Dr Martin Juneau, M.D., FRCPCardiologue et Directeur de la prévention, Institut de Cardiologie de Montréal. Professeur titulaire de clinique, Faculté de médecine de l'Université de Montréal. / Cardiologist and Director of Prevention, Montreal Heart Institute. Clinical Professor, Faculty of Medicine, University of Montreal.19 March 2018
The recent death of eminent American researcher Gerald Reaven, nicknamed the “father of insulin resistance,” is a good opportunity to recall the leading role this metabolic disorder plays in the development of type 2 diabetes and cardiovascular disease.
What is insulin resistance?
After a meal, insulin is secreted by the pancreas to signal to the body that circulating sugar levels need to be lowered, either by capturing it in the muscles and adipose tissue, or by promoting its storage in the liver. Under normal conditions, this mechanism is highly accurate and helps to keep the blood sugar level at an adequate level.
In people who are overweight, and especially those whose excess fat is located at the abdominal level, this insulin action is often disrupted and organs are no longer able to capture and store sugar effectively; they are said to be “insulin-resistant”. In its early stages, this insulin resistance often goes unnoticed because the pancreas is able to produce larger amounts of the hormone to compensate for this loss of effectiveness and thus allows organs to continue to collect and store enough sugar (see left portion of the figure). This compensatory hyperinsulinemia makes it possible to maintain blood glucose at approximately normal levels, but it unfortunately causes several metabolic abnormalities that can lead to the development of certain serious diseases. For example, excess insulin stimulates the production of triglycerides by the liver, which promotes the accumulation of fat and can result in the development of hepatic steatosis (fatty liver). Increased secretion of these fats into the bloodstream causes dyslipidemia, characterized by high triglycerides, an increase in LDL cholesterol, and a decrease in HDL cholesterol. Meanwhile, hyperinsulinemia increases sodium retention in the kidneys, contributing to the increased incidence of hypertension seen in insulin-resistant individuals.
All of these factors (dyslipidemia, hepatic steatosis, hypertension), combined with increased inflammation and a change in the properties of the endothelial cells lining the blood vessels (inflammation, procoagulant properties), make insulin resistance an important risk factor for cardiovascular disease.
Type 2 diabetes
In the longer term, overproduction of insulin can lead to pancreas depletion, which ultimately leads to the cessation of hormone production and the onset of type 2 diabetes, i.e., a state of chronic hyperglycaemia (see the right portion of the figure). This excess of blood sugar is very harmful to the blood vessels and significantly increases the risk of cardiovascular disease (heart attack and stroke) as well as damage to tissues whose function depends on the small blood vessels such as the retina, kidneys or nerves. Insulin resistance can be considered as a prediabetic state, the harbinger of diabetes developing insidiously.
“Excess abdominal fat should be considered as the first clinical sign of insulin resistance.”
Stay alert
One problem with insulin resistance is that it is often difficult to diagnose at an early stage, not only because it does not cause clinical symptoms but also because blood glucose is normal. As mentioned earlier, in the early stages of resistance, the pancreas offsets the loss of insulin efficiency by secreting larger amounts of the hormone, which is sufficient to maintain blood sugar at normal levels. Patients (just like their doctors) then have the false impression that they are in perfect health, even if in fact they are prediabetic and will become diabetic in the coming years if nothing is done. Overall, the studies suggest that a slight elevation of glycated haemoglobin (Hb1Ac 5.5% and above), a marker of chronic hyperglycaemia, may be a better approach for early detection of insulin resistance than tests that are used to measure blood glucose (fasting glucose, glucose tolerance). For example, it has recently been shown that people with normal fasting glucose but a HbA1c greater than 5.9% were eight times more likely to develop diabetes in the next four years than those whose HbA1c was less than 5.7%.
In short, it is important to remain vigilant and realize that excess fat, although somehow becoming the norm in our society (more than 60% of Canadians are overweight), is far from harmless. In practice, excess abdominal fat (waist circumference greater than 102 cm for men and 88 cm for women) should be considered as the first clinical sign of insulin resistance and an increased risk of developing type 2 diabetes, with disastrous consequences for cardiovascular health.
