“Eating nuts may help lower cholesterol levels,” BBC News reported.
This news story is based on a pooled analysis of 25 studies, looking at the effects of experimental nut diets on blood cholesterol and fat levels. A nut-enriched diet was found to be associated with both reduced total cholesterol and LDL (“bad”)-cholesterol. On average, in people who consumed 67g of nuts a day, total cholesterol was reduced by 5.1% and LDL-cholesterol by 7.4%. Nut-enriched diets had a lesser effect on reducing the cholesterol levels of people with higher BMIs.
These diets lasted between three and eight weeks, so it is unclear whether this reduction in cholesterol has any effect on the risk of coronary heart disease in the long term. Although nuts are low in saturated fat, they are nevertheless very high in fat and calories. Plain, unsalted nuts should be eaten in moderation as part of a healthy diet.
Where did the story come from?
The study was carried out by researchers from Loma Linda University in California and the Instituto de Salud Carlos III in Barcelona. Funding came from Loma Linda University and the International Tree Nut Council Nutrition Research and Education Foundation. Some of the researchers have also received funding from the California Walnut Commission, the Almond Board of California, the National Peanut Board and the International Tree Nut Council. The study was published in the peer-reviewed medical journal Archives of Internal Medicine.
The research was generally covered well by the newspapers, which all included the advice that people who want to increase their nut intake should avoid salty nuts.
What kind of research was this?
The researchers say that previous epidemiology studies have shown that frequent nut consumption reduces the risk of coronary heart disease. The aim of this research was to see whether it was possible to estimate the effects of nut consumption on the levels of different types of fat in the blood.
The researchers carried out a systematic review and a pooled analysis, in which they grouped the data from various published trials on the effects of nut consumption on cholesterol levels and blood fat. They also wanted to see whether other factors, such as a person’s age or the type of nut, affected the outcomes.
What did the research involve?
The researchers carried out a systematic search of a medical research database for scientific papers which looked at the effect of nuts on blood fat and cholesterol levels, and had been published between January 1992 and December 2004.
To be included, studies had to be based on humans, and either to have had a control group or to have taken stable baseline fat measurements from participants before they started the experimental diet. The experimental diets had to be supplemented solely by nuts, and to have lasted at least three weeks. The participants’ weight also had to have remained the same during the diet. The researchers excluded studies in which participants had taken fat or cholesterol-lowering medication.
Twenty-five studies of various design were suitable for inclusion in the analysis. In studies that had used a crossover design, in which participants received the experimental diet followed by the control diet or vice-versa, participants contributed two data points, one from when they were a control and one where they were receiving the experimental diet. This resulted in a total 1,284 data points and 583 participants.
The researchers looked at whether age, gender, body mass index (BMI) type of nut and type of diet modified the effect of nut consumption on cholesterol and fat measurements in the blood. They also considered the type of study design and the degree of control that the study’s investigator had over the participants’ overall diet, and whether this impacted the effect of nuts on the blood measurements.
For some analyses, the participants’ data was classified into groups. For example, cholesterol was classified as (low) less than 130mg/dL, (medium) 130-160 mg/dL, or (high) greater than 160 mg/dL. Other fats (triglycerides) were classified as less than 150 mg/dL or more than 150 mg/dL, and BMI was classified as normal weight (less than 25), overweight (25-30) or obese (more than 30).
What were the basic results?
Compared with control diets, nut diets were associated with a reduction in total cholesterol and levels of low-density lipoprotein (LDL) (“bad”) cholesterol. Nut consumption did not have an effect on the levels of high-density lipoprotein (HDL) (“good”) cholesterol, but it did increase the ratio of HDL compared to total cholesterol (p < 0.001).
Nut diets did not appear to affect blood triglyceride levels, except in participants who had high blood triglyceride levels before the study began, in whom blood triglyceride levels were reduced following the diet (p<0.05).
Age, gender and the type of nut did not influence the effects of nuts on blood cholesterol. However, BMI did have an effect. Participants with a lower BMI at the beginning of the study had lower cholesterol as an effect of eating nuts. Participants with higher LDL-cholesterol at the start of the study had a greater decrease of total cholesterol at the end of the diet.
Participants with LDL-cholesterol greater than 160 mg/dL at the start of the study were associated with a decrease of 17.5 mg/dL (approximately 11%) at the end. Having LDL-cholesterol at less than 130 mg/dL at the study start was associated with a decrease of 5.0 mg/dl (approximately 4% of 130).
Nut diets were associated with a 7.4% reduction in cholesterol and 9.6% reduction in LDL-cholesterol, compared to a western diet, 4.3% and 6.7% compared to a Mediterranean diet, and 4.1% and 6.0% compared to a low-fat diet.
The researchers then made estimates of how differing amounts of nuts would affect blood fat and cholesterol levels. They suggested that if a participant ate 71g of nuts per day as part of a 2,000-kcal diet (20% of dietary energy), this was associated with a 4.5% decrease in total blood cholesterol, and a 6.5% decrease in LDL-C.
How did the researchers interpret the results?
The researchers say that their study suggests that “increasing the consumption of nuts as part of an otherwise prudent diet can be expected to favorably affect blood lipid (fat/ cholesterol) levels (at least in the short term) and have the potential to lower CHD risk”.
They attempt to explain the association by saying that “nuts are rich in plant sterols, natural compounds that might contribute to cholesterol lowering by interfering with cholesterol absorption”. However, they also say that “more research is needed to answer the important question of why nuts are less effective in lowering blood cholesterol concentration among subjects with obesity”.
This study conducted a pooled analysis of 25 studies, which looked at the effect that a nut-supplemented diet had on cholesterol and fat levels in the blood. A nut-enriched diet was found to be associated with a decrease in blood cholesterol. There are a few limitations which affect how these results can be interpreted:
- Although the data pooled results from 25 studies the overall population was relatively small. The small size increases the likelihood that the differences observed were due to chance.
- The 25 studies had different diets to which the nut supplementation was added. The researchers did not give details of the energy, fat and cholesterol content of these background diets, which may have varied, potentially affecting the collective results. In addition, the studies varied on the degree of dietary control that the researchers had over the participants, such as whether they checked compliance to the diets or advised on other foods that should be eaten or avoided during the experimental nut diet.
- The experimental diets were typically between three and eight weeks long, which is a relatively short period of time. It is unclear what effect a nut-enriched diet would have over the longer term.
- It is unclear whether the reductions in cholesterol and LDL-cholesterol would be enough to lower the risk of coronary heart disease. Further analysis is needed to assess how much an individual would need to lower their cholesterol in order to lower their risk of coronary heart disease in the long term.
These preliminary results warrant further investigation into how to optimise our diets to lower blood cholesterol levels. As many nuts are heavily salted or coated with sugar and other vegetable oils, people are advised to choose raw unsalted nuts. They should also be aware that although nuts are low in saturated fat, they are nevertheless very high in fat and calories, and should be eaten in moderation as part of a healthy diet.