Conjugated linoleic acid – does it aid fat loss?

Is conjugated linoleic acid really a magic route to fat loss and a healthy old age?

Lies, damn lies and supplements! At least, that’s the way it can seem in the world of performance-enhancing sports supplements. All too often, the mere mention of a theoretical and untested benefit of a metabolic intermediate or obscure nutritional component in a scientific paper leads to a torrent of expensive products on the shelves, supported by quasi-scientific marketing aimed at the gullible consumer. When subjected to rigorous scientific scrutiny, many of these products don’t generate enough hard evidence to substantiate their conjugated linoleic acidims. In plain English, they simply don’t work!

However, it’s not always like that – witness the proven benefits of creatine for example – and now evidence is beginning to accumulate about the potential benefits of a derivative of linoleic acid. Reports suggest that supplementation with this derivative, known as conjugated linoleic acid or conjugated linoleic acid, may help promote fat loss in both animals and humans and, further, that conjugated linoleic acid may also act as an antioxidant, protecting the cells of the body against the free radical damage that is thought to be at the heart of ageing and degenerative disease.

Does this emerging evidence stand up to scrutiny and, if so, how can athletes manipulate their diet and supplementation régime to reap the benefits of conjugated linoleic acid?

Conjugated linoleic acid has been known about for some time, but interest in it became much more widespread when Pariza and co-workers identified it as a constituent of beef that appeared to possess anti-carcinogenic properties(1). Conjugated linoleic acid actually covers a whole group of closely related biologically active compounds called isomers, all of them derivatives of linoleic acid, which is one of the essential fatty acids and a common component in the diet. However, unlike linoleic acid, conjugated linoleic acid is only found in significant quantities in animal products such as cheese, milk and meat. This is because it is generated from linoleic acid in significant amounts in a process called ‘enzymatic isomerisation’, which occurs during the metabolism of linoleic acid by rumen bacteria, found in the gut of ruminant animals, such as cows and sheep. So far, at least eight conjugated linoleic acid isomers of linoleic acid have been identified, although only two are thought to possess significant biological activity: c9, t11 conjugated linoleic acid, which is the most common natural form, and the t10, c12 isomer.

Conjugated linoleic acid occurs in a number of animal foods, especially full-fat dairy produce, lamb and beef, although the actual amounts of conjugated linoleic acid present can be quite variable. Studies have shown that when cattle are fed diets rich in linoleic acid (such as sunflower and soyabean oils), the conjugated linoleic acid content of the milk they produce also increases(2). In addition, cows grazing pasture produce conjugated linoleic acid-enriched milk, especially when the grass is at an early growth stage. As a rule of thumb, however, most full-fat beef and dairy produce will contain between 3 and 7mgs of conjugated linoleic acid per gram of total fat content, 85-95% of which is present as the c9, t11 isomer.

There were early reports that the conjugated linoleic acid content of these foods could be increased by heat processing, pasteurisation and pan-frying. However, later studies suggested that conjugated linoleic acid content is increased by water loss rather than cooking per se and that the actual ratio of conjugated linoleic acid to total fat grams remains constant(3). The table below shows typical conjugated linoleic acid contents of various foods (expressed as mgs of conjugated linoleic acid per gram of total fat) and the percentage present as the c9, t11 isomer. Remember, though, that while some low-fat foods, such as yoghurt, have a favourable conjugated linoleic acid/total fat ratio, their low fat content means that the absolute amount of conjugated linoleic acid per portion will also be low.

