By Laura Cunningham
Canadians consume an average of 3400 mg of sodium daily, far more than the recommended allowance of 2300 mg(1), with 85% of men and 60% of women aged 9 to 70 consuming over the upper recommended limit (2). 90% of dietary sodium is consumed as sodium chloride (NaCl) (3), and sodium is necessary to assist in cell function and blood volume control, however overconsumption is responsible for approximately 30% of all hypertension (high blood pressure) diagnosed in Canada (4). Hypertension is well established as a major risk factor for heart disease, stroke, kidney failure, and blood vessel diseases of the eye and brain (5). Optimal blood pressure (BP) is when the systolic BP is equal to or less than 120 mmHg and the diastolic BP is equal to or less than 80 mmHg. In a doctor’s office, BP is considered high when the systolic BP is equal to or above 140 mmHg or diastolic BP is equal to or above 90 mmHg. When measured at home, BP above 135 mmHg systolic or 85 mmHg diastolic is considered high (6).
Although fast food is known to contain high levels of salt, its convenience and taste attract consumers across a broad spectrum. As a result, the Canadian fast food industry continues to expand each year, generating annual sales of approximately $26 billion in 2015 (7). Appraisal of Canadian fast food nutrition charts indicates that many fast food meals (main, side, drink, and dessert) are well over the recommended daily sodium allowance, and sodium levels of many items in Canada are higher than the same items sold by the same chains in other countries8. Studies evaluating a correlation between ingestion of a sodium load equivalent to a fast food meal and BP changes a short time afterward in healthy adults were not found in the literature. Thus, this study investigates whether consuming the equivalent amount of sodium in one average fast food meal (3000 mg) increases the BP of healthy adults shortly after consumption. Furthermore, to isolate the sodium component from other ingredients in the fast food meal, a drinkable beverage containing 3000 mg of sodium was designed.
The purpose of this experiment is to determine whether consuming the equivalent amount of sodium in a fast food meal (3000 mg) causes a change in the BP of healthy adults 30 minutes after consumption, compared to a control group consuming an equivalent volume of tap water.
i) If the experimental participants consume 250 mL of tap water, their systolic and diastolic BP will not increase from their baseline 30 minutes later.
ii) If the experimental participants then consume 250 mL of a sodium beverage, both their systolic and diastolic BP will increase from their baseline 30 minutes later.
18 healthy adult volunteer participants, 6 males and 12 females, between the ages of 23 and 60, were selected from among friends, family, former teachers, and medical professionals. Participants were contacted by email and telephone, and after receiving a Letter of Information, each provided written voluntary informed consent according to Youth Science Canada Ethics Guidelines. For each participant, gender, age, weight, and height were recorded. It was confirmed with the participant that pre-experimental protocol was followed: no consumption of alcohol, caffeine, tobacco, or salty food/ beverages for at least three hours prior to beginning either Part 1 or Part 2 of the experiment, and no exercise within 30 minutes of commencement of the experiment. BP measurement protocol was accurately followed: participant seated comfortably with no distractions such as TV or conversation, feet flat on floor, back rested against firm surface for minimum 5 minutes, arm on table, cuff at level of participant’s heart and placed on bare non-dominant arm, 3 cm above elbow.
Part 1 - Baseline BP and Water Control Experiment
1. Using a calibrated A & D Deluxe Connected Blood Pressure Cuff/Monitor (Model UA-651BLE), participant’s BP was taken three times, waiting one minute between each recording. These three recordings were averaged to obtain average baseline BP.
2. The participant was given 250 mL of room temperature tap water to drink within one minute.
3. After 30 minutes, the participant’s BP was taken three times, waiting one minute between each recording. These three recordings were averaged to obtain average post-water BP (this BP was used as a baseline for Part 2).
Part 2 - Sodium Beverage Experiment (Immediately following Part 1)
4. The experimental sodium beverage was prepared by mixing one 4500 mg packet of powdered Oxo chicken or beef bouillon (710 mg sodium) and 6026 mg salt (2290 mg sodium) or one half cube (5300 mg) of McCormick’s powdered vegetable bouillon (890 mg sodium) and 5553 mg salt (2110 mg sodium) into a cup with 250 mL boiled tap water and stirring well.
5. The participant was given a choice of sodium beverage (chicken/ beef/ vegetable) to drink within one minute.
6. After 30 minutes, the participant’s BP was taken three times, waiting one minute between each recording. These three recordings were averaged to obtain average post-sodium BP.
The mean change in BP (systolic and diastolic) between the baseline and 30 minutes post-water intake was calculated for each participant, and Student’s T-test was used to determine whether each mean change was statistically significant. The mean change in BP (systolic and diastolic) between the baseline (post-water intake) and 30 minutes post-salt intake was calculated for each participant, and Student’s T-test was used to determine whether each mean change was statistically significant.
On average, when a participant consumed 250 mL of tap water, neither systolic nor diastolic BP changed significantly 30 minutes after consumption. On average, when a participant consumed the equivalent amount of sodium found in an average fast food meal (3000 mg), both systolic and diastolic BP increased significantly 30 minutes after consumption (Figure 1).