Fortunately, insulin resistance is not an irreversible phenomenon: several studies show that people with glucose metabolism disorders can reverse the situation by simply changing their lifestyle. For example, a recent study reports that the adoption of a diet consisting primarily of low-fat plant foods is associated with a significant improvement in insulin sensitivity in overweight individuals. Being more active also seems beneficial: a study of 44,828 Chinese adults (20–80 years old) with above-average fasting blood glucose showed that people who were the most physically active were 25% less likely to develop type 2 diabetes.
Dr Martin Juneau, M.D., FRCPCardiologue et Directeur de la prévention, Institut de Cardiologie de Montréal. Professeur titulaire de clinique, Faculté de médecine de l'Université de Montréal. / Cardiologist and Director of Prevention, Montreal Heart Institute. Clinical Professor, Faculty of Medicine, University of Montreal.15 March 2018
Updated May 23, 2018
Nitrates (NO3–) and nitrites (NO2–) are mostly known to the public as undesirable residues of the agri-food chain as they are associated with potentially carcinogenic effects. Yet, these molecules are naturally found in fruits and vegetables (nitrates) as well as in the human body (nitrates and nitrites) where they contribute to important physiological functions, particularly in the cardiovascular system. Moreover, it has now been proven that dietary nitrates can be beneficial to cardiovascular health and sports performance, as will be discussed below.
Nitrates and Nitrites: Dangerous or Harmless?
During the curing process used to transform meats into charcuterie (ham, sausages, bacon, etc.), nitrite salt is added to stabilize the colour and taste of meats and to prevent the development of pathogenic microorganisms. Nitrite salt is in fact very effective in preventing the proliferation of bacteria, including the formidable Clostridium botulinum, which produces a powerful toxin that causes botulism, a very serious, sometimes deadly, paralytic illness. Nitrates and nitrites themselves are not carcinogenic; rather, it is N-nitroso compounds, such as nitrosamines, produced by the reaction between nitrites and meat protein that are. The curing process promotes the formation of nitrosamines due to the abundance of added nitrites, proteins and myoglobin whose heme group accelerates the reaction. Cooking at high temperatures (deep-frying) greatly accelerates the formation of nitrosamines. Government regulations limit the quantity of nitrites used to cure meats and requires the addition of neutralizing agents (antioxidants) in certain products, for example bacon. Nitrates naturally present in food mainly come from fruits and vegetables, which contain antioxidants, such as vitamin C and polyphenols that prevent the formation of N-nitroso compounds.
Up until about twenty years ago, nitrates and nitrites found in the human body were considered inert final products of the metabolism of nitric oxide (NO), a gas that acts as a signalling molecule and contributes to the regulation of blood flow and several other physiological functions. In the presence of oxygen, nitric oxide is produced in the endothelial cells that line blood vessels through the oxidizing reaction of the amino acid L-arginine into NO and L-citrulline. Several medications used to treat heart disease increase the signalling pathway of NO, either by increasing its bioavailability or by inhibiting its degradation. The most well-known are organic nitrates (e.g. nitroglycerine). They act by releasing NO rapidly and induce non-specific dilatation of both arteries and veins, which improves blood flow. Other pharmacological agents are phosphodiesterase-5 inhibitors, which are used to treat pulmonary hypertension and erectile dysfunction (e.g. sildenafil, better known by the brand name Viagra). Moreover, inhibitors of the HMG reductase enzyme (statins) and of the angiotensin-converting enzyme indirectly increase the bioavailability of NO.
