLA, a fatty acid found in some foods. It is technically a trans fat, but very different from industrial trans fats.
The main dietary sources of CLA are animal foods from ruminants, such as cows, goats and sheep.
The total amount of CLA in these foods varies greatly depending on what the animals ate (10).
Most people are already getting some CLA from their diet... the average intake in the U.S. is about 151 mg per day for women and 212 mg for men (12).
However... it's important to keep in mind that the CLA you find in supplements is NOT derived from natural foods.
It is made by chemically altering safflower and sunflower oils, which are unhealthy vegetable oils. The linoleic acid in the oils is turned into conjugated linoleic acid via a chemical process (13).
The balance of the different forms is heavily distorted in supplements. Foods are mostly c9, t11, while the supplements are very high in t10, c12, which is never found in large amounts in nature (14, 15).
For this reason, CLA taken in supplement form does not have the same health effects as CLA gotten from foods.
Bottom Line: The main dietary sources of CLA are ruminant animals like cows, goats and sheep. The type of CLA found in supplements is made by chemically altering vegetable oils.
The biological activity of CLA was first discovered in the year 1987 by a team of researchers who showed that it could help fight cancer in mice (16).
Later, other researchers discovered that it could also reduce body fat levels (17).
As obesity increased worldwide, people became more interested in CLA as a potential weight loss treatment.
That being said, cellular mechanisms and animal studies are fun and exciting (to me anyway), but what you probably want to know if it leads to actual pounds lost when taken by humans.
Bottom Line: CLA has caused massive amounts of fat loss in animal studies and many different biological mechanisms have been identified.
Fortunately, we have quite a few studies that have been done on CLA.
In fact, CLA may just be the most comprehensively studied weight loss supplement in the world.
Many of the studies are so-called randomized controlled trials, the gold standard of scientific experimentation in humans.
Some studies have shown that CLA can cause significant fat loss in humans (23).
In a big review paper that pooled the data from 18 controlled trials, CLA was found to cause modest fat loss (31).
The effects are most pronounced during the first 6 months, then it slowly plateaus for up to 2 years.
This is a graph from the paper. You can see how weight loss slows down with time:
According to this paper, CLA can cause an average fat loss of about 0.1 kilograms per week, or 0.2 pounds per week, for about 6 months.
Another review study published in 2012 found that CLA caused about 3 lbs (1.3 kg) more weight loss than placebo, a dummy pill (32).
A quote from their study:
"Our meta-analysis also revealed a small significant difference in fat loss favouring CLA ... The magnitude of these effects is small, and the clinical relevance is uncertain. Adverse events included constipation, diarrhea, and soft stools."
I agree... the weight loss effects may be statistically significant, but they are so small that they don't have any real-world meaning... and there is potential for side effects.
Bottom Line: CLA supplementation has been shown to cause fat loss, but the effects are small, unreliable and unlikely to make a difference in the real world.
In nature, CLA is mostly found in the fatty meat and dairy of ruminant animals.
Many long-term observational studies have been conducted, examining whether people who consume more CLA have a lower or higher risk of disease.
Additionally, studies in countries where cows eat grass show that people with the most CLA in their bodies have a lower risk of heart disease (36).
This may have something to do with the CLA, or other protective components in grass-fed animal products, like the Vitamin K2.
Of course, grass-fed beef and dairy products are healthy for various other reasons, so it is a good idea to consume them regularly.
Bottom Line: Numerous studies show that people who eat the most CLA have improved metabolic health and a lower risk of many diseases.
There is quite a bit of evidence that CLA found naturally in food is beneficial.
However, as I mentioned before, the CLA found in supplements is made by chemically altering linoleic acid from unhealthy vegetable oils.
The CLA in supplements is usually of a different form than the CLA in foods, being much higher in the t10, c12 type.
As is so often the case, some molecules and nutrients are beneficial when found in natural amounts in real foods, but become harmful when we start taking them in large doses.
According to some studies, this seems to be the case with CLA supplements.
There are also numerous studies, in both animals and humans, showing that despite lowering body fat, CLA can drive inflammation, cause insulin resistance and lower HDL (the "good") cholesterol (40, 41).
To be fair, many of the animal studies that showed these disturbing effects did use large doses, much higher than people regularly supplement with.
However, some of the studies were in humans, using reasonable doses, indicating that these are genuine concerns with CLA supplements.
CLA can also cause other less serious side effects like diarrhea, stomach pain, nausea and flatulence (42).
Bottom Line: The CLA found in most supplements is different from the CLA found naturally in foods. Several studies have shown disturbing side effects from CLA, such as increased liver fat.
Most of the studies used doses ranging from 3.2 to 6.4 grams per day.
Keep in mind that the risk of side effects increases as the dosage increases.
The FDA allows CLA to be added to foods and gives it a GRAS (Generally Regarded as Safe) status.
However, this is the same organizations that tells us that soybean oil and high fructose corn syrup are "safe" so take that with a grain of salt.
Personally I don't think losing a few pounds is worth the risk of increased liver fat and worsened metabolic health.
If you disagree and still want to take CLA supplements, then I recommend that you get regular blood tests to monitor liver function and other metabolic markers, to make sure that you're not harming yourself.
Although CLA from beef and dairy is beneficial, taking "unnatural" types of CLA made from chemically altered vegetable oils seems to be a bad idea.
Having a six pack is great, but there are other better ways to lose fat that won't give you fatty liver disease and diabetes in the process.