Conjugated Linoleic Acid weight loss

CLA

In by Raphikammer

Conjugated linoleic acid (CLA) consists of a group of positional

and geometric fatty acid (FA) isomers of linoleic

acid (C18:2; cis-9, cis-12 octadecadienoic acid).

CLA isomers are found naturally in ruminant meats and dairy

products due to biohydrogenation of linoleic or linolenic

acids in the rumen of these animals. Larger quantities of

CLA are chemically synthesized for use in dietary supplements

or fortified foods. Initially identified as a potential

anti carcinogen, CLA has been reported to prevent obesity,

diabetes, or atherosclerosis in different animal and

cell models, depending on the doses, isomers, and models

used. Potential mechanisms for preventing these diseases

include inducing cancer cell apoptosis, increasing

energy expenditure and delipidating adipocytes, increasing

insulin sensitivity, or reducing aortic lesions. However,

unequivocal evidence in human participants is

still lacking. Ironically, potential side effects of CLA

supplementation include chronic inflammation, insulin

resistance, and lipodystrophy. Long-term, well-controlled

clinical trials and more mechanistic studies are needed to

better understand the true potential health benefits versus

risks of consuming CLA isomers and their mechanisms

of action.

CHEMISTRY AND SYNTHESIS OF CLA

Natural Synthesis of CLA Isomers

CLA isomers are produced naturally in the rumen of ruminant

animals by fermentative bacteria Butyrivibrio fibrisolvens,

which isomerize linoleic acid into CLA isomers. A second pathway of CLA synthesis in ruminants

is in the mammary gland via -9-desaturase of trans-11, octadecanoic

acid (1). Thus, natural food sources of CLA are

dairy products including milk, cheese, butter, yogurt, and

ice cream and ruminant meats such as beef, veal, lamb, and

goat meat (2–4). The cis-9, trans-10 (9,11) isomer

(i.e., rumenic acid) is the predominating CLA isomer in

these products (∼80%), whereas the trans-10, cis-12 (10,12)

isomer represents approximately 10%. Although several

other isoforms of CLA have been identified, the 9,11 and

10,12 isomers appear to be the most biologically active

(5). Levels of CLA isomers in ruminant meats or milk can

be augmented by dietary manipulation, including feeding

cattle on fresh pasture (6) or by adding oils rich in linoleic

acid (e.g., safflower oil) or ingredients that alter biohydrogenation

of linoleic acid (e.g., ionophores) to their diet (7).

Structures of linoleic acid, cis-9, trans-11 CLA, and trans-10,

cis-12 CLA.

Chemical Synthesis of CLA Isomers

Because of the relatively low levels of CLA isomers in

naturally occurring foods that are high in fat content, the

chemical synthesis of CLA has been developed for making

supplements and for fortifying foods. CLA can be

synthesized from linoleic acid found in safflower or sunflower

oils under alkaline conditions, yielding a CLA mixture

containing approximately 40% of the 9,11 isomer and

44% of the 10,12 isomer (reviewed in Ref. 8). Commercial

preparations also contain approximately 4% to 10% trans-

9, trans-11 CLA and trans-10, trans-12 CLA, as well as trace

amounts of other isomers.

Conjugated Linoleic Acid

 CLA Content of Various Foods

Food mg/g fat Food mg/g fat

Meats/fish Dairy

  • Corned beef 6.6 Condensed milk 7.0
  • Lamb 5.8 Colby cheese 6.1
  • Fresh ground beef 4.3 Butter fat 6.1
  • Salami 4.2 Ricotta 5.6
  • Beef smoked sausage 3.8 Homogenized milk 5.5
  • Knackwurst 3.7 Cultured buttermilk 5.4
  • Smoked ham 2.9 American processed cheese 5.0
  • Veal 2.7 Mozzarella 4.9
  • Smoked turkey 2.4 Plain yogurt 4.8
  • Fresh ground turkey 2.6 Butter 4.7
  • Chicken 0.9 Sour cream 4.6
  • Pork 0.6 Cottage cheese 4.5
  • Egg yolk 0.6 Low fat yogurt 4.4
  • Salmon 0.3 2% milk 4.1
  • Vegetable oils Medium cheddar 4.1
  • Safflower oil 0.7 Ice cream 3.6
  • Sunflower oil 0.4 Parmesan 3.0
  • Peanut 0.2 Frozen yogurt 2.8
  • Sources: Based on values reported in Refs. 2–4; and the University of Wisconsin

Food Research Institute (Dr. Pariza, Director).

