Long-chain omega-3 fatty acids in red meat.
* Newly introduced Nutrient Reference Values indicate that most Australians need to increase their dietary intake of the long chain omega-3 polyunsaturated fatty acids (LC omega-3 PUFA), viz. eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA), to reduce the risk of chronic disease.
* Analysis of the 1995 National Nutrition Survey revealed that meat contributed almost as much as seafood to the LC omega-3 PUFA intake of adult Australians.
* Meat has a relatively high content of DPA, relative to EPA and DHA. Thus DPA accounts for 29% of the average LC omega-3 PUFA intake of adult Australians.
* Recent evidence suggests that DPA is just as important as EPA or DHA for delivering the health benefits associated with LC omega-3 PUFA.
* Current regulations, however, do not take account of the DPA content of foods in determining whether they qualify for an omega-3 content claim. Moreover, the DPA content of foods will not be considered in a proposed general level omega-3 health claim.
* Lean red meat is an important natural food source of LC omega-3 PUFA, the content of which can be influenced by modifying the composition of livestock feeds.
For more than a generation, Australians have been advised to reduce their consumption of saturated fat in order to reduce the risk of coronary heart disease. Animal fat, the predominant source of saturated fat in our diet, has been targeted, resulting in the widespread introduction and adoption of reduced fat products such as skimmed milk and trimmed cuts of meat. At the same time, there has been an increasing awareness of the distinctive qualities of unsaturated fats and, in particular, an appreciation of the relative attributes of monounsaturated fat as well as the omega-6 (n-6) and omega-3 (n-3) classes of polyunsaturated fat (PUFA). It is now apparent that, with the exception of trans fats, all unsaturated fats, when consumed regularly, have the potential to improve fasting blood lipid levels. (1) However, there are important differences between classes of unsaturated fat and their effects on blood lipids and cardiovascular (CV) risk. Unlike monounsaturated and omega-6 polyunsaturated fats, omega-3 fatty acids do not lower LDL-cholesterol but they lower fasting blood triglyceride levels and they can also inhibit clotting, facilitate blood flow and help to maintain a healthy heartbeat. (2-4) Thus they offer a multifactorial approach to counteract cardiovascular disease.
THE NEED FOR LONG CHAIN omega-3 PUFA
Subtle differences in molecular structure distinguish omega-3 from n-6 fatty acids (see Fig. 1). These structural differences confer important differences in biological activity on these molecules. As both these classes of PUFA mediate physiological functions yet neither is synthesised in humans nor are they interconvertible, each is considered to be essential for human diets. (5,6) Linoleic acid (LA), an n-6 fatty acid, and to a lesser extent [alpha]-linolenic acid (LNA), an omega-3 fatty acid, are both abundant in plant foods. As precursors for the synthesis of the physiologically important longer carbon chain n-6 fatty acid, arachidonic acid (AA), and the long chain omega-3 (LC n-3) PUFA, eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA), respectively, LA and LNA were thought to fulfil the dietary requirements to deliver these essential fatty acids. (7) However, it has become increasingly apparent from human bioavailability studies that, with the possible exception of vegetarians and pregnant/nursing women, the rate of conversion of LNA to EPA, DPA and DHA is limited (8,9) and may even be inadequate to sustain optimal health. (6) Hence Australians are now being advised to ensure that, in addition to LNA, they have an adequate intake (AI) of these LC omega-3 PUFA and, moreover, they are being encouraged to attain considerably higher suggested dietary targets (SDTs) to optimise their health status. The National Health & Medical Research Council's recently revised nutrient intake recommendations include, for the first time, AIs of 90 and 160mg/day and SDTs of 430 and 610 mg/day for LC omega-3 PUFA (i.e. EPA + DPA + DHA) for women and men respectively. (10) The AIs and SDTs represent the 50th and 90th percentiles of adult intakes respectively (see Table 1).