Table 1: Conjugated linoleic acid content of common foods
FOOD Typical mgs of conjugated linoleic acid
per gram of total fat
% of conjugated linoleic acid present as
c9, t11 isomer
Lamb 5.6 92
Homogenised cow’s milk 5.5 92
Low-fat yoghurt 4.8 84
Butter 4.7 88
Cottage cheese 4.5 83
Fresh ground beef 4.3 85
Sharp cheddar cheese 3.6 93
Chicken 0.9 84
Pork 0.6 82

Typical daily intakes of conjugated linoleic acid from food sources are very hard to estimate. Not only do conjugated linoleic acid contents vary, even among different samples of the same food, but conjugated linoleic acid intake will also depend very much on the lamb, beef and dairy content of an individual’s diet. Estimates range from just 102mgs per day (4) up to 500-1,500mgs per day (5)!

Much of the interest in conjugated linoleic acid has centred on its apparent ability to affect body composition favourably by increasing lean muscle mass and reducing body fat. Most of the early studies on conjugated linoleic acid showed promising results; specifically, adding supplemental conjugated linoleic acid to the diet appeared to improve the ratio of lean body mass to body fat. Although the mechanisms of possible action were (and still are) very poorly understood, it seemed that extra conjugated linoleic acid facilitated the accumulation of more lean tissue and less fat during periods of growth.

In a study carried out at the University of Wisconsin in 1997, mice fed a regular diet and supplemented with conjugated linoleic acid during a six-week period gained 65-73% less body fat than unsupplemented controls (6). The dose used was 2.5mgs of conjugated linoleic acid per calorie of food intake, which translates to about 6g per day for someone on a 2,500-calorie diet. Although the animals in the treatment group reduced their voluntary food intake by 9-13%, they actually maintained a higher level of energy expenditure than the control mice.

In a second study carried out that same year, Pariza looked at the effect of conjugated linoleic acid on fat metabolism and found evidence that conjugated linoleic acid reduces fat deposition in fat cells while boosting fat breakdown in muscle cells (7).

 But despite the excitement surrounding these studies, and their substantiation by other studies, there was still a big fly in the ointment: virtually all the studies were carried out on animals, with very little data on humans available. The joke in the scientific community was that CLA was a great weight loss supplement – for mice!

In the intervening years, conjugated linoleic acid (CLA) research on humans has gathered pace, but the results of studies are much more mixed. In one major trial, researchers from Norway studied a group of healthy but overweight men and women, who were given 1.7g, 3.4g, 5.1g, or 6.8g of conjugated linoleic acid (CLA) per day (8). All groups showed significant reduction in body fat and increase in lean muscle, but this effect was most significant for the two latter groups. It was also observed that all groups showed a reduction in blood fat and cholesterol.

Subsequently, Swedish researchers studied 25 obese men aged 39-64 for four weeks and found that those taking 4.2g of conjugated linoleic acid (CLA) per day showed an average loss in waist circumference of 1.4cm (9).

However, a large number of more recent studies have failed to demonstrate significant benefits of supplementation with conjugated linoleic acid (CLA), particularly in respect of body fat reduction. For example, a study of experienced resistance trainers taking 6g per day of supplemental conjugated linoleic acid (CLA) carried out last year failed to find any significant beneficial changes in total body mass, fat-free mass, fat mass, % body fat, bone mass, strength, serum substrates or general markers of catabolism and immunity during training (10). Meanwhile, earlier this year, a three-month double-blind placebo-controlled study failed to find any improvement in body weight maintenance after a period of weight loss (11). Interestingly, however, conjugated linoleic acid (CLA) seemed to aid the preferential regain of fat-free mass at both experimental doses of 1.8 and 3.6g per day.

Animals benefit more than humans

Other studies in humans have also found somewhat more evidence for a gain in fat-free mass than for a straightforward loss in body fat. And this distinction may help, in part, to explain the discrepancy between animal and human studies, since many of the animal subjects used have much faster metabolisms than humans, with tissue growth and development forming a major part of the metabolic processes.

On the whole, then, while there is some promising evidence, the scientific jury’s still out over conjugated linoleic acid (CLA) and body composition.

