Conjugated linoleic acid (CLA) is a slightly altered form of the essential fatty acid
linoleic acid.
Where is it found?
CLA is found mainly in dairy products and
also in beef and poultry,
eggs, and corn oil. Bacteria that live in
the intestine of humans can produce CLA from linoleic acid, but supplementation of a rich
source of linoleic acid did not produce increases in blood levels of CLA in one human
study.1 CLA is available as a supplement.
Why do dieters use it?*
Some dieters say that CLA
helps decrease appetite.
What do the advocates say?*
Research suggests that conjugated linoleic acid (CLA) may help to reduce body fat and
increase muscle. The research supporting CLA’s ability to help reduce body fat is good,
but more is needed. There are at least seven human studies (two are double-blind and the
others are controlled) showing significant reduction of abdominal obesity and body fat mass in
overweight and moderately obese people. However, since most of the studies involved a small
number of participants and were short in duration, larger double-blind studies are needed to
further document the benefits and mechanisms of action.
Although CLA promotes weight loss, which is good for heart health, it is important to
moniter cholesterol levels as it may reduce HDL (“good”) cholesterol.
How much is usually taken by dieters?
A double-blind trial found that exercising individuals taking 1,800 mg per day of CLA lost
more body fat after 12 weeks than a similar group taking a placebo.2 However, two
other studies found that amounts of CLA from 0.7 to 3.0 grams per day did not affect body
composition.34 Most double-blind trials have found that larger amounts
of CLA, 3.4 to 4.2 grams per day, does reduce body fat;567
however, one double-blind study of experienced strength-training athletes reported no effect
of 6 grams per day of CLA on body fat, muscle mass, or strength improvement.8
Are there any side effects or interactions?
Overweight volunteers who took 4.5 grams of
CLA per day for one year had an increase in their blood levels of lipoprotein(a), a risk
factor for heart disease.9 In a double-blind study of human volunteers,
supplementation with 4.2 grams per day of a mixture of cis-9,trans-11 CLA and trans-10,cis-12
CLA for three months increased the concentration of C-reactive protein, another risk factor
for heart disease.10 In a study of healthy volunteers, supplementing with 4.5 grams
of CLA per day for 12 weeks caused an impairment of blood vessel function (endothelial
dysfunction), which is believed to be associated with an increased risk of heart
disease.11 Taken together, these findings suggest that long-term use of CLA could
increase the risk of developing heart
disease.
In a double-blind study of people with type 2
diabetes, supplementing with 3 grams of CLA per day for eight weeks significantly
increased blood glucose levels by 6.3% and decreased insulin sensitivity.12 A
reduction in insulin sensitivity was also seen in a study of overweight men without diabetes
after treatment with 3 grams of CLA per day for three months.13 However, in another
study of obese men and women, supplementation with 6 grams of CLA per day for 24 weeks had no
significant effect on blood glucose levels or insulin sensitivity.14 Moreover, in a
study of young sedentary men, 4 grams of CLA per day for eight weeks improved insulin
sensitivity.15 Although the studies are conflicting, it would be prudent for people
who have, or are at risk of developing, diabetes to monitor their blood sugar levels during
long-term use of CLA. One unpublished human trial reported isolated cases of gastrointestinal
upset.16
At the time of writing, there were no well-known drug interactions
with conjugated linoleic acid.
*Dieters and weight-management advocates may claim benefits for CLA
based on their personal or professional experience. These are individual opinions and
testimonials that may or may not be supported by controlled clinical studies or published
scientific articles on CLA. For more complete and detailed information, including references
and safety information, see CLA as a nutritional
supplement.
References (To view, roll mouse over the "References" heading; to hide, click on the heading)
1. Herbel BK, McGuire MK, McGuire MA, et al. Safflower oil consumption
does not increase plasma conjugated linoleic acid concentrations in humans. Am J Clin
Nutr 1998;67:332–7.
2. Thom E, Wadstein J, Gudmundsen O. Conjugated linoleic acid reduces
body fat in healthy exercising humans. J Int Med Res 2001;29:392–6.
3. Mougios V, Matsakas A, Petridou A, et al. Effect of supplementation
with conjugated linoleic acid on human serum lipids and body fat. J Nutr Biochem
2001;12:585–94.
4. Zambell KL, Keim NL, Van Loan MD, et al. Conjugated linoleic acid
supplementation in humans: effects on body composition and energy expenditure. Lipids
2000;35:777–82.
5. Riserus U, Berglund L, Vessby B. Conjugated linoleic acid (CLA)
reduced abdominal adipose tissue in obese middle-aged men with signs of the metabolic
syndrome: a randomised controlled trial. Int J Obes Relat Metab Disord
2001;25:1129–35.
6. Smedman A, Vessby B. Conjugated linoleic acid supplementation in
humans—metabolic effects. Lipids 2001;36:773–81.
7. Blankson H, Stakkestad JA, Fagertun H, et al. Conjugated linoleic acid
reduces body fat mass in overweight and obese humans. J Nutr
2000;130:2943–8.
8. Kreider RB, Ferreira MP, Greenwood M, et al. Effects of conjugated
linoleic acid supplementation during resistance training on body composition, bone density,
strength, and selected hematological markers. J Strength Cond Res
2002;16:325–34.
9. Gaullier JM, Halse J, Hoye K, et al. Conjugated linoleic acid
supplementation for 1 y reduces body fat mass in healthy overweight humans. Am J Clin
Nutr 2004;79:1118–25.
10. Smedman A, Basu S, Jovinge S, Fredrikson GN, Vessby B. Conjugated
linoleic acid increased C-reactive protein in human subjects. Br J Nutr
2005;94:791–5.
11. Taylor JS, Williams SR, Rhys R, et al. Conjugated linoleic acid
impairs endothelial function. Arterioscler Thromb Vasc Biol 2006;26:307–12.
12. Moloney F, Yeow TP, Mullen A, et al. Conjugated linoleic acid
supplementation, insulin sensitivity, and lipoprotein metabolism in patients with type 2
diabetes mellitus. Am J Clin Nutr 2004;80:887-95.
13. Riserus U, Vessby B, Arnlov J, Basu S. Effects of cis-9,trans-11
conjugated linoleic acid supplementation on insulin sensitivity, lipid peroxidation, and
proinflammatory markers in obese men. Am J Clin Nutr 2004;80:279–83.
14. Whigham LD, O'Shea M, Mohede IC, et al. Safety profile of conjugated
linoleic acid in a 12-month trial in obese humans. Food Chem Toxicol 2004
Oct;42(10):1701–9.
16. Thom E. A pilot study with the aim of studying the efficacy and
tolerability of Tonalin CLA on the body composition in humans. Lillestrom, Norway: Medstat
Research Ltd., July 1997 [unpublished].
The information presented in Healthnotes is for informational purposes
only. It is based on scientific studies (human, animal, or in vitro), clinical
experience, or traditional usage as cited in each article. The results reported may not
necessarily occur in all individuals. For many of the conditions discussed, treatment with
prescription or over the counter medication is also available. Consult your doctor,
practitioner, and/or pharmacist for any health problem and before using any supplements or
before making any changes in prescribed medications. Information expires June 2009.