INTAKES AND SOURCES OF LONG CHAIN omega-3 PUFA
There is substantial evidence which indicates that LC omega-3 PUFA are essential not only for development, particularly of the nervous system, but for the maintenance of health throughout life. (2) However, by definition, only half of the adult population have an adequate dietary intake of LC omega-3 PUFA and 90% are eating less than is recommended to reduce the risk of chronic disease. (10) Fish and other seafood are the primary dietary source of LC omega-3 PUFA. (11) However, analysis of fatty acid intakes in the 1995 National Nutrition Survey shows that 43% of our LC omega-3 PUFA intake derives from meat, poultry and game and, in adolescents, this proportion increases to 49%, exceeding that of seafood (37%). (12) Not really surprising when one considers that Australians, on average, eat six times as much meat as fish. (11)
[FIGURE 1 OMITTED]
MEAT AS A SOURCE OF DPA
Much of the evidence substantiating health benefits of LC omega-3 PUFA, particularly cardiovascular benefits, (3) is based on dietary supplementation trials with fish or fish oil rich in EPA and/or DHA. Fish contains relatively little of the intermediate LC omega-3 PUFA, i.e. DPA, and there is even less in fish oil. Hence LC omega-3 PUFA intake recommendations for CV health usually specify EPA and DHA without reference to DPA. (3,13) Compared to fish, however, mammalian meat including beef and lamb has a relatively high proportion of DPA to EPA and DHA. (14) Table 2 shows the relative proportions of EPA, DPA and DHA in meat and fish, weighted according to the relative rates of consumption of different meat and fish products in Australia. Consequently, DPA has been shown to account for 29% of the total dietary LC omega-3 PUFA intake of Australian adults. (12) Hence we need to gain a better understanding of the physiological functions of DPA and how they compare to those of EPA and DHA.
PHYSIOLOGICAL EFFECTS OF INDIVIDUAL LONG CHAIN omega-3 PUFA
At this stage, the individual roles of EPA and DHA in mediating the health benefits attributed to LC omega-3 PUFA are still poorly understood. DHA is a key component of cell membranes, particularly in the nervous system, (15) and may influence the genetic expression of many mediators of metabolic functions through effects on transcription. (16) EPA can also influence a wide range of physiological functions by substituting for AA as the primary substrate for the synthesis of a vast family of eicosanoids. (17) This includes prostaglandins, leukotrienes and thromboxane, which mediate essential circulatory and immune functions. Moreover, both EPA and DHA are substrates for the recently identified resolvins which are important for tissue repair and immunity. (18) Whether DPA has an equivalent sphere of influence is yet to be determined.
Seal meat and seal oil are particularly rich sources of DPA. Interestingly, seal, not fish, was the main contributor to LC omega-3 PUFA intake in the traditional diet of Greenland Inuits, upon which the original epidemiological evidence for reduction of CV risk was based. (19) The few human intervention trials conducted with DPA-rich supplements indicate that DPA is equally if not more beneficial than either EPA or DHA for improving CV risk factors. (20-24) Recent clinical trials with seal oil have shown that it is more efficacious than fish oil in reducing plasma triglycerides (23) and indicate that DPA may have a specific inhibitory effect of on platelet aggregation. (24)
A ROLE FOR DPA?
It appears that DPA is the main end-point in the conversion of dietary LNA to LC omega-3 PUFA in humans. (8,9) Thus its physiological effects may be of significance not only to meat-eaters but also to vegetarians whose sole source of LC omega-3 PUFA is conversion from LNA. It has been estimated that 8% of ingested LNA is converted to both EPA and DPA but less than 0.1% is finally converted through to DHA. (8,9) Perhaps DHA accumulates in tissue pools with slower turnover such that the ongoing requirement for DHA is less than for EPA and DPA. This appears to be the case in erythrocytes, where DHA is bound to the inner layer of the plasma membrane and has a slower turnover than EPA which is bound to the outer layer and presumably more accessible to phospholipase activation. (25)
Clearly, more research is required on the specific roles of EPA, DPA and DHA. Meanwhile, for the purpose of dietary recommendations, there seems to be little reason for distinguishing between them. Nevertheless, DPA is not included in the LC omega-3 PUFA content required for a food to qualify for the omega-3 nutrition claim in the current Food Standards Code. (26) This ruling precludes many cuts of red meat from qualifying as a good source of omega-3, i.e. [greater than or equal to]60 mg of EPA + DHA/serve (see Table 3). Similarly, DPA is not considered in the US Food and Drug Administration's qualified heart health claim for omega-3, (3) nor will it be included in FSANZ's proposed pre-approved general level health claim for omega-3. (13) These standards favour seafood or, alternatively, foods enriched with LC omega-3 PUFA obtained from fish oil (with almost no DPA) over those with intrinsic DPA content.
