Motivation: The metabolic syndrome is characterized by a group of risk factors for
cardiovascular disease (CVD) that includes obesity, dyslipidemia, high blood pressure, insulin
resistance, glucose intolerance or non-insulin dependant diabetes mellitus, pro-thrombotic state
and pro-inflammatory state. The NHANES I11 study showed the prevalence of this syndrome
to be 24.0% in men and 23.4% in women in the USA. These figures translate to more than 47
million US residents having the metabolic syndrome. In the THUSA (acronym for Transition
and Health in the Urbanization of South Africans) study in South Africa it was found that
12% and 28.4% of men and women, respectively, of the black population of the North West
Province had three or more disturbances characterizing this syndrome. Therefore, it is evident
that the metabolic syndrome is a health problem not only for developed countries but also for
developing countries. As a result, this syndrome has been identified as a target for dietary
therapies to reduce the risk of CVD and type 2 diabetes.
Epidemiological studies have consistently demonstrated an inverse association between nut
consumption and coronary heart disease (CHD) morbidity and mortality in different
population groups. Nut consumption may not only offer protection against heart disease, but
also increase longevity. Recently, the benefits of nuts consumption were acknowledged by the
U.S. Food and Drug Administration when they approved a qualified health claim that eating
nuts (1.5 ounces/day ≈ 42.8 g/day) may reduce the risk of CHD. In this regard, the most
comprehensively studied mechanism involved the favourable lipid lowering effects of nuts.
There is, however, a lack of data in the literature regarding the effect of nuts on the metabolic
syndrome.
Objective: The main objective of this study was to examine the effects of a high walnut diet
and a high unsalted cashew nut diet on markers of the metabolic syndrome in humans. In
order to provide a foundational body of evidence for the aforementioned, a secondary
objective included conducting a systematic review that investigates the effects of nuts on the
lipid profile.
Methods: The main project consisted of a controlled feeding trial with a parallel, randomized
controlled study design on participants having the metabolic syndrome. Sixty-four subjects
having this syndrome (29 men, 35 women) with a mean (±SD) age of 45±10 y and who met
with the selection criteria were all fed a 3-week run-in control diet. After this period,
participants were grouped according to gender and age and then randomized into three
groups, namely, those that received a controlled feeding diet including walnuts (20% energy
(E), 60-100g/day; protein:carbohydrate:fat=18:42:40%E). or unsalted cashew nuts (20%E 66-
1 15g/day; protein:carbohydrate:fat=l9:44:37%E) or no nuts
(protein:carbohydrate:fat=20:47:33%E) for 8 weeks. The participants' physical activity and
weight were maintained for the duration of the study.
For the systematic review. human intervention trials that investigated the independent effects
of nuts on lipid concentrations were included. Medline and Web of Science databases were
searched from the start of the database to August 2004 and supplemented by cross-checking
reference lists of relevant publications. These papers received a rating based upon the
methodology as it appeared in the publication. No formal statistical analysis was performed
due to the large differences in study designs of the dietary intervention trials. The main
outcome measures for the systematic review, were percentage differences between treatment
and control groups for total blood cholesterol (TC), low-density lipoprotein cholesterol (LDLC),
high-density lipoprotein cholesterol (HDL-C) and triacyglycerols (TG).
Results: Regarding the main objective, we found that both the walnut and unsalted cashew
nut intervention diets had no significant effect on the lipid profile, serum fructosamine,
insulin, insulin sensitivity, insulin resistance, serum high sensitivity C-reactive protein, blood
pressure and serum uric acid concentrations when compared to the control dict. All three
groups experienced highly significant increases in serum insulin concentrations when
comparing the baseline to end (P<0.05). In turn, insulin resistance increased while insulin
sensitivity decreased in all three groups. Plasma glucose concentrations increased
significantly in the cashew nut group compared to the control group (P<0.05). By contrast,
serum fructosamine was unchanged in the cashew nut group while the control group had
significantly increased concentrations of this short-term marker of glycaemic control.
The literature search for the systematic review yielded 41 5 publications. After screening, 23
nut studies were included in the review with most of these studies including heart-healthy
diets. The majority of the studies were short (4-6 weeks) with only one study lasting 6
months. The number of subjects in most of the studies was sufficient to study the effects on
TC and LDL-C but not for HDL-C and TG. The results of three almond (50-100g/day), two
peanut (35-68g/day), one pecan nut (72g/day) and four walnut (40-84g/day) studies showed convincing evidence for a lipid lowering effect of TC between 2-1 6% and LDL-C between 2-
19%, when compared to their control diets. Currently, there are indications from inadequately
designed intervention studies that hazelnuts (lg/day/kg body weight) and pistachios (20%E)
may have a lipid lowering effect. At this stage the evidence for macadamia nuts is less
convincing. Furthermore, it is apparent that the components in nuts further reduce TC and
LDL-C concentrations beyond the effects predicted by equations based solely on dietary fatty
acid profiles.
Conclusions: In the controlled feeding trial, subjects displayed no improvement in the
markers of the metabolic syndrome after following a walnut or unsalted cashew nut diet
compared to a control diet while maintaining body weight (8 weeks). Finally, we suspect that
the dramatic increase in insulin resistance may have masked the protective effects of the
walnut and cashew nut diets in our subjects with the metabolic syndrome Further research is
warranted before a consensus can be reached.
From the systematic review it was concluded that the consumption of ≈50-100g (≈1.5-3.5
servings) of nuts five or more times/week as part of a heart-healthy diet with total fat content
(high in mono- and /or polyunsaturated fatty acids) of ≈ 35% of energy may significantly
decrease TC and LDL-C in normo- and hyperlipidemic individuals.
Recommendations:
A similar nut controlled feeding trial with some form of calorie restriction, should be done on
participants having the metabolic syndrome.
Future research should use randomized controlled studies with larger sample sizes and longer
duration to investigate the effects of nuts on HDL-C and TG concentrations. Also, studies
should investigate the effects on the lipid profile of mixed nuts and those individual nuts not
yet considered. In addition, the unique nutrient and non-nutrient composition of nuts requires
further research in order to elucidate the possible mechanisms responsible for the LDL-C
lowering effect / Thesis (Ph.D. (Nutrition))--North-West University, Potchefstroom Campus, 2005.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/690 |
| Date | January 2005 |
| Creators | Mukuddem-Petersen, Janine |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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