Using the home BP measurement of 135/85 as ‘high’, at baseline one participant was mildly hypertensive with a diastolic BP of 90. The salty beverage significantly increased the systolic and diastolic BP of the group (systolic 7.7mmHg, ± 9.2, p < 0.01) and (diastolic 3.6mmHg, ± 4.8, p < 0.01) compared to water which had no effect (systolic -0.7mmHg, ± 3.9, p=NS) and (diastolic -0.6mmHg, ± 4.5, p=NS), (Figure 2). The magnitude of this change was sufficient to advance six more participants into the hypertensive category 30 minutes after consuming the sodium beverage.
Drinking a beverage containing 3000 mg of sodium significantly increased both systolic and diastolic blood pressure 30 minutes after consumption, while drinking the same volume of tap water did not significantly affect blood pressure. These findings suggest that the sodium content in an average fast food meal (3000 mg) is sufficient to increase BP in healthy adults a short time after the meal is consumed.
Using the home high BP monitoring standard, one third of participants whose BPs were in the normal range before drinking the sodium beverage, were in the hypertensive range afterward, though sustained high BP over a longer timeframe is required for an individual to be considered hypertensive. Why a few participants showed little or no change in BP after consuming the sodium beverage might be explained by determining, in future studies, whether they are more salt-resistant than other participants who may be predisposed to salt-sensitivity.
Many fast food meals contain much more than 3000 mg of sodium, but this amount of sodium was used in the experimental design because most people could not easily tolerate a saltier beverage, the flavour was acceptable, and other ingredients in the bouillon are present only in very small or trace amounts. When a large amount of salt is disguised in food, it is more easily consumed. None of the participants felt unwell during or after the sodium beverage part of the experiment.
Many previous scientific studies have demonstrated that excessive dietary salt intake for long periods of time can cause hypertension, a major risk factor for heart disease, stroke, kidney failure, and blood vessel diseases of the eye and brain. It is estimated that reducing dietary sodium may decrease hypertension prevalence by 30% (9). In 2016, about 2 million Canadians had hypertension caused by excess dietary sodium (10). Further, 93% of Canadian children aged 4 to 8 years are exceeding the tolerable upper intake sodium level, putting them at risk for eventual hypertension (11). Fast food is commonly eaten because it is convenient for busy students and families and it is considered tasty. The flavour of the food is largely attributed to the sodium content, in the form of sodium chloride. Many people may not be aware that just one salty meal has the potential to increase their BP. Furthermore, few people even know the sodium content of various fast food products, and although occasional bursts of high sodium intake might not be harmful, habitually eating fast foods and prepared or packaged foods can lead to hypertension. Many perceive that a meal is healthy to eat if it is advertised as low sodium, yet many low sodium products still exceed the % DV or (% Daily Value) recommended by health officials (5%-15%) (12).
Much of the population, particularly those who have other risk factors, may be at increased risk by consuming just one high-sodium meal. Acute heart failure is an illness that hits suddenly and without any earlier symptoms and several risk factors, alone or together, increase the likelihood of heart failure. One risk factor such as high BP can be enough to trigger heart failure (13), and literature suggests that hospitalizations for heart failure spike after major holidays (14). Short-term changes in BP can also become chronic over time (15). A persistent high-sodium diet may result in hypertension (‘the silent killer’) quietly damaging vessels and organs for years before symptoms develop. The increased pressure of blood on arterial walls causes small tears that, when healed, cause scarring that reduces vessel compliance, limiting blood flow. Scarred areas also collect plaque causing vessel narrowing while other weakened areas enlarge and form aneurysms (16).
Furthermore, in Canada, years of public awareness regarding dietary sodium reduction have had little effect on consumer behaviour. Meaningful industry self-regulation has also been minimal. In 2013 Bill C-460, A Sodium Reduction Strategy for Canada failed in the House of Commons. The Strategy included sodium reduction goals for and increased accountability of fast food and other restaurants (17). The findings in this study may be relevant to Canadian public policy experts who met in 2017, to evaluate the food industry’s efforts to meet the sodium reduction targets for sodium in processed foods. Only 14% of food categories met the targeted reduction; 48% did not make any meaningful progress toward sodium reduction; and 30% lowered sodium content to levels similar to other foods in the same category (18).
Among healthy adults, consumption of a high-sodium beverage (3000 mg sodium), equivalent to the amount of sodium in an average fast food meal, caused a significant increase in both systolic and diastolic BP 30 minutes after the beverage was consumed. In the same participant group, there was no increase in BP after drinking the same volume of tap water. One third of participants progressed from a baseline normal BP to a high BP 30 minutes after drinking the sodium beverage, suggesting that the sodium content in an average fast food meal is sufficient to elevate BP in healthy adults. These results could provide useful information to a physician evaluating a patient with a typically normal BP who may have an elevated BP in the office setting, prompting the physician to consider asking the patient about consumption of a high sodium meal just prior to the appointment. Furthermore, these findings may be useful to those potentially at risk of acute heart failure due to sudden surges in BP, including transient increases due to consumption of meals high in sodium.
