Functional Fats for Weight Control: Conjugated Linoleic Acid, Diacylglycerols, and Medium-Chain Triglycerides – International Journal of Naturopathic Medicine

Abstract

Numerous functional foods have been examined for their potential as weight-loss agents. To evaluate the future of functional foods, the AECES model has been developed to verify the following: Acceptability, Ease of formatting, Cost-effectiveness, Efficacy and Safety. The goal of this review is to assess three oil-based weightloss functional foods, including: conjugated linoleic acid (CLA), diacylglycerols (DAG) and medium-chain triglycerides (MCT), in terms of the AECES model for consumer acceptability. First, CLA is an overall poor AECES model due to its weaknesses in the efficacy and safety aspects since most of the evidence of CLA is based on animal studies. Secondly, oils rich in DAG, specifically the 1,3-isoform, have an appropriate AECES model. Although, the efficacy still needs more research to determine the exact mechanisms of action for DAG-rich oils. Thirdly, MCT oils exhibit a good AECES model; nevertheless, the long-term efficacy of MCT needs to be further explored. The capability of these three functional oils as effective anti-obesity agents is substantial, yet further investigation should be conducted to determine the missing gaps in research and to accomplish satisfactory AECES model for market acceptance.

Obesity is at the forefront of global health issues as it directly contributes to many chronic illnesses. Excess weight is the result of an imbalance between energy intake (EI) and energy expenditure (EE), by which surplus EI is stored as triacylglycerol (TG) in adipose tissue. Overweight and obese consumers often turn to natural health products to help support and maintain their weightloss program. Although the weight management industry is large, most of the weight-loss supplements on the market have not been scientifically proven to be effective.(1) Recently, several natural health products have shown promise in the treatment of obesity, some of which are oil rich in conjugated linoleic acid (CLA), diacylglycerols (DAG) and mediumchain triglycerides (MCT).

The AECES model has been developed by experts in the nutrition field to determine the future of functional foods in the marketplace. Five criteria can be used to evaluate the potential of functional foods and nutraceuticals, including: acceptability, ease of formatting, cost-effectiveness, efficacy and safety.(2) This is known as the AECES model (Figure 1). A “good” AECES model includes the following characteristics (Table 1): easily incorporated into a suitable matrix and diet, acceptable cost to manufacturer and consumer, ability to produce a desired effect and lack of major side effects. All the components of the AECES model are closely interrelated sharing the same final goal: consumer acceptability of the functional food. The range of “appropriate” to “poor” in the AECES model would be assigned to functional foods that either lack data or provide some negative research results in one or more of the model criteria, which would lead to decreased consumer acceptance of the functional food. The purpose of this review is to examine the role of functional foods in health promotion, in relation to body weight and circulating lipid levels, such as oils rich in CLA, DAG and MCT oils. Particularly, this review is intended to evaluate these functional oils in terms of the AECES model for consumer suitability.

CONJUGATED LINOLEIC ACID (CLA)

Conjugated linoleic acid (CLA) is a collective term for a group of positional and geometrical conjugated dienoic isomers of linoleic acid that are found in dairy products and meat.3 The cis-9, trans-11 CLA is the principal dietary CLA form, but lower levels of the other isomers (trans-10, cis-12 CLA, trans-9, trans-11 CLA, and trans-10, trans-12-CLA) are present in food CLA sources. Naturally, CLA is produced in the rumen of ruminant animals by the fermentative bacteria that isomerize linoleic acid into CLA.4

Mechanisms of action of CLA include: enhanced thermogenesis, increased satiety, augmented fat oxidation, reduced fat cell size as well as fat deposition, increased apoptosis of adipocytes and altered preadipocyte differentiation.(5-6) Potentially, the combination or additive effects of all these mechanisms of action of CLA may lead to changes in weight and body fat, as no single mechanism fully explains CLA action.