OPTIMISING THE LONG CHAIN omega-3 PUFA CONTENT OF RED MEAT
As in all mammals, the LC omega-3 PUFA content of red meat is dependent on dietary fat consumption. Ruminants absorb LC omega-3 PUFA from their feed, albeit less effectively than monogastrics because of degradation in the rumen. Hence cattle which are predominantly pasture fed, as in Australia, tend to have significant levels of LC omega-3 PUFA (e.g. 90 mg/100 g for lean beef), whereas cattle which are predominantly grain fed, as in the USA, have considerably less. (27,28) This difference has significant implications for consumers attempting to meet target recommendations for LC omega-3 PUFA consumption, especially when dietary modelling is based on USDA compositional tables for fatty acid contents of red meat. (28,29) The increasing use of grain feedlots in Australia is likely to decrease the LC omega-3 PUFA content of beef, whilst increasing trans and saturated fat content. (27) The rate of absorption of LC omega-3 PUFA in ruminants will depend on the breed and age of livestock, as well as the composition of the diet.
Various attempts have been made to increase the LC omega-3 PUFA content of red meat by enriching ruminant feeds with LC omega-3 PUFA from marine sources. While this approach is effective in producing omega-3 enriched pork and poultry, (30) it is less promising for beef and lamb. Adding 5% fish oil to a beef cattle diet increased the LC omega-3 PUFA content of lean meat from 32 to 53 mg/100 g. (31) Similarly, adding 5% fishmeal increased LC omega-3 PUFA content from 19 to 43 mg/100 g, (32) whereas the same addition to a poultry feed increased the LC omega-3 PUFA content of breast meat from 17 to 68 mg/100 g. (30) Moreover, high PUFA intakes are well tolerated by monogastrics but can adversely affect ruminal digestion and impact negatively on health and development in ruminants. (33) Further research is needed to establish the most cost-effective feed formulations to optimise the LC omega-3 PUFA content of red meat and maximise the potential to deliver health benefits to both livestock and humans.
Red meat is an important dietary source of LC omega-3 PUFA, particularly DPA, and can contribute to the daily intake requirement for these essential fatty acids. Moreover, the content of LC omega-3 PUFA in red meat may be enhanced to a limited extent by feed modification. However, full exploitation of its nutritional benefit will depend on a better understanding of the individual contributions of EPA, DPA and DHA to potential health outcomes.