PHARMACOKINETICS AND EFFICACY OF CLA

Human and Animal Studies

As with other long chain unsaturated fatty acids (FA)s,

CLA is absorbed primarily in the small intestine, packaged

into chylomicrons, and distributed to extrahepatic tissues

having lipoprotein lipase (LPL) activity or returned to

the liver via chylomicron remnants or other lipoproteins.

The average daily intake of CLA is approximately 152 to

212 mg for nonvegetarian women and men, respectively

(9), and human serum levels range from 10 to 70 mol/L

after supplementation (10,11).

One major discrepancy between animal and human

studies is the dose of CLA administered (i.e., equal levels

of 9,11 and 10,12 isomers—referred to as a CLA mixture),

when expressed per unit body weight. For example,

most adult human studies provide 3 to 6 g/day of

a CLA mixture, whereas rodent studies provide 0.5% to

1.5% of a CLA mixture (w/w) in the diet. When expressed

per unit of body weight, humans receive approximately

0.05 g CLA/kg body weight, whereas mice received

1.07 g CLA/kg body weight, which is 20 times the human

dose based on body weight. Thus, part of the discrepancy

in results obtained from human and animal studies

is likely due to this large difference in the dose of CLA

administered. Supplementing humans with higher, or animals

with lower, doses of CLA would address this issue.

Other discrepancies in experimental designs include using

CLA isomer mixtures versus single isomers, duration

of CLA supplementation, and the age, weight, gender, and

metabolic status of the subjects or animals.

Cell Studies

In vitro studies have been conducted in a variety of cells

types, primarily using an equal mixture of 9,11 and 10,12

CLA, or each isomer individually. Doses used in cell

studies generally range between 1 to 100 M, reflecting

the concentration found in human participants following

supplementation. Results from these studies suggest

that these isomers are readily taken up by cells. For example,

we found that 10,12 CLA is readily incorporated

into neutral and phospholipid fractions of the primary

human adipocyte cultures and reduced lipid and glucose

metabolism (12). Similar to in vivo studies, 9,11 CLA acted

more like the linoleic acid controls.

ANTICANCER PROPERTIES OF CLA

CLA Reduces Tumor Growth

Pariza’s group initially discovered that CLA isomers in

fried ground beef acted as anticarcinogens (13). Subsequently,

numerous investigators have shown that CLA

mixtures or individual isomers decrease tumor cell growth

or increase cancer cell death in in vitro and in vivo models

of mammary, gastric, or skin cancer (reviewed in Ref. 14).

For example, feeding 0.8% to 1.0% individual CLA isomers

or mixtures block the initiation or progression of chemically

induced carcinogenesis in several rodent models

(15–17). A 5 M CLA mixture prevented cell growth and

cytokine production in transformed human keratinocyte like

cells (18). Proposed anticarcinogenic mechanisms

for CLA include decreasing nuclear factor (NF) B and

cyclooxygenase (COX) activity, thereby suppressing the

levels of prostaglandin (PG)E2, an inflammatory PG that

promotes the progression of certain forms of cancer and induces

human epidermal growth factor receptor 2 (HER2)

oncogene expression (19).

CLA Induces Apoptosis of Cancer Cells

Several groups have reported that CLA isomers cause

apoptosis or programmed cell death in cancer cells (reviewed

in Ref. 11). For example, 32 to 128 M CLA mixture

prevented rat mammary cancer cell growth through

apoptosis and decreased DNA synthesis in rat mammary

cancer cells (20). Moreover, 40 to 80 M 10,12 CLA induces

apoptosis in breast cancer cells (19,21,22). Proposed

proapoptotic mechanisms of CLA include inducing atypical

endoplasmic reticulum (ER) stress, leading to caspase-

12 activation (22).

In contrast to the cell and animal studies cited in

the preceding text, a recent prospective cohort study conducted

in Sweden found no evidence to support a protective

effect of CLA consumption on the development

of breast cancer in women (23). Furthermore, some studies

show that 10,12 CLA enhances the risk of developing

certain types of cancer (24). Thus, clinical studies examining

the effects of purified CLA isomers on preventing or

treating cancer, and safety issues, are needed.

ANTIOBESITY ACTIONS OF CLA

Due to the substantial rise in obesity over the past 30 years,

there is a great deal of interest in CLA as a weight loss

treatment, as it has been shown to decrease body weight

and body fat mass (BFM).

Conjugated Linoleic Acid weight loss

supplementation with a CLA mixture (i.e., 10,12 + 9,11 isomers in equal

concentrations) or the 10,12 isomer alone decreases BFM

in many animal and some human studies (reviewed in

Refs. 25 and 26). Of the two major isomers of CLA, the Martinez et al isomer is responsible for the antiobesity properties (27–31).

CLA Decreases Body Weight and Body Fat Mass

Park et al. (32) were one of the first groups to demonstrate

that CLA modulated body composition. Compared

with controls, male and female mice supplemented with a

0.5% (w/w) CLA mixture had 57% and 60% less BFM, respectively.

Since these findings, researchers have demonstrated

that CLA supplementation consistently reduces

BFM in mice, rats, and pigs.

For example, dietary supplementation with 1% (w/w) CLA mixture for 28 days

decreased body weight and periuterine white adipose tissue

(WAT) mass in C57BL/6J mice (36).

In humans, some studies show that CLA decreases

BFM and increases lean body mass (LBM), whereas others

show no such effects. For example, supplementation of 3

to 4 g/day of a CLA mixture for 24 weeks decreased BFM

and increased LBM in overweight and obese people (37).

On the other hand, supplementation of 3.76 g/day of a

CLA mixture in yogurt for 14 weeks in healthy adults had

no effect on body composition (38). Supplementation with

3.2 g/day of aCLAmixture decreased totalBFMand trunk

fat compared with placebo in overweight participants, but

not obese participants (39). These contradictory findings

among human studies may be due to the following differences

in experimental design: (i) mixed versus individual

CLA isomers, (ii) CLA dose and duration of treatment,

and (iii) gender, weight, age and metabolic status of the

participants.

These antiobesity effects of CLA do not appear to

be solely due to reductions in food intake in animals (40–

42) or humans (43,44). Several mechanisms by which CLA

decreases BFM will now be examined.

CLA Increases LBM

A recent meta-analysis of 18 human, placebo-controlled

CLA studies found that consuming a CLA mixture increased

fat-free mass (FFM) by 0.3 kg, regardless of the

duration or dose (45). When these same 18 studies were

examined for reductions in BFM, it was shown that CLA

supplementation decreased BFM by 0.05 kg/week for up

to one year (25). The average CLA mixture dose for these

studies was 3.2 g/day. Collectively, these meta-analyses

studies suggest that CLA supplementation of humans results

in a rather small but rapid increase in FFM or LBM,

and a much larger decrease in BFM over an extended period

of time. The effects of CLA on FFM orLBMin humans

mayvary depending on baseline body mass index, gender,

age, and exercise status of the participants.

Two proposed mechanisms by which CLA increases

LBM are via increasing bone or muscle mass. 10,12 CLA

supplementation for 10 weeks with a 0.5% (w/w) CLA

mixture increased bone mineral density (BMD) and muscle

mass in C57BL/6 female mice (46). CLA supplementation

has been proposed to increase BMD via increasing

osteogenic gene expression and decreasing osteoclast activity

(46,47). Furthermore, CLA supplementation alone

or with exercise increased BMD compared with control

mice (48). An alternative mechanism could be that CLA

decreases adipogenesis of pluripotent mesenchymal stem

cells (MSC) in bone marrow, and instead promotes their

commitment to become bone cells. Indeed, 10,12 CLA has

been shown to decrease the differentiation of MSC into

adipocytes and increase calcium deposition and markers

of osteoblasts (49). In contrast, 9,11 CLA increased

adipocyte differentiation and decreased osteoblast differentiation.

Consistent with these in vitro data, CLA mixture

supplementation of rats treated with corticosteroids prevented

reductions in LBM, BMD, and bone mineral content

(50). Increasing LBM is directly linked to an increase

in basal metabolic rate (BMR).

In addition to its effects on BMD, recent evidence

supports a role of CLA in increasing endurance and muscle

strength. For example, maximum swimming time until

fatigue was higher in CLA fed versus control mice

(51). Aging mice supplemented with a CLA mixture and

10,12 CLA had higher muscle weight compared with

9,11 CLA and corn oil controls (52). In addition, CLA

isomers increased levels of antioxidant enzyme activity,

ATP, and enhanced mitochondrial potential, indicating a

protective effect against age-associated muscle loss (52).

In humans, CLA increased bench-press strength in men

supplemented with 5 g/day for seven weeks who underwent

resistance training three days per week (53).

Furthermore, supplementation with CLA combined with

creatine monohydrate (C) and whey protein (P) led to

greater increases in bench-press and leg-press strength

than supplementation with C+P or P alone (54). Although

preliminary, these data suggest that CLA may enhance

exercise-induced muscle strength or prevent sarcopenia

or age-related muscle loss.

CLA Increases Energy Expenditure

CLA has been proposed to reduce adiposity by elevating

energy expenditure via increasing BMR, thermogenesis,

or lipid oxidation in animals (27,42,55). In BALB/c male

mice fed mixed isomers of CLA for six weeks, body fat

was decreased by 50% and was accompanied by increased

BMR compared with controls (42). Enhanced thermogenesis

may be associated with increased uncoupling of mitochondria

via uncoupling protein (UCP)s, which facilitate

proton transport over the inner mitochondrial membrane

thereby leading to dissipation of energy as heat instead

of ATP synthesis. UCP1 is highly expressed in brown adipose

tissue (BAT), and in WAT at lower levels. UCP3 is

expressed in muscle and in a number of other tissues,

whereas UCP2 is the form expressed at the highest level

across most tissues. Supplementation with a CLA mixture

or 10,12 CLA in rodents induced UCP2 mRNA expression

in WAT (29,56). Recently, it was demonstrated that CLA

increased mRNA and protein expression of UCP1 inWAT

(57). Similarly, CLA supplementation induced UCP gene

expression and elevated -oxidation in muscle and liver

(58–62).

CLA Increases Fat Oxidation

CLA has been shown to regulate the gene expression

or activity of proteins associated with FA oxidation in

adipose tissue, muscle, and liver. For example, CLA induced

the expression of carnitine palmitoyl transferase 1

(CPT1) in WAT of obese Zucker fa/fa rats (63). Additionally,

10,12 CLA increased the expression of peroxisome

proliferator-activated receptor (PPAR) coactivator-1

Conjugated Linoleic Acid 169

(PGC1) in WAT of mice (57). Consistent with these

in vivo findings, 10,12 CLA increased -oxidation in differentiating

3T3-L1 preadipocytes (64). Furthermore, 10,12

CLA treatment increased AMP kinase (AMPK) activity

and increased phospho-acetyl-CoA carboxylase (ACC)

levels in 3T3-L1 adipocytes, suggesting an increase in FA

oxidation and a decrease in FAesterification to triglyceride

(TG) (65).

In muscle, 10,12 CLA increased CPT1 expression in

hamsters fed an atherogenic diet (60). Supplementation of

a CLA mixture in high fat fed hamsters led to increased

CPT1 activity in muscle (66). A CLA mixture increased

CPT1b, UCP3, acetyl-CoA oxidase (ACO) 2, and PPAR

mRNA levels in skeletal muscle of Zucker rats (67). Consistent

with these data, 10,12 CLA increased mRNA levels

(63) and activity (68) of CPT1 in the liver. Additionally,

10,12 CLA increased hepatic peroxisomal fatty COactivity

(68), suggesting increased peroxisomal -oxidation in

addition to mitochondrial oxidation. These findings suggest

CLA may reduce adiposity through increased energy

expenditure via increased mitochondrial uncoupling and

FA oxidation in WAT, muscle, and liver.

At least one report demonstrates that CLA increases

FA oxidation in human participants (69). In this study,

overweight adults supplemented with 4 g/day of a CLA

mixture for six months had a lower respiratory quotient

(RQ), indicating an increase in FA oxidation compared

with placebo controls. However, others have shown no

effect of CLA on energy expenditure or fat oxidation in

humans (70,71). These discrepancies may be due to the

length of treatment, time period of measurement, and

time at which measurements are taken. For instance, CLA

treatment for four to eight weeks had no effect on energy

expenditure or FA oxidation, based on a 20-minute measurement

during resting and walking (70). In contrast, the

study by Close et al. (69) administered CLA for six months

and measured FA oxidation over a 24-hour period and

found that CLA increased FA oxidation and energy expenditure.

Thus, discrepancies in this area may be due to

insufficient duration of CLA treatment or measurements

of energy expenditure or FA oxidation.

CLA Decreases Adipocyte Size

Lipolysis is the process by which stored TG is mobilized,

releasing free fatty acids (FFAs) and glycerol for use by

metabolically active tissues. C57BL/6J mice fed 10,12 CLA

for three days had increased mRNA levels of hormone sensitive

lipase (HSL), a key enzyme for TG hydrolysis

(56). Consistent with these data, acute treatment withCLA

mixture or 10,12 CLA alone increased lipolysis in 3T3-L1

(32,72) and newly differentiated human adipocytes (73).

In vitro, a CLA mixture and to a greater extent 10,12 CLA

decreased TG content, adipocyte size, and lipid locule size

in adipocytes (74). Similarly, mice fed 1% CLA displayed

increased numbers of small adipocytes with a reduction in

the number of large adipocytes (75). Furthermore, a CLA

mixture reduced adipocyte size rather than cell number

in Sprague Dawley (40) and fa/fa Zucker rats (76). Thus,

CLA may reduce adipocyte size by increasing lipolysis.

CLA Decreases Adipocyte Differentiation

The conversion of preadipocytes to adipocytes involves

the activation of key transcription factors such as

PPAR and CAAT/enhancer-binding proteins (C/EBPs).

There is much evidence showing that CLA suppresses

preadipocyte differentiation in animal (77–79) and human

(12,80) preadipocytes treated with a CLA mixture or 10,12

CLA alone. 10,12 CLA treatment has been reported to decrease

the expression of PPAR, C/EBP, sterol regulatory

element-binding protein-1c (SREBP-1c), liver X receptor

(LXR), and adipocyte FA-binding protein (aP2),

thereby reducing adipogenesis and lipogenesis (12,29,79).

In rodents, supplementation of 10,12 CLA decreased

the expression of PPAR and its target genes (79,81–83).

In contrast, humans supplemented with a CLA mixture

had higher mRNA levels of PPAR in WAT, but no difference

in body weight or BFM (38). In mature, in vitrodifferentiated

primary human adipocytes or in mature

3T3-L1 adipocytes, 10,12 CLA treatment leads to a substantial

decrease in the expression and activity of PPAR

(82,83), and a decrease in PPAR target genes and lipid

content (80). This shows that 10,12 is not only able to inhibit,

but also to reverse the adipogenic process and indicates

that this may be mediated by suppression of PPAR

activity. In addition to its effect on PPAR, 10,12 CLA may

also directly impact the activity of other transcription factors

involved in adipogenesis and lipogenesis (i.e., LXR,

C/EBPs, SREBP-1c), which could contribute to CLA’s antiobesity

actions.

CLA Decreases Glucose and FA Uptake and TG Synthesis

Conversion of glucose and FAs to TG is a major function

of adipocytes. Genes involved in lipogenesis, such

as a LPL, ACC, fatty acid synthase (FAS), and stearoyl-

CoA desaturase (SCD), were decreased following supplementation

with mixed isomers of CLA or 10,12 CLA

alone (12,56,72,80). PPAR is a major activator of many

lipogenic genes including glycerol-3-phosphate dehydrogenase

(GPDH), LPL, and lipin as well as many genes encoding

lipid droplet-associated proteins, such as perilipin,

adipocyte differentiation-related protein (ADRP), and cell

death–inducing DNA fragmentation factor of apoptosislike

effector c (CIDEC) (84). Thus, the antilipogenic action

of 10,12 CLA may be explained by inhibition of PPAR activity.

In addition,CLArepression of expression of SREBP-

1 and its target genes may play an important role in delipidation.

Finally, CLA suppression of insulin signaling may

also impair insulin’s ability to activate or increase the

abundance of a number of lipogenic proteins including

LPL, ACC, FAS, SCD-1, and the insulin-dependent glucose

transporter GLUT4.

CLA Decreases Adipocyte Number

Apoptosis is another mechanism by which CLA may reduce

BFM. Apoptosis can occur through activation of the

death receptor pathway, ER stress, or the mitochondrial

pathway. A number of in vivo and in vitro studies have

reported apoptosis in adipocytes supplemented with a

CLA mixture or 10,12 CLA alone (56,64,85,86). For example,

supplementation of C57BL/6J mice with 1% (w/w)

CLA mixture reduced BFM and increased apoptosis in

WAT (75). Mice fed a high-fat diet containing 1.5% (w/w)

CLA mixture had an increased ratio of BAX, an inducer of

apoptosis relative to Bcl2, a suppressor of apoptosis (87).

170 Martinez et al.

Figure 2 Reported mechanisms by which 10,12 CLA decreases adipose

tissue mass and obesity.

Reported mechanisms by which CLA reduces adiposity

are shown in Figure 2.

ANTIDIABETIC PROPERTIES OF CLA

Feeding obese ob/ob C57BL/6 mice 0.6% 9,11 CLA for

six weeks improved plasma levels of glucose, TG, and

insulin and reduced the expression of markers of inflammation

and insulin resistance in WAT (88). Furthermore,

these authors demonstrated that 50 M 9,11 CLA prevented

tumor necrosis factor (TNF)-mediated insulin resistance

in 3T3-L1 murine adipocytes. Their data suggest

that 9,11 CLA improves insulin sensitivity by elevating

GLUT4 levels or translocation to the plasma membrane,

which are adversely affected by inflammation, thereby

facilitating glucose disposal. Similarly, Wistar rats fed a

high-fat diet supplemented with a 0.75% to 3.0% CLA

mixture for 12 weeks had lower plasma levels of glucose,

TG, and insulin compared with high-fat fed control rats

(89). The CLA mixture enhanced the expression of PPAR

target genes in WAT, which was proposed to be responsible

for the improvement in insulin sensitivity. Consistent

with these data, adiponectin, a WAT-specific, PPAR target

gene that reduces blood glucose by enhancing its oxidation

in liver and muscle, was increased in the plasma

of Zucker diabetic fatty (ZDF) rats fed a 1% CLA mixture

for eight weeks (55). Similarly, feeding 0.5% 9,11 CLA to

insulin resistant C57BL/6J mice improved insulin sensitivity

without affecting BFM (90). Conversely, these authors

found that feeding 0.5% 10,12 CLA lowered BFM

and increased LBM in these mice, but caused insulin resistance.

Other studies have also reported that 10,12 CLA

causes insulin resistance, especially in mice (81,99). Taken

together, these data suggest that 9,11 and 10,12 CLA have

opposite effects on insulin sensitivity, most likely due

to their opposing effects on the activity of PPAR, visa-

vis 9,11 CLA activates PPAR and 10,12 CLA inhibits

PPAR.

ANTIATHEROSCLEROTIC ACTIONS OF CLA

CLA has been reported to decrease risk factors of

atherosclerosis in several important animal models (reviewed

in Ref. 91). For example, feeding 0.5% mixed or

individual isomers of CLA to New Zealand White rabbits

fed a high saturated fat and cholesterol-rich diet reduced

blood lipids and atherosclerotic lesion area (92). Syrian

Golden hamsters fed a high saturated fat and cholesterolrich

diet containing 1.0% mixed CLA isomers (93), 0.9%

9,11 CLA (94) or 1.0% 10,12 CLA (95), had decreased aortic

lipid accumulation or fewer fatty aortic streaks compared

with controls. In apoE−/− deficient mice, feeding a 1.0%

CLA mixture decreased aortic lesion area, and reduced

macrophage infiltration and inflammatory gene expression

in the lesions (96). In contrast to these animal studies,

other animal and clinical trials with CLA mixtures have

yet to show beneficial effects on reducing risk factors for

atherosclerosis (reviewed in Ref. 97).

SAFETY

Adverse side effects have been reported for CLA supplementation

such as elevated levels of inflammatory

markers, lipodystrophy, steatosis, and insulin resistance.

