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Nutrition & Dietetics, School of Health Sciences, Faculty of Health, University of Newcastle, Callaghan, NSW, Australia and * Centre for Advancement of Food Technology and Nutrition, University of Newcastle, Ourimbah, NSW, Australia
2To whom correspondence should be addressed. E-mail: manohar.garg{at}newcastle.edu.au.
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ABSTRACT |
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 TOP ABSTRACT SUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • macadamia nuts • plasma cholesterol • hypercholesterolemia • LDL cholesterol • HDL cholesterol
Dietary factors known to alleviate plasma cholesterol levels in humans include (n-6) and (n-3) PUFA, dietary fiber and plant sterols. (n-6) PUFA, although associated with cholesterol-lowering effects, serve as a substrate for the formation in the platelets of thromboxane A2, which is a potent pro-aggregatory substance (1 ,2 ). Additionally, both (n-3) and (n-6) PUFA increase requirements for antioxidants to protect them from auto-oxidation and minimize oxidative damage in the body (3 –5 ). MUFA present in olive, sunola and canola oils have been shown to be cholesterol-neutral or even cholesterol-lowering in several studies (6 ,7 ). MUFA also reduce demands for antioxidants, and thus may alleviate oxidative stress in vivo (8 –10 ). Margarines and spreads enriched with MUFA and/or plant sterols with health claims to reduce plasma cholesterol concentrations are available in the supermarkets. The research on whole foods, containing factors with hypocholesterolemic potential, is scanty.
The macadamia nut, a tree nut native to Australia, contains 
75 g fat/100 g nuts and >85% of its energy from fat. Macadamia nuts contain higher levels of MUFA than any other food source known to date (>60 g/100 g of edible whole nuts) (11
). A recent study has demonstrated that macadamia nuts are a rich source (1.28 mg/g lipid) of plant sterols (12
). Diets containing high MUFA-rich foods have been shown to reduce plasma LDL cholesterol levels without any detrimental effects on HDL cholesterol (13
–15
). Recent studies have demonstrated that constituents other than fats in tree nuts may be responsible for their cholesterol-lowering effects (16
). Curb et al. (17
) demonstrated that a macadamia nut–based diet is nearly as effective as a moderately low fat diet (American Heart Association Step 1 diet) in reducing total plasma cholesterol and LDL cholesterol, in comparison to a typical American diet. The present study was carried out to examine the effects of replacing 15% energy intake by macadamia nuts (40–90 g/d) on plasma lipid and homocysteine levels as well as fatty acid composition in hypercholesterolemic men.
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SUBJECTS AND METHODS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Hypercholesterolemic men (n = 17; plasma cholesterol 6.1–7.5 mmol/L) were recruited. All subjects maintained their regular lifestyles, especially their physical activity. The Human Research Ethics Committee of the University of Newcastle approved these studies. All subjects signed an information and consent form agreeing to participate.
Intervention trial.
Freshly roasted, lightly salted and/or unsalted (subjects were allowed to choose) macadamia nut kernels (Macadamia integrifolia) were purchased from the Suncoast Gold Macadamias Ltd. (Queensland, Australia). Diabetics, smokers and people with thyroid disorders or high blood pressure or on cholesterol-lowering drug/nutritional therapies were excluded from the study. Subjects consuming >10 standard drinks/wk were also excluded. The intervention phase consisted of a 4-wk period of consuming macadamia nuts contributing 15% of the total daily energy intake. The absolute amount of macadamia nuts consumed ranged from 40 to 90 g/d, depending on subject’s energy intake. One-day supplies of nuts were packed in separate moisture-free bags. An extra color-coded bag of macadamia nuts was given to the subjects to share with family and friends to improve compliance. Compliance was measured by the nut count-back method and by examining the incorporation of 16:1(n-7) into plasma lipids after the 4-wk intervention period. Subjects were instructed to maintain their usual diet and level of physical activity. A 3-d food diary (including at least 1 d from the weekend) was completed by the subjects at the start (1 wk before commencement of the intervention) and toward the end (during wk 4) of the intervention period. Dietary records were analyzed for nutrient and energy intakes by a computer-based program (Foodworks 2.10.136, Xyris Software, Australia). Blood samples were collected from fasting subjects by venipuncture at entry to the study and at the conclusion of the intervention.
