Berryman CE, Grieger JA, West SG, et al. Acute consumption of walnuts and walnut components differentially affect postprandial lipemia, endothelial function, oxidative stress, and cholesterol efflux in humans with mild hypercholesterolemia. J Nutr. June 2013;143(6):788-794.

While walnuts (Juglans regia) contain a variety of compounds (polyphenols, γ-tocopherol, α-linolenic acid [ALA], linoleic acid, and L-arginine) that may contribute to their cardioprotective effect, there has been no previous research on the biological effects of whole walnuts versus different walnut components (i.e., skin, oil, defatted nutmeat) on these cardiovascular parameters. This randomized, controlled, postprandial, 4-period crossover study analyzed the effects of whole walnuts, walnut skin, walnut oil, and defatted walnut meat on postprandial lipid/lipoprotein responses, endothelial function, and oxidative stress in humans. In addition, the effect of postprandial intake of whole walnuts on ex vivo cholesterol efflux was evaluated.

The study was conducted at the General Clinical Research Center of the Pennsylvania State University in University Park, Pennsylvania. A small study group (n = 15) of 9 women and 6 men aged 21-60 years with a body mass index (BMI) of 25-39 kg/m², a low-density lipoprotein (LDL) cholesterol level of ≥ 110 mg/dL, and triglyceride (TG) levels < 350 mg/dL were recruited. The study had at least 1 week of separation between testing sessions. Each visit evaluated the oral ingestion of 1 treatment: (1) 85 g (3 oz) of ground, whole walnuts; (2) 34 g of ground, skinless, defatted nutmeat from walnuts; (3) 51 g of oil from skinless walnuts; or (4) 5.6 g of ground walnut skins. Unroasted English walnuts were blanched in hot water for 1 minute then submerged in ice-cold water for 30 seconds. Research staff removed the skins, ground the skinless walnuts with a mortar and pestle, and extracted the oil from the nutmeat with hexane. Diet Jell-O^® was used as a food carrier for the whole walnuts and walnut components.

Subjects were instructed on how to consume low-antioxidant foods 3 days prior to each test visit to help maximize the potential for observing effects with the high-antioxidant walnut and walnut component treatments. After a 12-hour overnight fast, subjects consumed 1 of 4 walnut-Jell-O test meals. Blood samples were taken at 30, 60, 120, 240, and 360 minutes following consumption of the test meal; and an endothelial function test was done immediately prior to the 240-minute blood draw. Twenty subjects were randomly assigned to the study. One subject withdrew due to a vagal response following the blood draw; 1 subject was excluded due to noncompliance with the antioxidant and fasting requirements; and 3 subjects withdrew due to nausea following the walnut meal, the inability to consume the low-antioxidant diet, and work commitments. Nine subjects reported having loose stools/diarrhea between 1 and 9 hours post-walnut oil treatment.

The mixed models procedure was used to test the effects of treatment, time point, visit, and their interaction (treatment × time point and treatment × visit). There was a treatment × time point interaction for TGs. Peak TG responses for the oil and whole nut treatments were observed at 120 and 240 minutes post-meal. Changes in TGs were greater following the oil and whole nut treatments compared to the nut skin treatment. There was an 89% reduction in the postprandial TG area under the curve (AUC) for the nut skin treatment relative to both the oil and whole nut treatments. There were no treatment × time point interactions or treatment effects for total, LDL, or high-density lipoprotein (HDL) cholesterol.

The post-meal ferric reducing antioxidant potential (FRAP) response differed by treatment. The nutmeat treatment tended to lower mean FRAP from baseline (P = 0.054) such that nutmeat differed from the oil and nut skin treatments (P < 0.01). There were no significant treatment × time interactions or treatment effects for plasma total thiols and malondialdehyde. The authors observed that differences in plasma FRAP may be due to a lack of bioactive components in the nutmeat versus the oil, skin, and whole walnut treatments.

There were no treatment effects on arterial stiffness; however, treatment effects for endothelial function, measured by the Reactive Hyperemia Index (RHI), were observed. The walnut oil treatment preserved endothelial function compared with whole walnut and skin treatments. The nut skin treatment actually reduced the RHI compared with baseline (P = 0.02) such that the oil and nut skin treatments differed (P = 0.01).

The heart rates of the participants differed across treatments (P < 0.01). The postprandial heart rate was greater with the oil (P = 0.01) and whole nut (P = 0.02) treatments compared to the nut skin treatment. The oil and whole nut treatments also increased heart rate from baseline (P < 0.05 for both).

Cholesterol efflux is an important step in the reverse cholesterol transport pathway and, independent of apolipoprotein A-1 (apoA-1) and HDL, is a strong predictor of both carotid intima-media thickness and coronary artery disease. Acute consumption of walnuts increased cholesterol efflux by 3.3% in cells cultured with postprandial serum relative to the fasting baseline (P = 0.02). Cholesterol efflux was positively correlated with plasma total cholesterol (r = 0.78), LDL cholesterol (r = 0.70), and HDL cholesterol (r = 0.43) concentrations (P ≤ 0.02 for all). In cells cultured with apolipoprotein B (apoB)-depleted serum, there were no significant changes in cholesterol efflux.

The authors suggest that walnut oil may have elicited a higher endothelial response and a more rapid TG response due to the increased bioavailability of ALA, γ-tocopherol, and polyphenols when unhindered by a complex food matrix. Endothelial function decreased with walnut skin consumption, which the authors attribute to a possible lack of antioxidant absorption due to the food matrix used or the lack of ALA in the skins. Further studies could be conducted to examine the potential difference in the bioavailability of ALA and other compounds in a more accessible food form compared with that of a complex food matrix, such as whole walnuts.

The authors conclude that, to their knowledge, this is the first study to directly compare and evaluate the cardioprotective effects of individual walnut components versus whole walnuts. While the sample size was small (n = 15) and the design lacked a control group, significant effects for some of these walnut components on TGs, heart rate, endothelial function, cholesterol efflux, and FRAP were observed and should be verified in a larger, more diverse population against a control group.