spoon

 

New Zealand

 

Why you should eat walnuts

Professor GP Savage,

Food Group, ALSD, Lincoln University, Canterbury.

Nuts, especially walnuts, are full of heart-friendly nutrients such as magnesium, vitamin E, phytosterols and fibre. Nuts help to reduce high blood triglycerides and a regular intake is associated with a lower incidence of heart disease. In part, this is because the fatty acids that nuts contain are mainly polyunsaturated. Randomized clinical trials (i.e. very carefully run experiments using human subjects) and many observational studies have shown that nuts, especially walnuts, can be a valuable part of a normal, lipid lowering or weight controlling diets.

In the past nuts were traditionally valued as a healthy food but some decades ago their value was discounted because of concerns about total fat and total energy intakes.1 This view has now been reversed as more understanding about the role of fats in health and the importance of unsaturated fatty acids has become more apparent. Nuts are now seen as an important food that has a protective effect against heart disease. Nuts are rich in monounsaturated fatty acids and they typically have polyunsaturated to saturated fatty acid ratios (P:S ratios) which range from 6-10.2

The fact that an increased intake of nuts improves rather than impairs lipid profiles in the blood is now clear from the results of a number of human trials on normal, hyperlipidaemic and diabetic subjects. A recent study in Australia of 58 adults suffering from Type 2 diabetes showed that the consumption of 30 g walnuts/day (that is 5 or 6 nuts) resulted in a 10% fall in low density lipoprotein cholesterol (LDLc) in the blood and a significant improvement (an increase of 30%) in the high density lipoprotein cholesterol (HDLc).3

An even more interesting study was carried out recently in Spain on people who had moderately high blood cholesterol levels. A walnut diet (10 walnuts/day) was compared with a control Mediterranean diet which contained olive oil, olives and avocados.4 The people who ate the walnut containing diets had significantly improved endothelium-dependent vasodilation (this is a measure of the artery's ability to vasodilate and is one of the recognised intermediate mechanisms by which the progress of arterial atherosclerosis can be monitored). These people also had lower levels of vascular cell adhesion molecule-1 in their blood.

Simply this means that walnuts lower the lipids and cholesterol (LDLc) in the blood that contribute to heart disease while elevating cholesterol that is protective (HDLc). Atherosclerosis develops because of high lipids in the blood and narrows the blood vessels making heart attacks and strokes more likely. Walnuts contain nutrients which help to keep the blood vessels healthy by maintaining their ability to widen (vasodilation).

There is very consistent epidemiological evidence showing that there is a positive association between higher nut consumption and reduced incidence of heart disease.5-8 Fraser (1999) showed that this association was very strong - a 30% fall in cardiovascular disease risk was estimated as a result of substituting 30 g nuts/day for an energy-equivalent carbohydrate. He observed a 45% reduction if nuts were substituted for saturated (animal) fat in the diet.8 In addition a higher nut intake is also associated with a lower incidence of diabetes and gall bladder stones.9-10

The impact on the addition of nuts to the diet and its effect on body weight is interesting. In the past dietitians would have suggested that nuts are a high fat (or energy) food which should be reduced or perhaps eliminated from the diet. This is rather a simplistic approach to fat being a "problem" in weight management. There is however contradictory evidence on whether nut consumption actually reduces weight more than other energy-reduced diets.3,10,11 Randomised clinical trials have consistently shown that a reasonable nut intake does not increase body weight.11,12 It is possible that the consumption of nuts leads to an increased satiety or their slightly slower rate of digestion because of their complex carbohydrate and fibre content may counter their high energy content.

