Heat waves are sporadic events of high temperatures, which can have serious consequences on human life. More than 70,000 people died during the heat wave that hit Europe in 2003, and another 10,860 died during a heat wave in Russia in 2010. The criteria for defining a heat wave vary from country to country. In Canada, a heat wave occurs when it is 30°C or higher for at least three consecutive days. It has been estimated that the average temperature of our planet will increase by 1°C by 2100 if we reduce greenhouse gas (GHG) emissions or 3.7°C if we do not. In 2000, about 30% of the world’s population was exposed to heat waves for at least 20 days a year. By 2100, it is expected that this proportion will increase to about 48% if we drastically reduce GHG emissions and 74% if we continue to increase GHG emissions.
When it is very hot, humid or both, the excess heat absorbed by the body must be dissipated by the skin and the respiratory system in order to maintain body temperature at 37°C: this is the thermoregulation process. The hypothalamus initiates a cardiovascular response by dilating blood vessels to redistribute blood to the body surface (the skin) where heat can be dissipated into the environment. Sweating is activated, allowing heat to dissipate by evaporation (600 kcal/hour). When it is very hot and humid, the evaporation of sweat is greatly reduced and the body struggles to maintain an adequate temperature. Heat stroke is a serious and life-threatening condition, which is defined as a body temperature above 40°C, accompanied by neurological signs such as confusion, seizures or loss of consciousness. The main risk factors for heat stroke are shown in Table 1.
Table 1. Risk factors for heat stroke. From Yeo, 2004.
|Extremes of age (younger than 15, older than 65)
|Skin-altering conditions (psoriasis, eczema, burns)
|Lack of air conditioning in home
|Living in a multi-storey building
|Low socioeconomic status
|Occupations with prolonged exertion and environmental exposure to temperature extremes (e.g., athletes, military workers, miners, steel workers, firefighters, factory workers, rescue workers)
· Impaired thermoregulation (diuretics, beta blockers, anticholinergics, phenothiazines, alcohol, butyrophenones)
· Increased metabolic heat production (benzotropin, trifluoperazine, ephedra containing dietary supplements, diet pills, amphetamines, cocaine, ecstasy)
|Previous history of heat-related illness
|Prolonged sun exposure
|Wearing heavy or excessive clothing
In a review of the literature on the causes of death during heat waves, 5 physiological mechanisms disrupting 7 vital organs have been identified (brain, heart, intestines, kidneys, liver, lungs, pancreas). The authors have identified 27 different ways in which heat-activated physiological mechanisms can lead to organ failure and ultimately death.
1- Ischemia. When the human body is exposed to heat, the hypothalamus initiates a cardiovascular response by dilating the blood vessels to redistribute blood to the body surface (the skin) where heat can be dissipated into the environment. This compensatory process can lead to an insufficient supply of blood to the internal organs (ischemia) and consequently to a lack of oxygen (hypoxia).
2- Toxicity due to thermal shock. High body temperature causes stress the body reacts to by producing stress proteins and free radicals that damage cells. This damage, combined with that caused by ischemia, affects the functioning of several organs.
3- Inflammatory response. Erosion of the intestinal mucosa allows bacteria and endotoxins to enter the bloodstream, leading to sepsis and activation of a systemic inflammatory response. If hyperthermia persists, the exaggerated inflammatory response causes damage to various organs.
4- Disseminated intravascular coagulation. Systemic inflammation and damage to the vascular endothelium caused by ischemia and heat shock can initiate this harmful mechanism. The proteins responsible for the control of coagulation become overactive and this can lead to the formation of clots that block the blood supply to vital organs. Depletion of blood clotting proteins can lead to subsequent bleeding (even in the absence of injury), which can be fatal.
5- Rhabdomyolysis. This is the rapid degradation of skeletal muscle cells caused by heat shock and ischemia. Muscle proteins such as myoglobin are released into the bloodstream and are toxic to the kidneys and can lead to kidney failure.
The heart is hit hard
In the heart, the combination of ischemia, heat shock cytotoxicity, and hypokalemia (potassium deficiency caused by excessive sweating) can lead to cardiac muscle breakdown. This myocardial injury increases the risk of cardiac arrest due to loss of myofibrils and reduced efficiency of the body in controlling heart rate and blood pressure. Stress on the heart can be exacerbated by dehydration, which thickens the blood and causes vasoconstriction, increasing the risk of coronary thrombosis and stroke. In the pancreas, erosion of the endothelial lining allows leukocytes to infiltrate the tissue, exacerbating inflammation. In the brain, the permeability of the blood-brain barrier allows toxins and pathogens to enter, increasing the risk of neuronal damage. All these physiological responses are interconnected in such a way that the failure of one organ can lead to negative effects on others, initiating a vicious cycle of deterioration that often leads to permanent damage, long-term recovery, or death.