Since 2001, we know that endogenous nitrites are an important alternative source of NO, particularly when oxygen levels are low, as is the case with blood microcirculation (see Figure 1). At that time, it was thought that the intake of nitrates and nitrites from food sources had no effect on blood vessels, since it was not thought that this intake could increase the circulating concentration of nitrites. We now know that dietary nitrates are quickly absorbed in the small intestine, about 75% of nitrates are excreted by the kidneys, and what is left becomes highly concentrated in the salivary glands (10 times the plasma concentration). When nitrates are secreted in saliva, they are converted to nitrites by the commensal bacteria, then swallowed with the saliva and absorbed into intestinal circulation. The circulating nitrites can be transformed into nitric oxide by different enzymes (reductases).
Figure 1. Formation and recycling of nitrates (NO3–), nitrites (NO2–) and nitric oxide (NO). Adapted from Woessner et al., 2017. In the presence of oxygen, endothelial nitric oxide synthase (eNOS) catalyzes the oxidation of L-arginine to NO. NO can also be quickly oxidized into nitrites and nitrates. A secondary source of vascular NO is obtained through diet. Consumption of foods high in inorganic nitrates (green leafy vegetables, beetroot) has been shown to increase plasma nitrate concentration,which can be secreted in saliva and reduced to nitrite by commensal bacteria in the mouth. Nitrites can then be further reduced to NO (and other biologically active nitrogen oxides) via several mechanisms that are expedited under hypoxic conditions. Hence, although some of the circulating nitrates and nitrites are excreted in the kidneys, they can also be recycled back to NO.
Dietary Sources of Nitrates
About 85% of dietary nitrates (NO3–) come from vegetables, and the rest mostly from drinking water. Dietary nitrites (NO2–) mostly come from cured meats (charcuterie). Vegetables can be grouped into 3 categories according to their nitrate content (see Table I). Vegetables high in nitrates (>1000 mg/kg) belong to the Brassicaceae (arugula), Chenopodiaceae (beetroot, spinach), Asteraceae (lettuce), and Apiaceae (celery) families. Most commonly eaten vegetables have medium levels of nitrates (100–1000 mg/kg), whereas onions and tomatoes contain very little nitrates (<100 mg/kg). Juicing vegetables is a popular and convenient way to increase vegetable consumption, and several commercial juices are available on the market. Whereas the nitrate content of homemade fresh juice is negligible, it increases dramatically after two days at room temperature, but remains low if stored in the refrigerator at 4 °C. The conversion of nitrates to nitrites in juices prepared at home is due to the presence of bacterial enzymes (reductases), which is less problematic in commercially prepared juices since they are lightly pasteurized.
Table I. Nitrate content in vegetables and water. Source: Lidder & Webb, 2012.
*Note: To facilitate the selection of vegetables to build a diet, the authors recommend using “nitrate units” (1 unit = 1 mmol) to ensure sufficient nitrate intake in order to benefit from the hypotensive effects or to improve exercise performance, and also to avoid consuming more nitrates than recommended (4.2 units for an adult weighing 70 kg).
The acceptable daily intake (ADI) established by the European Food Safety Authority for nitrates is 3.7 mg/kg (0.06 mmol/kg), which corresponds to about 260 mg (4.2 mmol) daily for an adult weighing 70 kg. This ADI was established by dividing the maximum harmless dose for rats and dogs by 100. According to estimations, Europeans consume 31–185 mg of nitrates daily and 0–20 mg of nitrites daily. Based on the moderate recommendation to eat 400 g of a variety of fruits and vegetables per day, the dietary intake of nitrates is about 157 mg/day. Several countries currently recommend a diet high in nitrates for people with heart disease. The DASH diet (Dietary Approach to Stop Hypertension), for example, with its emphasis on fruits and vegetables, whole grains, lean meats (poultry, fish) and nuts, provides a significant level of nitrates. In a clinical study, the DASH diet (rich in fruits and vegetables) lowered blood pressure in subjects with hypertension almost as much as a monotherapy with antihypertensive medication. In fact, it has been suggested that the cardioprotective effects of fruits and vegetables observed in epidemiological studies are caused by the high nitrate content of green leafy vegetables.
