The importance of dietary carbohydrates

Forty years ago carbohydrates (CHO) were regarded as a simple energy source whereas they are now recognized as important food components. The human diet contains a wide range of CHO, the vast majority of which are of plant origin. Modern techniques based on chemical classification of dietary CHO replaced the traditional by difference measurement. They provide a logical basis for grouping into categories of specific nutritional importance. The physiological effects of dietary CHO are highly dependent on the rate and extent of digestion and absorption in the small intestine and fermentation in the large intestine, interactions which promote human health. Current knowledge of the fate of dietary CHO means that the potentially undesirable properties of many modern foods could be altered by using processing techniques that yield foods with more intact plant cell wall structures. Such products would more closely resemble the foods in the pre-agriculture diet with respect to the rate of digestion and absorption of CHO in the small intestine. The potentially detrimental physiological consequences of eating sugars and starch that are rapidly digested and absorbed in the small intestine suggest that, as fibre, the form, as well as the amount of starch should be considered. Increasing consumer awareness of the relationship between diet and health has led to demands for more widespread nutrition labelling. The entry carbohydrate is required in most countries, and the value is usually obtained by difference and used in the calculation of energy content. However, the value provides no nutritional information per se. Food labels should provide values that aid consumers in selecting a healthy diet.

Nutrition and cataract in low-income Mexicans: experience in an Eye camp.

Surgical Eye-camps for cataract treatment of low-income adult Mexicans have been undertaken over the last 10 years. Despite the high prevalence of cataracts among these subjects, no assessment of their nutritional or health status has ever been made. We compare the results obtained for 81 adults (44 men and 37 women) who received treatment in May 1997 with those for a "control" group of age and sex-matched but affluent individuals in Mexico City. alpha-Tocopherol and beta-carotene were assessed and analysed by HPLC and colorimetric procedures, respectively. The plasma tocopherol to cholesterol ratio did not reveal deficiencies of this vitamin, and only 5 patients (2 men and 3 women) had low beta-carotene plasma levels. The patients had high BMI values, with 32% of men and 30% of women overweight, and 2% and 14%, respectively, obese, with higher glucose, cholesterol and triglyceride values reflecting enhanced insulin resistance and lipid abnormalities. The alkaline phosphatase values were elevated suggesting that many of these blind patients are osteomalacic because they now remain indoors. Although it has been suggested that an adequate intake of carotenes and tocopherol are associated with absence of cataract, this appears not to be the case in our study population. Surveys in Mexico have revealed, however, a highly prevalent deficiency of other vitamins such as niacin and riboflavin, both of which have been proved to be protective against cataract. It appears that nutritional deficiencies, obesity, incipient diabetes and lipid disorders co-exist in modern Mexico. We have identified a need for research to aid the design of preventive nutritional approaches at the population level that could be applied in parallel with ongoing surgical treatment.

Unusually high prevalence rates of obesity in four Mexican rural communities.

OBJECTIVE: To establish the nutritional status of previously studied rural populations. DESIGN AND SUBJECTS: A total of 139 households with 245 males and 301 females from four relatively isolated Mexican rural communities were randomly selected to be surveyed in 1996. RESULTS: Underweight was not a problem in either children or adults. In children <5 y only three (4.2%) were stunted but the age- and sex-specific distributions of body mass index (BMI) in children showed 17% of boys and 19% of girls exceeded the proposed International Obesity Task Force limits for classifying the overweight. Triceps skinfold values were similar to NHANESI values for white USA children. Of the adult men 42% were overweight (BMI 25.0-29.9) and 9% obese; 40% of adult women were overweight and a further 33% obese. Adjusting BMI values with corrected total heights by relating them to measured knee height reduced the BMI of women >50 y by 2.0 units; the male data were essentially unchanged. The prevalence of abdominal obesity in women, based on waist measurements and WHO cut-off points was high with 25% of women having elevated values despite a normal BMI; 43% of the overweight women had substantial increases in waist measurements, indicative of high risk, as did 91% of obese women. The men's waist measurements were greater in relation to both BMI and body fat but the prevalence of values in excess of the suggested sex-specific WHO limits was less than half that of women. CONCLUSIONS: The high prevalence of overweight and obesity is now evident in poor and relatively isolated rural communities of Mexico. SPONSORSHIP: The Chronic Disease Office, from the Ministry of Health in Mexico partially financed this study.

The starch and total sugar content of Mexican fruit and vegetables.

The starch and total sugar contents of 20 types of fruit, 28 types of vegetables and six different herbs, grown in Mexico, were analysed. The selection was based on dietary surveys to identify those foods most widely consumed. Starch was determined by an enzymatic method whilst total sugar was determined gravimetrically. The foods were grouped according to the Southgate classification. Fruits contained little starch (range 0-4 g/100 g fresh weight (FW) except in the case of the plantain (31 g/100 g FW starch), whereas vegetables showed a higher concentration with tubers in the range 10-20 g/100 g FW starch. Legumes contained 0-5 g/100 g FW; amongst the capsicum group the chilli poblano had the highest concentration at 1.3 g/100 g FW starch. The concentration of sugars in fruits ranged from 0.6 g/100 g FW to 21.1 g/100 g FW.

Salt intake and blood pressure in rural and metropolitan Mexico.