Meanwhile, scientists are currently researching a number of other possible benefits of conjugated linoleic acid (CLA), including its antioxidant properties (which would make it a valuable weapon against both cancer and heart disease), its ability to improve insulin function (with positive implications for diabetes) and its role as an immune stimulant. So, even if more research on body composition and conjugated linoleic acid (CLA) is needed, isn’t there enough evidence of other benefits to justify using it anyway?

Unfortunately, not all the changes noted with conjugated linoleic acid (CLA) supplementation have been beneficial. Some of the animal studies have produced an increase in liver weight and increased insulin, suggesting a reduced sensitivity to insulin. More worryingly, this impairment of insulin sensitivity has also been observed in humans (12). Why does this matter to athletes? Well, a reduced sensitivity to insulin makes it harder for the body to absorb glucose and amino acids into muscle tissue (exactly what athletes don’t want) and is also associated with an increased risk of diabetes and heart disease.

Another problem centres on the t10, c12 isomer – the variety less prevalent in food. Recent animal studies appear to show that this is the isomer responsible for reducing body fat, and some commercially available conjugated linoleic acid (CLA) preparations have started to appear with increased proportions of this isomer. However, in a large human study on obese men last year, t10, c12 supplementation increased markers of both oxidative stress (suggesting increased oxidative damage to cell membranes) and inflammation (13). Inflammation is frequently a response to tissue injury, and some of the inflammatory markers measured in this study are highly correlated with an increased risk of heart disease.

Faced with such a mixed bag of evidence, it’s hard to make firm recommendations, but here goes:

  1. There is no point in trying to increase your dietary conjugated linoleic acid (CLA) because, even if all the theoretical benefits of conjugated linoleic acid (CLA) were confirmed, you’d have to consume at least 500g of (mainly saturated) fat each day to get meaningful amounts (3-plus grams). That’s over 4,500 fat calories each day – hardly conducive to fat loss! If you want to boost your conjugated linoleic acid (CLA) intake to the levels used in scientific studies, you will have to take supplements;
  2. Since the research indicates that conjugated linoleic acid (CLA) may be more effective at increasing lean muscle mass than simply reducing body fat, you would probably do better to use supplemental conjugated linoleic acid (CLA) in a strength or mass-building phase rather than continuously. Since there’s very little data on long-term supplementation with conjugated linoleic acid (CLA), this would probably be prudent anyway;
  3. In terms of the actual conjugated linoleic acid supplement you use, it’s probably wise to stick to those whose isomer content is similar to naturally occurring conjugated linoleic acid – ie consisting predominantly of c9, t11 conjugated linoleic acid. While the t10, c12 isomer may exert a much more potent fat-reducing effect, there are simply too many unanswered questions over its possible health risks to recommend using large amounts at present;
  4. Finally, because conjugated linoleic acid is known to share metabolic pathways with linoleic and alpha-linolenic acid (the essential fats or EFAs), if you supplement with conjugated linoleic acid it is worth ensuring that your dietary intake of these oils is up to scratch. Good sources of EFAs include seeds, such as sunflower and pumpkin, nuts, such as walnuts and hazelnuts, wheatgerm, cold-pressed seed oils and most unrefined whole grain cereals.


  1. Cancer Res, 43:2444-2446; 1983
  2. J Nutr 128:881-885, 1998
  3. J Agric Food Chem, 42(8):1757-1760, 1994
  4. J Am Col Nutr, 17(5):526 (abstract 118), 1998
  5. J Dairy Technol, 49:93-97, 1994
  6. FASEB Journal, 11(3): A599, 1997
  7. FASEB Journal, 11(3): A139, 1997
  8. J Nutr, 130:2943-8, 2000
  9. Int J Obes Relat Metab Disord, 2001 Aug; 25(8): 1129-1135
  10. J Strength Cond Res, 16(3): 325-34, 2002
  11. Int J Obes Relat Metab Disord, 27(7):840-7, 2003
  12. Lipids, 38(2): 133-7, 2003
  13. Circulation, 106(15): 1925-9, 2002
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