The choice of fruits and vegetables eaten can have an important impact on the quantity of dietary nitrates. For example, it is estimated that a DASH diet that only includes fruits and vegetables with low nitrate levels would provide 174 mg of nitrates and 0.41 mg of nitrites, whereas choosing fruits and vegetables high in nitrates can provide up to 1222 mg of nitrates and 0.35 of nitrites. This estimation indicates that the dietary intake of nitrates can vary up to about 700%, according to dietary choices. An excessive intake of nitrates, which is very rare, can cause methemoglobinemia, a disease or intoxication where the level of methemoglobin (a type of hemoglobin that cannot bind oxygen) is too high. Infants (<3 years) are much more susceptible than older children and adults to this disease. In children it is sometimes called “blue baby syndrome.” In adults, this intoxication is rare because their diet cannot contain nitrates in high enough quantities to cause the disease. However, infants can get sick by consuming 200 g of spinach high in nitrates/per day. The American Academy of Pediatrics recommends not giving children foods (purees) containing vegetables (e.g. spinach, beetroot, green beans, carrots) before the age of three months.
A prospective study published in 2018 revealed an association between urinary nitrate and the prevalence of heart disease and the risk of mortality. A concentration of nitrates in urine that was 10 times higher was associated with a 33% decreased risk of hypertension and a 39% decreased risk of stroke. However, there was no association between the concentration of nitrates in urine and the risk of myocardial infarction. Moreover, a ten-fold increase of urinary nitrates was associated with a reduction in all-cause mortality (–37%) and a reduction in cardiovascular mortality (–56%). Despite concerns that nitrates can be converted to nitrites and N-nitrosamines and become carcinogenic, nitrates in urine were not associated with cancer prevalence or cancer mortality. Future studies should evaluate whether nitrate supplements can prevent or reduce the prevalence of heart disease and premature death.
The Effect of Nitrates on Blood Pressure
A study published in 2008 (randomized, placebo-controlled, crossover design) evaluated the effects of a diet high in nitrates on blood pressure in healthy, non-smoking and physically active participants. A diet high in nitrates led to a significant decrease in average blood pressure (3.2 mm Hg) and diastolic blood pressure (3.7 mm Hg), when compared to a diet low in nitrates. In this study, the daily dose of nitrate supplements taken corresponded to that normally contained in 150–250 g from vegetables high in nitrates, such as spinach, beetroot and lettuce. The authors note that the decrease in blood pressure observed in their study was similar to that observed in the DASH study in the healthy group that ate a diet rich in fruits and vegetables, when compared with the group that consumed few fruits and vegetables. In another study, drinking 500 ml of beetroot juice led to an even more significant decrease in systolic (~10.4 mm Hg) and diastolic (~8 mm Hg) blood pressure, when compared to the group that ingested the placebo (500 ml of water, crossover study). This effect was temporally correlated with the transient increase in plasma nitrite concentration. Interrupting the enterosalivary conversion cycle of nitrates to nitrites (by asking participants to spit out all their saliva for 3 hours after ingesting beetroot juice) completely prevented the increase of plasma nitrite concentration, and the decrease in blood pressure. This latter finding confirms that the decrease in blood pressure caused by the consumption of beetroot juice is due to the conversion of nitrates found in beetroot juice to nitrites.
Hypertension, Type 2 Diabetes, Hypercholesterolemia, Obesity
Even though the effect of nitrates on the decrease in blood pressure in healthy subjects was consistently reported in several studies, this is not always the case in studies among subjects with a chronic disease. In a British study of 68 subjects with hypertension, the blood pressure of those who drank 250 ml of beetroot juice daily for a month was lower by 8 mm Hg, compared to those who consumed beetroot juice depleted of nitrates (placebo). In a similar study, also among hypertensive subjects, no decrease in blood pressure was observed, even though the consumption of beetroot juice resulted in a considerable increase in plasma nitrite concentration. In another study of diabetics, there were no effects of dietary nitrites (beetroot juice) on blood pressure, endothelial function, and insulin sensitivity. However, supplementing the diet with beetroot juice significantly reduced systolic blood pressure of overweight or obese participants, aged 55 to 70, when compared to supplementation with blackcurrant juice, which was very low in nitrates. Finally, a study among 69 participants with hypercholesterolemia showed that the intake of dietary nitrates improved vascular function when compared to the group that received the placebo. The reason for variability of the results obtained in these clinical studies is unknown. Length of the treatment, medications used to manage hypertension, methods used to measure blood pressure, and differences between cohorts (e.g. age, BMI, diminished response to NO in certain diseases) are among possible explanatory factors.