A selected group of 155 Mexican adults aged 20-64 years were studied to investigate the role of sodium (Na) intake in explaining blood pressure (BP) differences in a rural town and urban Mexico City. The subjects had their BP, height, weight and skinfolds measured and they collected 3 continuous 24 h urines. Adjusted for age differences, average BPs were significantly higher (p < .05) for the urban (112.7 systolic: 73.6 diastolic mmHg) than for the rural group (108.4 systolic: 70.8 diastolic mmHg). They were also higher for men (111.8 systolic: 74.3 diastolic mmHg) than for women (109.6 systolic: 70.2 diastolic mmHg), the diastolic BP difference being significant (p < 0.05). The average daily Na excretion was also higher in the urban (122.2 mmol/day) than in the rural community (98.0 mmol/day) (p < 0.01). Potassium excretion rates showed similar differences. The differences in sodium excretion and blood pressure among communities were particularly marked in those over 30 years of age. The means for the four community-sex groups had the same rank order for both BP and Na. However, although some large surveys have suggested that half the observed differences in BP might be explained by different Na intakes, in this study the relationship between Na excretion and BP did not achieve statistical significance. Differences in the body mass index (BMI) accounted for 41% of the observed variance in BP.

Defining cooking salt intakes for patient counselling and policy making.

The role of salt (NaCl) in the development of high blood pressure has been a matter of debate, however, the Intersalt Study showed that sodium (Na) intake in various areas of the World is related to the slope of blood pressure with age. Accurate amounts of the total salt intake or that coming from a particular source are needed, both, for physicians who need to consider the salt intake of their patients and for public health workers who are in charge of the implementation of public health programs where salt is used as a carrier of other nutrients. An analysis of the literature suggests that exaggerated values for total salt intakes have often been obtained from indirect estimates; discretionary salt use, i.e. home-cooking salt has invariably been overestimated. A method is described for measuring the contribution of cooking salt to total salt intake since it is a confusing area where inappropriate methods have been used to assess its contribution. The method described is based on the use of small amounts of lithium carbonate fused with NaCl. Validation experiments were undertaken to determine the naturally occurring lithium (Li) in a number of foods including fresh, frozen and tinned vegetables, and the use of Li tagged salt for cooking vegetables and for direct use in cooked foods. We also assessed whether Li was taken up proportionally with Na into foods during cooking. In general vegetables contained variable but only small amounts of Li except aubergine and spinach, and Li was taken up proportionally with Na in a variety of vegetables. Results showed that 36, 35 and 21% of the salt added during cooking was recovered in carrots, runner beans and potatoes respectively, the rest being discarded in the cooking water. This suggest that about a third of salt added during the cooking of vegetables will be ingested by the household. Attempts to rely simply on the total use of household salt supplies will clearly exaggerate, markedly, the true intake of individuals.

Accurate assessment of the quantitative significance of different sources of salt in the diet.

A metabolic study was conducted to assess the validity of using lithium tagged salt as a technique for monitoring the sources of salt in the diet. Discretionary sources, table and cooking salt, were separately labelled and studied, the table salt being available ad libitum whereas cooking salt intakes were controlled. The study showed that lithium excretion in the urine did provide an accurate measure of the amount of the labelled salt ingested. Subsequent analysis suggest that Li is not excreted readily in sweat or faeces so it can be used on its own to ensure the completeness of a series of 24h urines. Latin American studies on salt sources in the diet are needed as a base for programmes of primary prevention of hypertension.

Dietary fibre, what it is and how it is measured.

Carbohydrates are the major component of the human diet and are an important source of energy. The World Health Organization recommends that 50-70% of ingested carbohydrates should be in the form of polysaccharides such as starch. A small proportion of dietary carbohydrate is in the form of non-starch polysaccharides (NSP) (Dietary Fibre). Dietary Fibre is a medically important component of the diet since epidemiological evidence links it with the etiology of various diseases. Scientists have engaged in trying to understand the mechanism by which dietary fibre prevents disease. This article highlights the lack of consensus on its chemical definition and the advantages and disadvantages of the two main methods used to measure it. These are the enzymic gravimetric method (AOAC) that measure fibre as the weight of residual matter following enzymic treatment of the food; and the enzymic chemical method that identifies and measures fibre from its chemical components. The latter method, proposed by Englyst and Cummings measures dietary fibre as NSP and gives detailed information about its components. This is important for interpreting epidemiological and physiological studies. The precise and confident measure of the different components of carbohydrates is important in Latin America. It will allow a coherent, scientific and rational approach to the role of carbohydrates in health.

A metabolic unit for studies on human nutrition.

In clinical nutrition we are used to dealing with the clinical management of patients, but of equal or greater importance is the study of how nutrition affects the development of diseases or modifies its manifestations. This field is complex and links physiological studies of nutrition to the epidemiological analyses which form the basis of thinking in public health in Mexico today. Thus a number of studies have investigated the nutritional risk factors leading to the development of diseases such as heart disease and cancer (1-10). This epidemiological research requires the difficult task of accurately assessing the food consumption of individuals: with poor methodologies the chances of erroneous results are very high. This has implications for both group and individual comparison. Physiological studies on the effects of highly controlled changes in food intake on risk factors then allows the epidemiology to be interpreted in metabolic terms. In this paper we illustrate some of the benefits of metabolic studies and some of the requirements for this successful conduct.