Plasma was separated from red blood cells within 1 h of collection by centrifugation at 3000 x g for 10 min and analyzed for total cholesterol, HDL cholesterol and triglycerides by colorimetric enzymatic assays using a Hitachi autoanalyzer (Hitachi 747; Hitachi-Boehringer, Mannheim, Germany). LDL cholesterol values were calculated. Aliquots of plasma were frozen at -80°C for determination of homocysteine and fatty acid concentrations.
Fatty acid analysis.
The lipid profile of the plasma was determined according to a modification of the method of Lepage and Roy (18 ). Briefly, 100 µL of plasma was mixed with 2 mL of methanol:toluene (4:1 v/v) containing 4 mg/L of 21:0 as an internal standard, and 0.12 mg/L butylated hydroxytoluene as an antioxidant, in a glass culture tube. This was followed by the slow addition of 200 µL acetyl chloride with continuous shaking. The samples were methylated by heating at 100°C for 60 min and extracted samples stored in glass vials sealed with a Teflon-lined cap for subsequent analysis by gas chromatography.
Fatty acid methyl esters (FAME) were quantified using a Hewlett–Packard 6890 gas chromatograph (Hewlett–Packard, Palo Alto, CA) equipped with a 30 m x 250 µm x 0.25 µm capillary column (J&W DB-225; J&W Scientific, Folsom, CA) flame ionization detector (19 ). Chromatographic conditions were as follows: inlet temperature, 250°C; inlet pressure, 97.5 kPa; split ratio, 20:1; carrier gas as hydrogen (ultrahigh purity) at 1.6 mL/min. The chromatographic conditions were as follows: initial temperature of 170°C held for 2 min; ramped to 190°C at a rate of 10°C/min; held there for 1 min; ramped to 220°C at a rate of 3°C/min; and held at 220°C for 15 min. FAME were identified and quantified by comparison with authentic FAME standards (Nu-Chek Prep, Elysian, MN).
Homocysteine assay.
Plasma total homocysteine was measured by use of a commercially available assay kit (Drew Scientific DS30 Hcy homocysteine assay kit, Cumbria, UK) (20 ). After extraction, the samples were analyzed on a Hewlett–Packard 1100 Series HPLC system by use of a Hewlett–Packard 1046A programmable fluorescence detector, and Hewlett–Packard Chem Stations Rev. A:08.03 [847] software. The chromatographic conditions were as follows: column; Hewlett–Packard ODS Hypersil, 5 µm x 125 mm x 4 mm; column temperature, 36°C; mobile phase, 50 mmol/L KCl; 10 mmol/L heptanesulfonic acid sodium salt; 13 mmol/L HCl, 2% (v/v) methanol; flow rate, 1.0 mL/min; injection volume, 20 µL; excitation, 400 nm; emission, 503 nm.
Statistical analysis.
The means ± SEM are presented for each measurement. A two-tailed, paired Student’s t test was used to compare baseline values with postintervention values. Differences were considered significant at P < 0.05.
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RESULTS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Plasma MUFA such as 16:1(n-7), 18:1(n-7) and 20:1(n-9) were significantly elevated in all subjects after macadamia nut consumption, further confirming the high degree of compliance (Table 2 ). The concentration of palmitoleic acid, 16:1(n-7), for which the unique dietary source is macadamia nut, was elevated by treatment (Table 2) . PUFA were not affected by the intervention. Some SFA (20:0 and 22:0) were elevated following intervention.
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DISCUSSION |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The results presented suggest that consumption of macadamia nuts, despite their being high in fat, has no detrimental effects on plasma cholesterol levels. A randomized crossover trial in a free-living population fed a macadamia nut–based monounsaturated fat diet, demonstrated a 5% reduction in plasma cholesterol compared to a "typical American" diet high in saturated fat (17 ). In the present study, macadamia nut consumption was associated with a proportional increase in MUFA intake and proportional reductions in PUFA and SFA consumption. These changes could contribute to the cholesterol-lowering potential of macadamia nuts in individuals with higher than normal plasma cholesterol (>6.0 mmol/L), as evident from the data presented here. In the present study, the beneficial effects were 2-fold, that is, a reduction in total and LDL cholesterol and an increase in HDL cholesterol, resulting in a 7.9% reduction in the plasma total to HDL cholesterol ratio. In contrast, previous studies (17 ,21 ) reported no difference in HDL cholesterol after macadamia nut intervention compared to individuals consuming the AHA Step 1 or high carbohydrate diets. The total fat and monounsaturated fat intakes increased during macadamia nut intervention in the present study, which may be responsible for the increase in HDL cholesterol. It was previously shown that a reduction in fat intake lowers HDL cholesterol levels and vice versa (22 –24 ) and that inclusion of MUFA in low fat diets may reduce HDL cholesterol less than the low fat diet alone (25 ).
PUFA in the plasma were not affected by macadamia nut intervention. MUFA, especially 16:1(n-7) from macadamia nuts, were incorporated into plasma lipids, indicating that the subjects were compliant. Previous studies did not report fatty acid profiles of plasma after intervention with macadamia nuts. This study demonstrated that because 16:1(n-7) is uniquely present at substantially higher levels in macadamia nuts than in any other food source, the plasma level of this fatty acid is a reliable measure of compliance. One question that was previously raised (16
) is whether nuts contain constituents other than fats that may affect blood cholesterol. A recent analysis of the published data on the effects of incorporating tree nuts on plasma lipids and lipoproteins revealed that when subjects consumed test diets containing nuts, there was an 25% greater cholesterol-lowering response than that predicted by the equations. This suggests that there are non–fatty acid components in nuts with cholesterol-lowering effects. In this respect, it is noteworthy that macadamia nuts contain substantial levels of plant sterols (1.28 mg/g lipid), which were previously shown to inhibit cholesterol absorption (26
,27
). Plant sterols contributed by macadamia nuts were 50–115 mg/d, depending on subjects’ energy intakes. It has been established that plant sterols are effective as cholesterol-lowering agents only when present in gram quantities (26
,27
). However, the possibility of synergies between MUFA, fiber, polyphenols and plant sterols in whole macadamia nuts to reduce plasma cholesterol cannot be ruled out.
In summary, inclusion of macadamia nuts as part of a healthy diet favorably altered the plasma lipid profile, despite an increase in the dietary fat content. Interestingly, the beneficial effects were observed in a cohort (hypercholesterolemics) in need of reducing plasma cholesterol. Indeed, the extent of reduction in plasma cholesterol in a short-term intervention of 4 wk is not sufficient to exempt patients from additional drug/nutritional interventions, but may be of value as an adjunct to current known cholesterol-lowering therapies. An intervention trial involving macadamia nut consumption for a longer duration (
6 mo) is warranted. Regardless, the degree of alterations in the blood lipid profile extends beyond those observed with an AHA Step 1 diet because of the dual beneficial effects on LDL and HDL cholesterol. These results, in association with previously published reports on the beneficial effects of tree nuts on biomarkers of coronary artery disease, allow a prudent recommendation for the inclusion of macadamia nuts as part of a heart-healthy diet.
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FOOTNOTES |
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3 Abbreviations used: BMI, body mass index; FAME, fatty acid methyl esters; SFA, saturated fatty acids.
Manuscript received 7 November 2002. Revision accepted 7 January 2003.
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LITERATURE CITED |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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