To prevent heat stroke (according to Peiris et al., JAMA, 2014):
- Schedule outdoor activities during cool times of the day.
- Drink plenty of fluids. Avoid drinks with too much sugar or alcohol, which can cause dehydration.
- Wear loose-fitting, light-coloured clothing.
- Acclimate to new hot environments, over many days if possible.
- Be aware of medication side effects. If taking medications, be aware of those that may cause fluid losses, decrease sweating, or slow the heart rate. Common medications include those used for depression, blood pressure and heart disease, and coughs and colds.
- Never leave an impaired adult or a child in a car unattended.
What to do if you suspect a heat stroke
Call 911 if you notice these signs of heat stroke: body temperature over 40°C; accelerated heart rate; accelerated breathing; hot and red skin; nausea or vomiting; change of mental state (confusion, headache, difficulty in articulating words, convulsions or coma).
What to do while you wait for help:
- Move the individual out of the heat.
- Remove clothing to promote cooling.
- Position the person on his or her side to minimize aspiration.
- Immerse the individual in cold water or apply cold, wet cloths or ice packs to the skin (neck, armpits, and groin areas, where large blood vessels are located) to lower the body temperature.
- Continue cooling the individual until the body temperature reaches 38.4°C to 39°C (101°F to 102°F).
- Do not give any fluids to the person because it is not safe to drink during an altered level of consciousness. If the person is alert and requests water, give small sips.
- Avoid aspirin and acetaminophen; they do not help with cooling.
Berries are becoming increasingly popular in our diet, whether consumed fresh, frozen, dried or canned, and in related products such as jams, jellies, yogurts, juices and wines. Berries provide significant health benefits because of their high content of phenolic compounds, antioxidants, vitamins, minerals and fibres. Recognizing these health benefits has recently led to a 21% increase in world berry production.
The generic term “berries” is sometimes used to refer to small fruits, but from a botanical point of view, if some berries are genuineberries (blueberries, bilberries, cranberries, currants, lingonberries, elderberries), others are polydrupes (raspberries, blackberries), and the strawberry is a “false fruit” since the achenes (the small seeds on the outer surface of the strawberry) are the actual fruits of the strawberry. Berry fruits are rich in phenolic compounds such as phenolic acids, stilbenes, flavonoids, lignans and tannins (see the classification and structure of these compounds in Figure 1). Berries are particularly rich in anthocyanidins, pigments that give the skin and flesh of these fruits their distinctive red, blue or purple colour (Table 1).
Figure 1. Classification and chemical structure of phenolic compounds contained in berries. Adapted from Parades-López et al., 2010 and Nile & Park, 2014.
Like most flavonoids, anthocyanidins are found in nature as glycosides (compounds made of a sugar and another molecule) called anthocyanins. These anthocyanins can be absorbed in their whole form (linked to different sugars) both in the stomach and in the intestine. Anthocyanins that reach the large intestine can be metabolized by the microbiota (intestinal flora). The maximum concentration of anthocyanins in the bloodstream is reached from 30 minutes to 2 hours after eating berries. However, the maximum plasma concentration (1–100 nmol/L) of anthocyanins is much lower than what is measured in intestinal tissues, indicating that these compounds are metabolized extensively before entering the systemic circulation as metabolites. After administering a radiolabelled anthocyanin to humans, 35 metabolites were identified, 17 in blood, 31 in urine and 28 in feces. Thus, it is likely that these metabolites, rather than the intact molecule, are responsible for the health benefits associated with anthocyanins.
Table 1. Content of phenolic compounds, flavonoids, and anthocyanins of different berries. Adapted from Parades-López et al., 2010 and Nile & Park, 2014.
