Polyphenols constitute one of the most numerous and ubiquitous groups of plant metabolites and are an integral part of both human and animal diets. Ranging from simple phenolic molecules to highly polymerized compounds with molecular weights of greater than 30,000 Da, the occurrence of this complex group of substances in plant foods is extremely variable. Polyphenols traditionally have been considered antinutrients by animal nutritionists, because of the adverse effect of tannins, one type of polyphenol, on protein digestibility. However, recent interest in food phenolics has increased greatly, owing to their antioxidant capacity (free radical scavenging and metal chelating activities) and their possible beneficial implications in human health, such as in the treatment and prevention of cancer, cardiovascular disease, and other pathologies. Much of the literature refers to a single group of plant phenolics, the flavonoids. This review offers an overview of the nutritional effects of the main groups of polyphenolic compounds, including their metabolism, effects on nutrient bioavailability, and antioxidant activity, as well as a brief description of the chemistry of polyphenols and their occurrence in plant foods.

Polyphenols: Chemistry, Dietary Sources, Metabolism, and Nutritional Significance

Polyphenols are the most abundant antioxidants in the diet and are widespread constituents of fruits, vegetables, cereals, dry legumes, chocolate, and beverages, such as tea, coffee, or wine. Experimental studies on animals or cultured human cell lines support a role of polyphenols in the prevention of cardiovascular diseases, cancers, neurodegenerative diseases, diabetes, or osteoporosis. However, it is very difficult to predict from these results the effects of polyphenol intake on disease prevention in humans. One of the reasons is that these studies have often been conducted at doses or concentrations far beyond those documented in humans. The few clinical studies on biomarkers of oxidative stress, cardiovascular disease risk factors, and tumor or bone resorption biomarkers have often led to contradictory results. Epidemiological studies have repeatedly shown an inverse association between the risk of myocardial infarction and the consumption of tea and wine or the intake level of some particular flavonoids, but no clear associations have been found between cancer risk and polyphenol consumption. More human studies are needed to provide clear evidence of their health protective effects and to better evaluate the risks possibly resulting from too high a polyphenol consumption.

Dietary polyphenols and the prevention of diseases

Flavonoids are a group of polyphenolic compounds, diverse in chemical structure and characteristics, found ubiquitously in plants. Therefore, flavonoids are part of the human diet. Over 4,000 different flavonoids have been identified within the major flavonoid classes which include flavonols, flavones, flavanones, catechins, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Flavonoids are absorbed from the gastrointestinal tracts of humans and animals and are excreted either unchanged or as flavonoid metabolites in the urine and feces. Flavonoids are potent antioxidants, free radical scavengers, and metal chelators and inhibit lipid peroxidation. The structural requirements for the antioxidant and free radical scavenging functions of flavonoids include a hydroxyl group in carbon position three, a double bond between carbon positions two and three, a carbonyl group in carbon position four, and polyhydroxylation of the A and B aromatic rings. Epidemiological studies show an inverse correlation between dietary flavonoid intake and mortality from coronary heart disease (CHD) which is explained in part by the inhibition of low density lipoprotein oxidation and reduced platelet aggregability. Dietary intake of flavonoids range between 23 mg/day estimated in The Netherlands and 170 mg/day estimated in the USA. Major dietary sources of flavonoids determined from studies and analyses conducted in The Netherlands include tea, onions, apples, and red wine. More research is needed for further elucidation of the mechanisms of flavonoid absorption, metabolism, biochemical action, and association with CHD.