Heart Failure
A recent study (randomized, placebo-controlled, crossover design) shows that dietary nitrate supplementation (beetroot juice) increases exercise performance in people with heart failure with reduced ejection fraction. Here is a summary of the study and the main results. After consuming 140 ml of concentrated beetroot juice, the plasma nitrate and nitrite concentration of subjects increased on average by 15 times (1469%) and 2 times (105%), respectively, and the concentration of nitric oxide (a gas) in breath increased by 60%. This effect was not observed with the placebo, a beetroot juice previously depleted of nitrates and that could not be differentiated from the original beetroot juice (packaging, colour, texture, taste and smell) by the study subjects. Two hours after consuming the beetroot juice, subjects exercised for a few minutes on an ergometer stationary bike in a semi-reclined position at various intensities. Respiratory gas exchange was measured continuously. Heart rate, blood pressure and perceived fatigue were evaluated during the last 30 seconds of each phase. Consumption of nitrates had no effect on the ventilatory response, or exercise efficiency, heart rate, and blood pressure. However, compared to the placebo group, the subjects that ingested the beetroot juice were able to reach peak oxygen consumption (VO2 peak) that was higher by 8%, and increased, on average, their duration of effort to exhaustion by 7%. These findings suggest that dietary nitrate intake could be a valuable addition to the management of exercise intolerance among patients with heart failure with reduced ejection fraction.
Nitrates and Athletic Performance
Several studies have been conducted on the impact of nitrate supplementation on the performance of amateur and competitive athletes. In one study, 10 young men drank concentrated beetroot juice or a placebo and, after 2.5 hours (to coincide with the maximum concentration of circulating nitrites), did moderate to high intensity physical activity. When compared to the placebo group, consuming 70 ml of beetroot juice had no effect on athletic performance, but ingesting 140 ml or 280 ml of juice reduced oxygen consumption during moderate physical activity by 1.7% and 3.0%, whereas average time–to-task failure(at very high intensity) increased from 8 min 18 s to 9 min 30 s (14%) and from 8 min 13 s to 9 min 12 s (12%), respectively. Such an increase (12–14%) can seem enormous, but in fact translates to about a 1 to 2% reduction in time to complete a race, for example. In an elite sport, a 1% difference is considered very significant, reducing the time it takes to race a 1,500-metre distance by about 2 seconds and that of a 3,000-metre distance by about 4–5 seconds, for example. Other studies have shown a reduction in oxygen consumption (for the same effort) and an improvement in performance for walking, running, rowing, and cycling, through nitrate supplementation (beetroot juice or NaNO3–). A meta-analysis of 17 of these studies shows that nitrates give a small advantage in performance for time to exhaustion tests, and have a slight beneficial, but not statistically significant, effect on performance during time trials. Another meta-analysis, published in 2016, including 26 randomized, placebo-controlled studies, indicates that nitrate supplementation significantly reduces oxygen consumption for a given effort during a moderate to high intensity exercise in healthy individuals, but not in people with a chronic disease.
Beetroot juice and other supplements with high nitrate levels are obviously not a cure-all. It is better to adopt a global approach to stay healthy, i.e. exercise daily and follow a healthy diet (Mediterranean for example) and eat several servings of fruits and vegetables every day, including green vegetables rich in nitrates, fibre, minerals and vitamins.