Exactly why nuts have all these positive effects is still a matter of some conjecture. It is quite clear that the mono- and polyunsaturated fatty acid content of nut oils are important factors. Walnut oil in particular, contain 73% polyunsaturated fatty acids, higher than any other nut oil and more modest levels of monounsaturated fatty acids (18%).13 As well as containing these lipids, walnut oil is nutrient-dense and contains significant amounts of antioxidants such as vitamin E, phytosterols and polyphenols. Whole nuts also contain minerals such as magnesium, potassium and copper as well as dietary fibre.2,14,15

Phytosterols occur in a wide range of foods and their most well known property is lowering blood cholesterol levels.16 Savage and Dutta (2001) showed that walnuts grown in New Zealand contained a mean of 125 mg total desmethyl sterol/100 g nuts which was higher than the values for hazelnuts and macadamia nuts.16

Nuts also contain high levels of the essential amino acid, arginine. Arginine can be broken down to nitric oxide in the body and nitric oxide has been shown to be a powerful vasodilator (i.e it tends to open up the blood vessels).5,8,14

In many clinical experiments patients have been given the recommendation to eat say, 30 g nuts/day. Generally very little other dietary advice is then given. It is clear, however, from these experiments that when this amount of nuts is added to the diet there is a tendency for the patients to modify their diet a little. They tend to reduce their intake of other fats in their diet, if these fats happen to contain saturated fatty acids then there is an extra bonus from eating nuts.

Nuts, apart from being part of a healthy diet, have an important role in the management of patients who have high blood cholesterol and lipids levels. For instance, nuts were shown by Jenkins et al., (2003) to be equally effective as treatment with one of the statins, lovastatin.17 This is a major observation, evidence that a food is as effective as a medicine is very encouraging. The problem is that foods do not receive the same level of promotion or advertising.

References

1. Tunstall-Pedoe H. Nuts to you (..and you, and you). Eating nuts may be beneficial-though it is unclear why. Br Med J 1998 317(7169): 1332-1333.
2. Maguire LS et al. Fatty acid profile, tocopherol, squalene and phytosterol content of walnuts, almonds, peanuts, hazelnuts and the macadamia nut. Int J Food Sci Nutr 2004 55(3):171-178.
3. Tapsell LC et al. Including walnuts in a low-fat/modified-fat diet improves HDL cholesterol ratios in patients with type 2 diabetes. Diabetes Care 2004 27 (12): 2777-2783.
4. Ros E et al. A walnut diet improves endothelial function in hypercholesterolemic subjects: a randomised crossover trial. Circulation 2004 109(13): 1609-1614.
5. Alert CM. et al. Nut consumption and decreased risk of sudden cardiac death in the Physicians' Health Study. Arch Intern Med 2002 24; 162: 1382-1387.
6. Ellsworth JL. et al. Frequent nut intake and risk of death from coronary heart disease and all causes in postmenopausal women: the Iowa Women's Health Study. Nutr Metab Cardiovasc Dis 2001 11(6):372-377.
7. Hu FB. et al. Nut consumption and risk of coronary heart disease: a review of epidemiologic evidence. Curr Atheroscler Rep 1999; 1(3):204-209.
8. Fraser GE. Nut consumption, lipids, and risk of a coronary event. Clin Cardiol. 1999 22(7 Suppl) 111, 11-15.
9. Jiang R. et al. Nut and peanut butter consumption and risk of type 2 diabetes in women. J Am Med Assoc 2002; 288: 2554-2560.
10. Tsai CJ. et al. A prospective cohort study of nut consumption and the risk of gallstone disease in men. Am J Epidemiol. 2004 160(10): 961-968.
11. St-Onge MP. Dietary fats, teas, dairy, and nuts: potential functional foods for weight control? Am J Clin Nutr 2003 81(1): 7-15.
12. Sabeté J. Nut consumption and body weight. Am J Clin Nutr 2003 78(3) Suppl) 647S-650S.
13. McNeil DL and Savage GP. Some New Zealand sources of healthy oils- olive and nut oils. Proc Nutr Soc NZ 2004 25: 10-18.
14. Strahan TM et al. Nuts for cardiovascular protection Asia Pacific Clin Nutr 2004; 13(Suppl): S33.
15. Kris-Etherton PM et al. Nuts and their bioactive constituents: effects on serum lipids and other factors that affect disease risk Am J Clin Nutr 1999 70(3 Suppl) 504S-511S.
16. Savage GP Dutta PC. 2001 Desmethyl composition of some common nuts. Proc Nutr Soc NZ 2004 26: 87-91.
17. Jenkins DJ et al. Effects of a dietary portfolio of cholesterol-lowering foods vs lovastatin on serum lipids and c-reactive protein J Am Med Assoc. 290(4): 502-510.