We are indebted to Ms Sally Record for the expert analysis of nutritional data presented in a confidential report to Meat & Livestock Australia Ltd. (34)
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Peter HOWE, (1) Jon BUCKLEY (1) and Barbara MEYER (2)
(1) Nutritional Physiology Research Centre and ATN Centre for Metabolic Fitness, School of Health Sciences, University of South Australia, Adelaide, South Australia and (2) School of Health Sciences, Metabolic Research Centre and Smart Foods Centre, University of Wollongong, Wollongong, New South Wales, Australia
Table 1 Deciles of fatty acid intakes (mg/day) estimated from 1995 National Nutrition Survey adapted from Howe et al. (34) Fatty acid 10th 20th 30th 40th median 60th Women 18:2 n-6 LA 2.809 4.052 5.175 6.226 7.405 8.681 20:4 n-6 AA 0.008 0.031 0.050 0.069 0.088 0.111 18:3 omega-3 LNA 0.298 0.421 0.511 0.608 0.710 0.826 20:5 omega-3 EPA 0.001 0.006 0.012 0.018 0.024 0.034 22:5 omega-3 DPA 0.000 0.005 0.013 0.022 0.034 0.045 22:6 omega-3 DHA 0.002 0.006 0.010 0.015 0.021 0.029 Total LC omega-3 0.005 0.027 0.047 0.066 0.091 0.120 (EPA+DPA+DHA) Men 18:2 n-6 LA 4.373 6.068 7.721 9.144 10.691 12.514 20:4 n-6 AA 0.030 0.063 0.091 0.119 0.151 0.185 18:3 omega-3 LNA 0.494 0.644 0.781 0.910 1.058 1.227 20:5 omega-3 EPA 0.005 0.015 0.023 0.034 0.043 0.057 22:5 omega-3 DPA 0.003 0.017 0.032 0.046 0.061 0.081 22:6 omega-3 DHA 0.005 0.011 0.018 0.026 0.036 0.048 Total LC omega-3 0.027 0.059 0.089 0.122 0.160 0.206 (EPA+DPA+DHA) Fatty acid 70th 80th 90th Women 18:2 n-6 LA 10.242 12.167 15.991 20:4 n-6 AA 0.141 0.179 0.244 18:3 omega-3 LNA 0.975 1.195 1.570 20:5 omega-3 EPA 0.046 0.066 0.117 22:5 omega-3 DPA 0.061 0.082 0.124 22:6 omega-3 DHA 0.043 0.067 0.206 Total LC omega-3 0.161 0.228 0.424 (EPA+DPA+DHA) Men 18:2 n-6 LA 14.891 17.923 23.095 20:4 n-6 AA 0.226 0.284 0.386 18:3 omega-3 LNA 1.438 1.742 2.278 20:5 omega-3 EPA 0.075 0.107 0.172 22:5 omega-3 DPA 0.106 0.141 0.205 22:6 omega-3 DHA 0.067 0.103 0.270 Total LC omega-3 0.267 0.380 0.605 (EPA+DPA+DHA) Table 2 Relative LC omega-3 PUFA content of meat and fish (mg/100 g)* adapted from Howe et al. (34) Beef Lamb Pork Poultry Fish* EPA 45 40 14 14 247 DPA 71 83 28 18 66 DHA 13 20 16 15 415 Total LC omega-3 129 143 58 47 728 * contributions of different types of meat and fish were weighted according to their relative rates of consumption in the 1995 National Nutrition Survey. Table 3 LC omega-3 PUFA content of selected cuts of red meat (mg/100 g) adapted from Mann et al. (14) EPA + DHA EPA + DPA + DHA Beef Mince Hamburger 72.32 166.05 Beef Mince Regular 23.14 68.61 Beef Mince Low Fat 40.14 94.20 Beef Lean rump steak 36.12 89.32 Beef Lean round steak 27.32 62.71 Beef Lean topside & silverside roast 18.51 48.44 Beef Lean fillet (sirloin, scotch fillet, T- 39.81 89.34 bone) Beef Lean blade steak 56.16 114.99 Veal Lean leg steak 33.64 63.25 Veal Lean cutlet 34.81 65.27 Veal Lean stir-fry & diced 31.68 66.19 Lamb mince 67.63 148.18 Lamb Lean leg, tenderloin mini roast & chump 31.51 71.05 chop Lamb Lean loin chop 32.37 70.50 Lamb Lean forequarter chop & EasyCarve 59.57 116.17 shoulder Mutton Lean baking Leg 51.68 100.93 Mutton Lean casserole 76.63 134.86
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|Title Annotation:||Section 2: Key nutrients delivered by red meat in the diet|
|Author:||Howe, Peter; Buckley, Jon; Meyer, Barbara|
|Publication:||Nutrition & Dietetics: The Journal of the Dietitians Association of Australia|
|Date:||Sep 1, 2007|
|Next Article:||Red meat and food guides.|