Studies have shown that CLA, specifically the trans-10, cis-12 isomer, can reduce body weight and fat mass. Most animal studies associated with feeding CLA have shown that CLA lowers body fat and energy retention as well as increases energy expenditure, thereby decreasing weight(7-10); yet, others have shown no effects on weight.(11-14) This may be due to the dose or the CLA isomers used in animal studies. Results demonstrate that body weight and/or fat mass of animals were not affected when they were supplemented with low amounts of CLA mixture (0.5% in the diet), which contained about equal amounts of the trans-10, cis-12 isomer and cis-9, trans-11 isomer .11,13 Yet, weight gain was similar to control when high amounts of CLA mixture with mostly the cis-9, trans-11 isomer were given.(7,9) However, most human studies have not been able to replicate the magnitude of weight lost.(15-28) Only a few human studies suggest that CLA supplementation has reduced body fat(15-17,19,20,22) and other studies did not show any effect.(29-32) The variety of species used in studies may also explain the discrepancy of results obtained.

In animals, CLA supplements appear to have some undesirable side effects such as induced insulin resistance as well as fatty liver and spleen.(33-36) These animal studies also demonstrate that CLA may have detrimental effects on plasma lipids. Human studies also show evidence that CLA may adversely influence health, in particular insulin sensitivity and blood lipids, but the results are conflicting.(16,17,21,23,37) CLA is widely available in capsule form that improves its oxidative stability,38 therefore having an appropriate matrix, cost, and sensory quality for consumers. However, the efficacy of CLA is questionable because the animal evidence is more convincing than the human data. The lack of clarity on the mechanism of action can explain the inconsistencies in the research results. In addition, human studies should be carried out to determine the long-term effects of CLA and whether any adverse outcomes occur. In summary, the data available from literature demonstrates a poor AECES model. More research is needed to investigate the efficacy and the safety aspects before CLA will have optimal consumer acceptance.

DIACYLGLYCEROLS (DAG)

All cooking oils naturally contain small quantities of diacylglycerols (DAG), ranging from 0.8% in rapeseed oil to 9.5% in cottonseed oil.39 In addition, DAG is produced in the digestive tract as a metabolic intermediate, as 1,2-diacyl-sn-glycerol (1,2-DAG) or 2,3-diacyl-sn-glycerol (2,3-DAG), after the ingestion of TG.40 In recent years manufacturers have developed an enzymatic process to produce 1,3-diacyl-sn-glycerol (1,3-DAG) by migration of the acyl group with the reverse reaction of the 1,3-specific lipase. DAG oil can be easily incorporated into food products since it is similar in taste, appearance, and fatty acid composition to other oils.(41)

It is the specific structural differences of DAG isomers and not the fatty acid composition of DAG or TG that appear to explain the different action on lipid metabolism and body weight.(42) The main end products of lipase action on 1,3-DAG are glycerol and free fatty acids, which may be less readily re-synthesized to chylomicron TG. Moreover, larger amounts of fatty acids from digested DAG may be released into the portal circulation rather than being incorporated into chylomicrons, compared with TG oils.(43) In addition to producing lower TG content of chylomicrons, lower serum TG levels in a fasted state and in the postprandial state occur after DAG ingestion.(44-51) This hepatic exposure to fatty acids by increasing DAG intake may lead to greater fat oxidation by the liver than following TG intake.(52-58) Enhanced fat oxidation may lead to increased satiety.(59) Thus, decreasing caloric intake may induce a decrease in weight and fat loss in long-term DAG feedings.

While certain studies indicate that 1,3-DAG has a positive outcome in animal(44-46,54-56,60) and human trials, (47- 51,60-62) other studies show no effect on body weight63-67 or TG levels.(57,63-64,68) This lack of effect may be due to insufficient doses used (10% in the diet) or the heterogeneity across subjects used including overweight or obese versus normal weight individuals. Overweight and obese subjects could have defective fat oxidation; thus, higher fat oxidation may produce greater weight loss. Although the use of DAG oils for weight control is promising, much remains to be clarified regarding the mechanism of dietary DAG.