Canadians of all ages consume too much sodium. More robust efforts on the part of government and the food industry are needed to reduce sodium in processed, packaged, and fast food in Canada, to fully address the high rates of chronic disease in our communities.
1 Health Canada. Food and Nutrition - Sodium in Canada. Available at: http://www.hc-sc.gc.ca/fn-an/nutrition/sodium/index-eng.php. Accessed April 8, 2017.
2 Campbell N., Johnson J., Campbell T. Sodium Consumption: An Individual’s Choice? Int J Hypertens. 2012;2012[Article ID 860954]: 6 pp.
3 Mattes RD, Donnelly D. Relative contributions of dietary sodium sources. J Am Coll Nutr. 1991;10:4:383-393.
4 Campbell N., Willis K.J., L’Abbe M., et al. Canadian Initiatives to Prevent Hypertension by Reducing Dietary Sodium. Nutrients. 2011;Aug; 3(8):756-764.
5 Mayo Clinic. High Blood Pressure Dangers: Hypertension’s Effects on Your Body. Available at: https://www.mayoclinic.org/diseases-conditions/high-blood-pressure/in-depth/high-blood-pressure/art-20045868. Accessed July 16, 2018.
6 Public Health Agency of Canada. Chronic Diseases – Hypertension. Available at: http://www.phac-aspc.gc.ca/cd-mc/cvd-mcv/hypertension-eng.php. Accessed April 8, 2017.
7 Statista: The Statistics Portal. Sales of quick-service restaurants in Canada from 2010 to 2015 (in billion Canadian dollars). Available at: https://www.statista.com/statistics/323626/sales-of-quick-service-restaurants-in-canada/. Accessed April 8, 2017.
8 Dunford E., Webster J., Woodward M., Czernichow S., Yuan W., Jenner K., Mhurchu C., Jacobson M., Campbell N., Neal B. The variability of reported salt levels in fast foods across six countries: opportunities for salt reduction. CMAJ. June 12, 2012;184(9):1023-1028.
9 Joffres M.R., Campbell N, Manns B, Tu K. Estimate of the benefits of a population-based reduction in dietary sodium additives on hypertension and its related health care costs in Canada. Can J Cardiol. 2007; May 1; 23(6): 437-443.
10 CBC News. Salt is worse than sugar, says Canada’s leading sodium doctor. Available at: https://www.cbc.ca/news/canada/calgary/salt-hypertension-health-concerns-1.3354724. Accessed July 17, 2018.
11 Health Canada. Sodium in Canada. Available at: https://www.canada.ca/en/health-canada/services/food-nutrition/healthy-eating/sodium.html. Accessed July 17, 2018.
12 Health Canada. Using the Nutrition Facts Table: % Daily Value. Available at: https://www.canada.ca/content/dam/canada/health-canada/migration/healthy-canadians/alt/pdf/publications/eating-nutrition/label-etiquetage/fact-fiche-eng.pdf. Accessed July 17, 2018.
13 Franks, I. Acute heart failure: What you need to know. Medical News Today. [Newsletter]. Fri Jun 2, 2017. Available at: https://www.medicalnewstoday.com/articles/317737.php. Accessed July 17, 2018.
14 Shah M., Bhalla, V., Patnaik S., et al. Heart failure and the holidays. Clin Res Cardiol. 2016;105:865.
15 Mayo Clinic Staff. High blood pressure dangers: Hypertension’s effects on your body. [Mayo Clinic website]. Available at: http://www.mayoclinic.org/diseases-conditions/high-blood-pressure/in-depth/high-blood-pressure/art-20045868. Accessed April 8, 2017.
16 Health 24 – Cholesterol. Atherosclerosis. Feb 27, 2018. Available at: https://www.health24.com/Medical/Cholesterol/Cholesterol-associated-disorders/Atherosclerosis-20120721. Accessed July 17, 2018.
17 Open Parliament. Bill C-460 (Historical). Sodium Reduction Strategy for Canada Act. Available at: https://openparliament.ca/bills/41-1/C-460/. Accessed July 17, 2018.
18 Government of Canada.Sodium Reduction in Processed Foods in Canada: An Evaluation of Progress toward Voluntary Targets from 2012 to 2016.. Available at: https://www.canada.ca/en/health-canada/services/food-nutrition/legislation-guidelines/guidance-documents/guidance-food-industry-reducing-sodium-processed-foods-progress-report-2017.html Accessed July 17, 2018.
About the Author
I am a 15-year-old travel enthusiast and I attend St. Thomas Aquinas Regional High School, North Vancouver, where I play clarinet in concert band and saxophone in jazz band. I also enjoy singing in a chamber choir, writing for my school’s student newspaper, and volunteering at a senior’s care centre and local library. For the past 9 years I have spent a great deal of time with horses, primarily as an equestrian rider and volunteer at summer riding camps. Studying science satisfies my curiosity about the world and it’s complexities, providing a never-ending source of questions and answers. My project, Salt in Fast Food: No Great Shakes For Your Health, won a Gold Award and Grand Award (Junior Division) at the Greater Vancouver Regional Science Fair and a Bronze Medal at the 2017 Canada Wide Science Fair.