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By Ellen Feldman, MD
Altru Health System, Grand Forks, ND
Dr. Feldman reports no financial relationships relevant to this field of study.
SYNOPSIS: A long-term, large-scale prospective study reveals an association between increased consumption of nuts, decreased weight gain, and decreased risk of obesity.
SOURCE: Liu X, Li Y, Guasch-Ferré M, et al. Changes in nut consumption influence long-term weight change in US men and women. BMJ Nutr Prev Health 2019;bmjnph-2019-000034. doi:10.1136/bmjnph-2019-000034. [Epub ahead of print].
• This analysis incorporated data from three prospective, longitudinal studies: 27,521 men from the Health Professionals Follow-Up Study; 61,680 women from the Nurses’ Health Study; and 55,684 women from the Nurses’ Health Study II.
• Weight change, changes in total nut consumption, and consumption of individual types of nuts were among the data collected.
• The association between these two parameters was investigated over consecutive four-year periods for 20-24 years.
• An increase in total nut consumption by 0.5 servings daily was associated with significantly less weight gain (P < 0.01) and lower risk of obesity (P = 0.0036).
Almost 40% of U.S. adults are classified as obese, with a body mass index (BMI) ≥ 30 kg/m2.
Because obesity is directly related to a range of disorders, such as type 2 diabetes mellitus, obesity-related cancers, and cardiovascular disorders, identification of measures to control or prevent obesity is a growing public health priority.1 Yet, obesity remains a complex condition to tackle. The etiology is multi-pronged and includes a range of sociologic, genetic, and even psychological factors. Adulthood only seems to complicate matters further: U.S. adults gain an average of one pound yearly.1,2
Primary prevention programs addressing obesity and weight gain in adults have focused on activity and dietary changes, including an emphasis on decreasing soda consumption. These interventions may be effective; there has been a slight drop in sugary drink consumption and concurrent measurable increase in nut consumption among the public. Nut consumption has risen from 0.5 servings daily in 1999 to 0.75 servings daily in 2012. However, there are concerns that despite the known health benefits of nuts, their high fat content is a factor in weight gain.3,4
In this study, Liu et al carefully investigated the relationship of changes in nut consumption and concurrent weight change by analyzing responses from ongoing, long-term studies involving nutrition every four years over a 20- to 24-year period.
Liu et al used three well-known, longitudinal, prospective studies conducted via responses to self-reported questionnaires. The Health Professionals Follow-Up Study (HPFS) began in 1986 and enrolled more than 50,000 male health professional adults age 40-75 years.5 The Nurses’ Health Study (NHS) began in 1976 with more than 100,000 nurses age 35-55 years, and the Nurses’ Health Study II (NHS II) began in 1989 with a similar number, but a younger set of nurses (age 24-44 years).6 Notably, the participants in all of the studies are primarily Caucasian, well-educated, and involved in healthcare.
These ongoing studies all collect information about nutrition, health habits, and disease via questionnaires mailed biannually, including a food frequency questionnaire (FFQ) every four years. The FFQ contains questions about nut consumption frequency, with a serving defined as 28 g, or 1 oz., of nuts. Although peanuts technically are legumes, they were included as part of overall nut consumption and measured individually as well. Peanut butter also was included, with a serving defined as one tablespoon (15 g).5,6
For the purposes of this analysis, Liu et al converted the questionnaire frequency measures to servings/day and looked not only at total nut consumption, but also at consumption of tree nuts (including walnuts, but not peanuts), walnuts, other tree nuts, peanuts, and peanut butter. The researchers then looked at the difference in weight over each four-year interval when the FFQ was administered.
Exclusions for this study included participants who did not complete the FFQ, those who were missing a BMI at baseline, or those who had specific medical diagnoses at baseline, such as cancer or diabetes mellitus. The final numbers of participants eligible for analysis for this study included 27,521 men from the HPFS and 117,364 women from the NHS and NHS II combined.
The multivariable analysis was comprehensive; data were adjusted for factors including age, BMI, physical activity, and relevant health habits, including quality of overall diet and caloric intake.
The results from the HPFS and NHS were analyzed from 1986 until 2010, while the NHS II results included data from 1991 until 2011. During this time span, the average weight gain among participants across all studies was 0.71 pounds yearly. Total nut consumption increased in each of the groups.
Increasing total consumption of nuts by more than 0.5 serving a day, or about 12 almonds daily, was associated with 0.419 pound less weight gain over a four-year period. Table 1 shows more specific data for each type of nut. All values in this category have a P < 0.01 (except for changes in peanut consumption in the NHS only).
Table 1 also depicts the risk estimates for both weight gain and becoming obese associated with increasing total and specific nut consumption by more than 0.5 servings daily.
The authors of this impressive study looking at long-term data regarding nut consumption and weight change showed findings consistent with several other shorter-term studies from the same respondent pools (HPFS, NHS, and NHS II) and from a five-year European study.7,8 One strength of the Liu et al study includes combining the cohorts to look at data from both men and women. The study design, looking at weight changes associated with nut consumption every four years over a 24-year period, allowed time for a dietary change to affect weight and to observe such changes over a significant period of time. The data collected in the questionnaires permit adjustment for many other lifestyle changes and health factors.
Still, this remains an observational study. Causation is not assured and confounding by unknown or unanticipated factors is a real possibility. Additionally, generalization is limited by the homogeneity of the study population.
Another strength of the study is that the specificity of the questionnaires allowed analysis based on nut type. Future studies need to identify and analyze association of weight changes with specific preparation and presentation of nuts (e.g., raw, salted, packaged, incorporated in foods, etc.). The data on peanut and peanut butter consumption suggest that even these calorie-dense food items were not associated with obesity or significant weight gain.
It is important to note that this study did not measure absolute weight loss, as the focus of this work is on measuring changes in weight gain over time. Teasing out more specifics in the trajectory of weight will be a helpful component for future studies.
Liu et al speculated that the connection between nut consumption and lower risk of weight gain may be multifactorial, including the mechanical effort in chewing, high fiber content, and increase in resting energy expenditure due to the high content of unsaturated fatty acids in nuts. Certainly, future investigations will work toward defining a clear etiological pathway.
Future studies also are needed to determine if there is an upper boundary for quantity of nut consumption (i.e., when consumption of nuts is no longer associated with reduced risk of weight gain). Researchers for this study looked at an increase of > 0.5 servings of nuts consumed daily, but did not identify if there is such a point at which increased consumption of nuts becomes associated with weight gain.
For now, we can advise patients that an increase of nut consumption of at least 0.5 serving (0.5 oz.) daily is associated with reduced weight gain over time. In concrete terms, this may translate to one of the following: 12 almonds, nine medium-sized cashews, four brazil nuts, or 17 peanuts.9 It is reasonable to inform patients that although the jury is still out on specifics, tree nuts appear to have a stronger association with this pattern than peanuts or peanut butter. Incorporating this information into a general visit can serve as a bridge to a more general discussion regarding diet and the essential role of nutrition in health.
Financial Disclosure: Integrative Medicine Alert’s Executive Editor David Kiefer, MD; Peer Reviewer Suhani Bora, MD; Associate Editor Journey Roberts; Editor Jason Schneider; Relias Media Editorial Group Manager Leslie Coplin; and Accreditations Manager Amy M. Johnson, MSN, RN, CPN, report no financial relationships relevant to this field of study.