|Berries (genus and species)||Phenolic compounds||Flavonoids||Anthocyanins
|(mg/100 g fresh fruit)||(mg/100 g fresh fruit)||(mg/100 g fresh fruit)
|Raspberry (Rubus ideaous)||121||6||99
|Blackberry (Rubus fruticosus)||486||276||82–326
|Strawberry (Fragaria x. ananassa)||313||–||54
|Blueberry (Vaccinium corymbosum)||261–585||50||25–495
|Bilberry (Vaccinium myrtillus )||525||44||300
|Cranberry (Vaccinium macrocarpon)||315||157||67–140
|Redcurrant (Ribes rubrum)||1400||9||22
|Blackcurrant (Ribes nigrum)||29-60||46||44
|Elderberry (Sambucus nigra)||104||42||45-791
|Red cranberry (Vitis vitis-idea)||652||74||77
Biological activities of berries
Data from in vitro and animal experimental models indicate that the phenolic compounds in berries may produce their beneficial effects through their antioxidant, anti-inflammatory, antihypertensive, and lipid-lowering activities, which could prevent or mitigate atherosclerosis. Perhaps the best-known of the biological activities of phenolic compounds is their antioxidant activity, which helps protect the body’s cells from damage caused by free radicals and counteract certain chronic diseases associated with aging. According to several studies using in vitro and animal models, berries also have anti-cancer properties involving several complementary mechanisms such as induction of metabolic enzymes, modulation of the expression of specific genes and their effects on cell proliferation, apoptosis (programmed cell death, an unsettled process in cancer cells), and signalling pathways inside the cell.
In a prospective study conducted in China with 512,891 participants, daily consumption of fruit (all types of fruit) was associated with an average decrease in systolic blood pressure of 4.0 mmHg on average, a decrease of 0.5 mmol/L of blood glucose concentration, a 34% reduction in the risk of major coronary events and a 40% reduction in the risk of cardiovascular mortality. These results were obtained by comparing participants who ate fruits daily to those who did not consume them at all or very rarely. In this study, there was a strong dose-response correlation between the incidence of cardiovascular events or cardiovascular mortality and the amount of fruit consumed. Studies suggest that among the constituents of fruit, it is the flavonoids, and especially the anthocyanins, that are responsible for these protective effects.
A number of prospective and cross-sectional studies have examined the association between the consumption of anthocyanins and cardiovascular risk factors (see this review). In four out of five studies that examined the risks of coronary heart disease or nonfatal myocardial infarction, anthocyanin consumption was associatedwith a reduction in coronary artery disease risk from 12% to 32%. The impact of anthocyanins on the risk of stroke was investigated in 5 studies, but no evidence of a protective effect was found in this case.
With respect to cardiovascular risk factors, studies indicate that higher consumption of anthocyanins is associated with decreased arterial stiffness, arterial pressure, and insulinemia. The decrease in blood pressure associated with the consumption of anthocyanins, -4 mmHg, is similar to that seen in a person after quitting smoking. The effect of anthocyanins on insulin concentration, an average reduction of 0.7 mIU/L, is similar to the effects of a low-fat diet or a one-hour walk per day. A decrease in inflammation has been associated with the consumption of anthocyanins and flavonols, a mechanism that may underlie the reduction of cardiovascular risk and other chronic diseases.
Randomized controlled trials
A systematic review and meta-analysis of 22 randomized controlled trials, representing 1,251 people, report that berry consumption significantly reduces several cardiovascular risk factors, such as blood LDL cholesterol , systolic blood pressure , fasting glucose concentration , body mass index , glycated haemoglobin , and tumour necrosis factor alpha , a cytokine involved in systemic inflammation. In contrast, no significant changes were observed for the other markers of cardiovascular disease that were tested: total cholesterol, HDL cholesterol, triglycerides, diastolic blood pressure, ApoAI, ApoB, Ox-LDL, IL-6, CRP, sICAM-1,and sICAM-2.
Another systematic review published in 2018 evaluated randomized controlled trials on the effects of berry consumption on cardiovascular health. Among the 17 high-quality RCTs, 12 reported a beneficial effect of berry consumption on cardiovascular and metabolic health markers. Four out of eleven RCTs reported a reduction in systolic and/or diastolic blood pressure; 3/7 studies reported a favourable effect on endothelial function; 2/3 studies reported an improvement in arterial stiffness; 7/17 studies reported beneficial effects for the lipid balance; and 3/6 studies reported an improvement in the glycemic profile.
Berries and cognitive decline
Greater consumption of blueberries and strawberries was associated with a slowdown in cognitive decline in a prospective study of 16,010 participants in the Nurses’ Health Study aged 70 or older. Consumption of berries was associated with delayed cognitive decline of approximately 2.5 years. In addition, nurses who had consumed more anthocyanidins and total flavonoids had a slower cognitive decline than participants who consumed less.
The exceptional content of phenolic compounds in berries and their positive effects on health remind us that the quality of food is not just about nutrients: proteins, carbohydrates, lipids, vitamins and minerals; a wide variety of other molecules found in plants are absorbed from the intestines and routed through the bloodstream to all cells in the body. While not essential nutrients, phytochemicals such as flavonoids can contribute to better cardiovascular health and healthier aging.