DAG oil studies do not indicate any severe adverse health effects related to its consumption.(66,70-71) However, it still remains to be seen how DAG oil intake will affect humans on a long-term basis as well as synergistically with other nutrients.(72)

Overall, DAG oils are easily incorporated into foods without affecting palatability, but have slightly higher costs than conventional oils.(41) The AECES model for DAG shows it being a generally appropriate functional food for weight control; however, DAG oil has not yet been a huge success with consumer acceptance due to conflicting studies on the efficacy of the product. Overall, DAG oil demonstrates potential as a weight loss agent, but future research is needed to elucidate mechanisms responsible for its action on weight loss.

MEDIUM-CHAIN TRIGLYCERIDES (MCT)

Medium-chain triglycerides (MCT) have unique characteristics relating to energy density and metabolism giving them advantages over more common long-chain triglycerides (LCT).(73) Human consumption of MCT oils is low since naturally occurring sources of MCTs are rare; however, those sources include milk fat, palm kernel oil and coconut oil.

MCTs are less energy dense and highly ketogenic compared to LCTs. First, the energy density of MCTs is less than that of LCTs due to their shorter chain length. MCTs provide about ten percent fewer calories than LCTs; 8.3 Cal per gram for MCTs versus 9 Cal per gram for LCTs.(74) MCTs also differ from LCTs in their metabolic pathway because they are easily oxidized and utilized as energy, with little tendency to deposit as body fat. (73) Consequently, the intake of MCTs can decrease caloric intake and potentially decrease body weight and body fat in the long term.

The literature supports that MCT oil increases energy expenditure and decreases body fat in the majority of studies in both animals(75-81) and humans.(82-89) In addition, MCTs may have a greater effect in overweight subjects as opposed to normal weight or obese subjects.(85,90) Overall, short-term intakes of MCT oil have been shown to promote weight loss; however, chronic intakes of MCTs have shown various effects on energy expenditure, body weight, and fat mass.(89- 93) Yet, appetite control may play a bigger role in weight loss in long-term feedings of MCTs. The exact mechanism for the satiating effects of MCT is unknown, but may perhaps be explained by the distinctive energy density of MCT or the increase in fat oxidation.(86,88,89,93) These studies suggest that replacing LCT with MCT oil could generate body fat loss over long periods of time, with or without reduced energy intake.

Studies provide varying results concerning the influence of MCT on lipid metabolism such as increased TG concentrations.(94-97) In addition, several studies have reported that MCTs do not affect blood cholesterol levels(94,95,98-100); however, others have reported hypercholesterolemic effects of MCTs(96,97,101) due to their high saturated fat content. Therefore, the incorporation of other functional foods, such as conventional oils, essential fatty acids or plant sterol may minimize the risk of negative effects of MCT on blood lipids while optimizing decreases in body weight and body fat accumulation.(98) In addition, the ingestion of MCT incorporated into the diet does not appear to cause any adverse symptoms.(102)

MCTs are easily included in food products without negatively affecting their taste or producing undesirable effects.(102) MCT production is cost-effective compared to other oil-based functional foods. The short-term efficacy of MCT is proven; however, long-term effects of MCT still need to be examined more carefully. Overall, MCT shows a good AECES model and demonstrates the greatest potential for use as a functional fat for weight control.

CONCLUSION

In summary, significant interest exists in the development of oil-based functional foods for obesity. These natural health products need to be carefully compared against several norms, such as the AECES model, in order to optimize consumer use and acceptance. In general, CLA has the weakest AECES model; therefore, the least potential as a weight loss agent due to the lack of consistency between studies and the uncertain safety issues. Secondly, DAG oils demonstrate certain potential, but more research is needed to explain the mechanism of action. Finally, MCT-rich oils have demonstrated the greatest potential as a weight loss agent and thus the strongest AECES model. Overall, edible oils such as CLA, DAG, and MCT demonstrate potential consumer use and acceptance as natural health products to treat obesity.

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