Flavonoids—Chemistry, metabolism, cardioprotective effects, and dietary sources

Objective To provide current global and regional estimates of anaemia prevalence and number of persons affected in the total population and by population subgroup. Setting and design We used anaemia prevalence data from the WHO Vitamin and Mineral Nutrition Information System for 1993-2005 to generate anaemia prevalence estimates for countries with data representative at the national level or at the first administrative level that is below the national level. For countries without eligible data, we employed regression-based estimates, which used the UN Human Development Index (HDI) and other health indicators. We combined country estimates, weighted by their population, to estimate anaemia prevalence at the global level, by UN Regions and by category of human development. Results Survey data covered 48.8 % of the global population, 76.1 % of preschool-aged children, 69.0 % of pregnant women and 73.5 % of non-pregnant women. The estimated global anaemia prevalence is 24.8 % (95 % CI 22.9, 26.7 %), affecting 1.62 billion people (95 % CI 1.50, 1.74 billion). Estimated anaemia prevalence is 47.4 % (95 % CI 45.7, 49.1 %) in preschool-aged children, 41.8 % (95 % CI 39.9, 43.8 %) in pregnant women and 30.2 % (95 % CI 28.7, 31.6 %) in non-pregnant women. In numbers, 293 million (95 % CI 282, 303 million) preschool-aged children, 56 million (95 % CI 54, 59 million) pregnant women and 468 million (95 % CI 446, 491 million) non-pregnant women are affected. Conclusion Anaemia affects one-quarter of the world's population and is concentrated in preschool-aged children and women, making it a global public health problem. Data on relative contributions of causal factors are lacking, however, which makes it difficult to effectively address the problem.

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Worldwide prevalence of anaemia, WHO Vitamin and Mineral Nutrition Information System, 1993-2005

A longitudinal population study of 1462 women aged 38-60 was carried out in Gothenburg, Sweden, in 1968-9. In univariate analysis the ratio of waist to hip circumference showed a significant positive association with the 12 year incidence of myocardial infarction, angina pectoris, stroke, and death. The association with incidence of myocardial infarction remained in multivariate analysis and was independent of age, body mass index, smoking habit, serum cholesterol concentration, serum triglyceride concentration, and systolic blood pressure. The relation between the ratio of waist to hip circumference and the end points of myocardial infarction, angina pectoris, stroke, and death was stronger than for any other anthropometric variable studied.

https://www.bmj.com/content/bmj/289/6454/1257.full.pdf

Distribution of adipose tissue and risk of cardiovascular disease and death: a 12 year follow up of participants in the population study of women in Gothenburg, Sweden.

The previous literature on menstrual blood loss studies was reviewed. A new study was conducted to ascertain the variation of menstrual blood loss at various ages and to establish the upper normal limit of menstrual blood loss. The study involved 476 women of all ages in Goteborg Sweden. Amenorrheic women were excluded from the study. Measurement of hemoglobin concentration plasma iron concentration and hematocrit levels indicated wide variation among women. No statistically significant differences in menstrual blood loss were observed among age groups except that women 50 and older had the highest and women 15 and under the lowest mean loss levels. It was concluded that a loss of 60-80 ml is normal; losses above 80 ml should be considered pathological.

Menstrual blood loss--a population study. Variation at different ages and attempts to define normality.

Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate.

Estimation of available dietary iron

INTRODUCTION 123 MEASUREMENTS OF DIETARY BIOAVAILABILITY OF IRON 124 Earlier Methods 124 Extrinsic Tag Method 125 BIOAVAILABILITY OF NON-HEME IRON 126 Individual Factors Influencing Bioavailability 126 Iron status 126 Pregnancy 127 Disease states 128 Dietary Factors Influencing lron Bioavailability 128 Bioavailability of iron in single foods 128 Pool concept and concept of bioavailability 129 Diet factors that particularly influence bioavailability of non-heine iron 130 Bioavailabilio~ of iron in meals from different countries 132 Bioavailability of fortification iron 133 BIOAVAILABILITY OF HEME IRON 140 Individual Factors 140 Dietary Factors 141 ASSESSMENT OF BIOAVAILABILITY OF IRON IN THE WHOLE DIET ...... 141 Earlier Studies 141 Implications of the Pool Concept 142 Estimations Based on Bioavailable Nutrient Density 143 CONCLUSIONS 144

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Leif Hallberg

Papers

Menstrual blood loss--a population study. Variation at different ages and attempts to define normality.

Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate.

Estimation of available dietary iron

Bioavailability of dietary iron in man.

Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron