Double-blind controlled dietary cross-over intervention with differentially fertilised intact lettuce leaves shows acute reduction in blood pressure in young adults, associated with faster uptake of nitrate than of phenolics

Abstract

Purpose: To compare acute effects on blood pressure (BP) of ingestion of visually similar lettuce with controlled high and low content of either nitrate or phenolic compounds.

Methods: In a randomised cross-over design, 19 healthy participants (22-31 years) received 50 g of lettuce containing either 530 mg (8.4 mmol) nitrate + 11 mg (0.03 mmol) phenolic compounds (HNLP); or 3 mg nitrate (0.05 mmol) + 77 mg (0.2 mmol) phenolic compounds (LNHP), obtained by differential fertilisation. Ambulatory BP was recorded along with plasma, salivary and urinary nitrate and nitrite and plasma concentrations of cyclic guanosine monophosphate (cGMP), phenolic metabolites, Trolox equivalent antioxidant capacity (TEAC) and ferric reducing antioxidant power (FRAP).

Results: Compared with LNHP, 3 h post ingestion of HNLP, plasma nitrate increased 0.31 ± (95%CI) 0.12 mM (+ 240%), and salivary nitrate 5.5 ± 1.4 mM (+ 910%); accumulated urinary nitrate excretion increased 188 ± 72 mg (+ 296%) (all P < 0.001). Systolic BP was reduced 4.9 ± 4.2 mmHg (P = 0.031) between 3 and 6 h after ingestion of HNLP compared with LNHP; systolic BP differences were negatively correlated (P = 0.004) with differences in saliva nitrate concentrations. LNHP increased plasma phenolics at 6 h, predominantly 3'-methoxycinnamic acid-4'-glucuronide (ferulic acid-4'-glucuronide), 116%, 204 ± 138 nM more than HNLP (P = 0.001); increased cGMP 14% (P = 0.019); and reduced FRAP 3.1% (P = 0.009).

Conclusion: The acute BP difference within 6 h of consumption matched the plasma/saliva nitrate peak, not the slower changes of plasma phenolics. This is the first double-blind controlled dietary intervention demonstrating differential effects on human physiology by consumption of an intact plant food, where compositional differences were obtained by controlling growing conditions, indicating potential opportunities for health claims relating to precision/vertical farming.

Clinical trial registration: The trial was retrospectively registered on ClinicalTrials.gov, with identifier NCT02701959, on March 8, 2016.

Keywords: Chlorogenic acid; Nitrate; Nitrite; Phytochemicals; Young healthy adults.

Conflict of interest statement

The authors declare that they have no conflict of interest.

© 2022. The Author(s).

Figures

Fig. 1

The design of each session in the intervention trial. At least 3 weeks between the two sessions; cross-over, with randomised, double-blind sequence of treatments; restrictions on nitrate-rich foods for 24 h before each session, and no mouthwash throughout the study period. Automated BP measurements from before lettuce consumption until 24 h after. Standard meals consumed 12 h before, 4 h after and 12 h after lettuce consumption. Samples at − 1, 9, 12 and approx. 22 h collected by the participants at home, all other samples collected at the research facility

Fig. 2

CONSORT flow chart for the cross-over design. The participant whose data were excluded consistently excreted substantially more NO3− in the urine than was provided in the intervention and other foods

Fig. 3

The effect of HNLP and LNHP lettuce consumption on 24-h ABPM systolic and diastolic BP. Average values for each 3-h interval (values recorded up to 3 h before) are shown as circles, SEM are shown as error bars, *Indicates a significant difference (P = 0.031). N = 19, each data point consisting of an average of 3–6 measurements (less frequent during the night)

Fig. 4

The effect of HNLP and LNHP lettuce consumption on the concentrations of a plasma NO3−, b salivary NO3− and NO2−, c urinary NO3− and NO2−, all in µM. Urinary NO3− excretion (d) is in mg/h and accumulated excretion in mg. Data are all shown on log scales, as mean ± 95% CI, with N = 19

Fig. 5

Differences between values after HNLP and LNHP lettuce consumption for salivary NO3− and corresponding values for systolic blood pressure, as an average value for each person during each time period where saliva samples were collected. Small white circles are individual measurements (n = 190, with 10 values from each person), large patterned circles show the average BP difference for each 2-mM range of salivary NO3−-values. The dotted line is the regression trend line with formula and R2 value (with P = 0.0036), the two dashed lines show the 95% CI corresponding to the trend

Fig. 6

The effect of HNLP and LNHP lettuce consumption on plasma concentrations of the sum of all measured phenolic compounds. a shows absolute values, b changes from baseline. Data expressed as mean ± 95% CI, N = 19

Fig. 7

The effect of HNLP and LNHP lettuce consumption on a FRAP (ferric reducing antioxidant power) and b TEAC (Trolox equivalent antioxidant capacity). Data expressed as mean ± 95% CI

Fig. 8

The effect of HNLP and LNHP lettuce consumption on the plasma concentration of cGMP, cyclic guanosine monophosphate. a shows absolute values, b changes from baseline. Data expressed as mean ± 95% CI