Most adverse side effects are due to the 10,12 CLA

isomer.

CLA Increases Markers of Inflammation

Treatment with 10,12 CLA increases the expression or

secretion of inflammatory makers such as TNF, interleukin

(IL)-1, IL-6, and IL-8 from adipocyte cultures

(56,73,80,81,83). Moreover, CLA increases the expression

of COX-2, an enzyme involved in the synthesis of PGs,

and the secretion of PGF2 (79,98). These inflammatory

proteins are known to antagonize PPAR activity and insulin

sensitivity (87,98–100).

Consistent with these in vitro data, 10,12 CLA

supplementation increases the levels of inflammatory

cytokines and PGs in humans (101,102). For example,

women supplemented with 5.5 g/day of a CLA mixture

for 16 weeks had higher levels of C-reactive protein

in serum and 8-iso-PGF2 in urine (44). 10,12 CLA

supplementation in mice resulted in macrophage recruitment

in WAT (81). In contrast, 9,11 CLA exhibits antiinflammatory

actions (6).

CLA Causes Insulin Resistance

Insulin resistance has been reported in vivo (56,102–104)

and in vitro (12,73,79,98) following supplementation with

a CLA mixture or 10,12 CLA alone. For example, 10,12

CLA supplementation of 3.4 g/day for 12 weeks in obese

men with metabolic syndrome increased serum glucose

and insulin levels and decreased insulin sensitivity (103).

Supplementation with a CLA mixture in type-2 diabetics

increased fasting plasma glucose levels and reduced

insulin sensitivity (102). Mice fed 1% (w/w) 10,12 CLA

displayed elevated fasted and feeding plasma insulin

levels and had reduced insulin sensitivity (75). Consistent

with these data, the mRNA levels of adiponectin,

a key adipokine associated with insulin sensitivity, decrease

following supplementation with 10,12 CLA in vivo

(36,81,100) and in vitro (79,82,105,106).

CLA Causes Lipodystrophy

The combination of inflammation and insulin resistance

results in reduced FA and glucose uptake in WAT,

leading to ectopic lipid accumulation in the blood (hyperlipidemia),

liver (steatosis), or muscle. CLA-mediated

hyperlipidemia and steatosis has been reported in several

animal studies (36,76,107). For example, 1% (w/w)

CLA time-dependently increased insulin levels and led

Conjugated Linoleic Acid 171

Figure 3 Reported mechanisms by which CLA reduces the risk of cancer,

obesity, diabetes, and atherosclerosis.

to increased liver weight and liver lipid accumulation in

C57BL/6J mice (36). Aging C57BL/6J mice fed 0.5% 10,12

CLA displayed increased insulin resistance and liver hypertrophy

(107).

US Regulatory Status

Recently, the FDA approved CLA as GRAS (generally recognized

as safe) for use in foods and beverages (not to

exceed 1.5 g/serving) due its potential favorable effects.

However, the use ofCLAas a dietary supplement or ingredient

should be cautioned based on the aforementioned

safety issues.

CONCLUSIONS

There is an abundance of evidence in animals suggesting

that CLA consumption may reduce the incidence or risk

of developing cancer, obesity, diabetes, or atherosclerosis,

depending on the type and abundance of CLAisomer consumed

and the physiological status of the animal model

(Fig. 3). Data on the antiobesity properties of 10,12 CLA

in animals, especially mice, are the most reproducible.

However, these potential benefits are not without risks,

as the 10,12 isomer is associated with increased levels of

inflammatory markers, lipodystrophy, and insulin resistance.

More clinical studies are needed to determine the

efficacy of CLA isomers in humans, and more mechanistic

animal and cell studies are needed to determine the precise,

isomer-specific mechanisms of action of CLA, and

potential side effects.

ACKNOWLEDGMENTS

This work was supported by NIH NIDDK R15 DK 059289,

NIH NIDDK/ODS R01DK063070, USDA-NRI 199903513,

and NCARS 06771 awards to Michael McIntosh, NRSA

NIH Fellowships to Kristina Martinez (F31DK084812)

and Arion Kennedy (F31DK076208), and a United Negro

College Fund-Merck predoctoral Fellowship to Arion

Kennedy.

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