Monday November 10 2008
A handful of almonds a day can fight heart disease and cancer, according to the Daily Express. The newspaper said that the “miracle nut” had been given to people with raised cholesterol levels for a month and had lowered their “bad cholesterol” levels, with results similar to taking statins.
The study the news report was based on was small, on only 27 people, did not compare the nuts with statins, and did not look at the effect on cancer at all. The finding relating to cholesterol is actually from a 2002 study and this report focuses on the study's findings about the effects of almonds on markers of oxidative stress.
Together the 2002 and 2008 research gives some indication of the short term effects of almonds on body chemistry, particularly cholesterol and markers of oxidative stress. But it provides no good evidence that eating almonds reduces the risk of heart disease or cancer.
Where did the story come from?
Dr David Jenkins and colleagues from St Michael’s Hospital in Toronto, the Almond Board of California and other research centres in Canada and the USA carried out this research. The study was funded by the Canada Research Chair Endowment, the USDA Agricultural Research Service, and the Almond Board of California. The study was published in the peer-reviewed Journal of Nutrition.
What kind of scientific study was this?
This was a randomised controlled crossover trial looking at the effects of almonds on certain biochemicals in the blood of people with high blood lipid (fat) levels. The researchers say that previous studies suggested that nut consumption is associated with a reduced risk of coronary heart disease (CHD).
Although this may be partly due to their ability to reduce cholesterol levels in the blood, the reduction in CHD cannot be fully explained by this. The researchers thought that antioxidants in the nuts might also be contributing to the reduction in risk effect. To test this idea they looked at whether nut consumption reduced the levels of chemical markers of oxidative damage in the blood.
The researchers enrolled 43 men and postmenopausal women who had high levels of LDL -C (“bad” cholesterol) in their blood but were otherwise healthy. All of the participants were instructed to follow the National Cholesterol Education Program Step 2 diet before starting the study. This program aims to achieve a diet in which less than 7% of energy comes from saturated fat, and whihc contains less than 200mg per day of dietary cholesterol.
After at least two months on this diet, the participants received one of three different supplements. These were either whole raw unblanched almonds (73g daily), wheat muffins (147g daily) that contained the same amount of fibre, saturated and polyunsaturated fatty acids as the almonds, and similar levels of protein, or “half and half” of each supplement. All the participants were assigned to receive all three diets for one month each, in a random order, with at least a two week break between each different treatment. During each of the treatment periods, the participants reduced their food intake to ensure that their total energy intake remained the same, and were instructed not to eat any other nuts or nut products.
Dietary counselling was given at the beginning and after the first and second week of each treatment period. The participants recorded their diets including how much of the almonds and muffins they ate. Any uneaten almonds or muffins were returned to the researchers and weighed. After the second and fourth week of each diet, blood samples were taken and levels of various chemicals were measured, for example vitamin A (an antioxidant), alpha and gamma tocopherol (forms of vitamin E, which is also an antioxidant), and malondialdehyde (MDA, a chemical that indicates oxidative damage in the body).
Urine samples were also taken at the fourth week and levels of the chemical isoprostane were measured. This chemical also indicates the presence of oxidative damage in the body. The participants were also weighed before and after each treatment period. The researchers compared the levels of these chemicals during the three different treatment periods. The analyses took into account the order in which the participants received the treatments, their gender and their total cholesterol levels.
What were the results of the study?
Only 27 participants (63% of the original number) completed all three treatment periods and were analysed. The average age was 62 years. Two of the people who withdrew did so because of food allergies, one because of abdominal discomfort, and the remaining left for reasons unrelated to treatment.
The researchers found that almond supplementation reduced the levels of markers of oxidative damage in the blood and urine (MDA and isoprostane) compared to the control (wheat muffin) supplementation. The half-almond, half-muffin diet reduced levels of the marker of oxidative damage in the urine (isoprostane) compared to control, but not of the marker in the blood (MDA). There were no differences in levels of vitamin A, or alpha and gamma tocopherol (vitamin E) between the treatments.
The researchers also found that the full almond supplementation reduced levels of LDL-C (“bad cholesterol”) compared to control, but the half dose did not. The levels of cholesterol at the end of treatment were: 4.22 mmol/L with control (wheat muffins), 4.10 mmol/L with half dose almonds and muffins, 4.01mmol/L with full dose almonds. However, these results were not the focus of this publication, as they had already been published in 2002.
What interpretations did the researchers draw from these results?
The researchers concluded that the antioxidant activity of almonds was confirmed by their effect on two biological markers of oxidative damage in the body. They say this antioxidant activity may contribute to the reduction in CHD risk seen with nut consumption.
What does the NHS Knowledge Service make of this study?
Previous observational studies have suggested that eating nuts may reduce CHD risk. However, this study did not investigate this, but instead looked at how nuts might have this effect. Although the study provides some evidence that antioxidant activity may have a role, it is too small to base firm conclusions on.
It is important to note that this study does not tell us anything about the effect of nuts compared with other healthy diets – only about their effect on certain biochemicals compared with wheat muffins – not statins as implied by some news reports. In addition, the people in this study were following a low fat diet, and some of them were on statins (cholesterol lowering drugs); these approaches are the mainstay of treating high cholesterol.
In order to confirm the effects of nut consumption on CHD risk, a randomised controlled trial should ideally be carried out. Without this kind of evidence, people who may want to eat nuts, but should be aware that nuts are energy dense foods, and so they should reduce their food consumption to compensate for this. Nuts that have been roasted in oil or salted should be avoided or eaten in moderation, as these could effectively cancel out any positive effects almond consumption might have.