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.

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.

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.

Are the proposed limits of energy intake:basal metabolic rate and dietary nitrogen:urinary nitrogen ratios suitable for validation of food intake?

The validity of 7 d weighed records of diet obtained for pre-menopausal Mexican women was assessed by two independent methods: the energy intake:BMR (EI:BMR) and the dietary N:urinary N (DN:UN). For the latter, complete urine collections are required and completeness was assessed from measurements of para-aminobenzoic acid (PABA) excretion. There were forty-six adult female subjects in the study, thirty-four were from Mexico City and twelve were from a rural population in the Central Highlands, Mexico. However, data were rejected from five urban women for whom the PABA excretion data suggested incomplete urine collection on four or more days. BMR was measured with Oxylog portable O2 consumption meters, and physical activity level was assessed from a self-completed activity diary. An approximate relationship between the EI:BMR ratio and the DN:UN ratio suggested that the rejection limits on the EI:BMR ratio recommended by are wider than the limits on the DN:UN ratio recommended by. Using the recommended cut-off points for EI:BMR but wider limits for DN:UN, twenty-one and twenty-five women respectively had acceptable intake records by the two methods, and sixteen of them by both methods. In conclusion the modification of the DN:UN limits to 0.92 and 1.70 to set acceptable intake values makes the use of measurements of N and energy balance comparable. Urine values with PABA recoveries greater than should be rejected, as should UN values validated by less than 3 d.

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.

The dominance of salt in manufactured food in the sodium intake of affluent societies.

Statistical analyses suggest that 25-50% of the salt intake of Western populations is derived from the discretionary use of cooking and table salt. Yet direct estimates of discretionary salt use by a lithium technique show that in one community in Britain this source contributed only 15% to total intake. The estimates of discretionary salt use in Finland, the United States, and Britain have been exaggerated because salt losses in cooking water were not considered. Only about a quarter of cooking salt actually enters the consumed food; allowance for this in statistical calculations makes data on dietary intake similar to those assessed from urinary sodium excretion. Daily salt intake in Britain averages about 10.7 g for adult men and 8.0 g for women, figures similar to those from countries in northern Europe. The natural salt content of food provides about 10% intake, the remaining 75% being derived from salt added by manufacturers; drinking water provides a negligible amount. Any programme for reducing the salt consumption of a population should therefore concentrate primarily on a reduction in the salt used during food processing.

The potential use of lithium as a marker for the assessment of the sources of dietary salt: cooking studies and physiological experiments in men.

Lithium was investigated for its possible use as a marker for identifying the various sources of NaCl in the diet. Micromolar concentrations of lithium can be detected in various vegetables, tap water and also in urine specimens of adult volunteers. The lithium content of vegetables varied from 6.1 to 24.5 mumol of lithium/kg dry weight, with the exception of spinach and aubergines which had much higher concentrations. The excretion of the element in 24 h urine specimens ranged from 2 to 4 mumol of lithium/day. Experiments were performed to assess whether both lithium and sodium would penetrate foods at the same rate during cooking. The rates of penetration into food for both elements were proportional to their concentration in the cooking water despite a sodium/lithium ratio of 50:1. Physiological experiments were conducted to investigate the handling of small doses of lithium by the body. A dose of 250 mumol of lithium was chosen as optimal and given orally to healthy volunteers in either single or continuous aqueous doses of lithium carbonate. The recoveries of oral lithium in urine were 92 +/- 5% (SD) and 97 +/- 4 (SD) (n = 5) for single and continuous doses respectively. The daily addition of 100 mmol of oral NaCl to the diet of volunteers receiving a standard dose of lithium did not affect urinary lithium excretion rates nor the final recovery of the administered lithium. These studies suggest that lithium carbonate may be a useful marker for the uptake of NaCl into cooked food; after eating lithium-enriched food the monitoring of urinary lithium output may then be used to quantify the amount of sodium derived from the specific foods.

A test of the validity of the lithium-marker technique for monitoring dietary sources of salt in man.

A lithium-tagged salt for assessing the sources of salt in the human diet was prepared by fusing lithium carbonate and sodium chloride at 900 degrees C followed by grinding and sieving the fused salts to a defined grain size; magnesium carbonate was added as a hygroscopic agent. To validate the use of this tagged salt, which replaced alternately table salt and cooking salt in daily use, a 44 day metabolic study was conducted on five volunteers. Measurements of sodium and lithium balance throughout the study showed that 93% of both dietary sodium and dietary lithium were excreted in the urine and 1.7% of dietary lithium and 2% of dietary sodium in the faeces. Collections of sweat for 48 h at intervals throughout the study showed that only 1.7% of dietary lithium and 1.4% of dietary sodium were recovered in sweat. Thus both elements were excreted equivalently by the three main routes. The apparent mean sodium retention over the whole period of dietary control was 7.4 +/- 4.4 mmol/day or 3.4% of the total intake. The excretory patterns of lithium after a period of lithium ingestion were exponential, thus allowing a method to be developed for the prediction of total lithium output from a shorter period of urine collection. This study suggests that the lithium method is suitable for epidemiological use to measure the sources of dietary salt.

An assessment of the sources of dietary salt in a British population.

An epidemiological study was conducted in the market town of March, Cambridgeshire, to assess the quantitative importance of cooking and table salt to total dietary salt intake by the use of a fused mixture of lithium carbonate and sodium chloride. Men and women aged 20-60 participated in a 12 day study with sequential 24 h urine collections to assess salt sources over a 7 day period. Total salt consumption estimated from urinary chloride excretion amounted to 10.6 +/- 0.55 (SEM) g in 33 men and 7.4 +/- 0.29 (SEM) g in 50 women. The cooking salt eaten was only 0.45 +/- 0.09 (SEM) g in men and women, with men eating more table salt (0.77 g/day) than women (0.46 g/day). Discretionary sources, i.e. cooking and table salt use, contributed only 15% to the total intake. Salt from manufacturing foods and catering in purchased food therefore provided on average 85% of total salt intake. These results are consistent even when an allowance is made for the slightly poorer pouring quality of the lithium-tagged salt. The importance of food as a source of salt was reflected in the significant relationship between the weight of the individual and the amount of salt eaten (for males P less than 0.05 and for females P less than 0.001). Cooking salt consumption did not relate to the amount of salt derived from purchased food nor did table salt use relate to the amount of salt in cooked foods.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel technique for assessing the sources of dietary sodium.

Li can be used to track Na during the cooking and eating process as it is handled by the body in a manner similar to Na. A new approach to study the sources of salt using Li is described. When this technique was applied in an epidemiological context using 12 consecutive 24-hour urine collections, we found that at least 83% of the salt ingested by a population in Cambridgeshire, England, was derived from processed food. Only a small amount of the total salt intake came from cooking or from using salt on the table. The total amount of salt eaten was not as high as was previously estimated indirectly. There was a clear sex difference in salt intake which in part related simply to the finding that salt intake correlates with body weight of an individual. This study implies that those who wish to advocate a reduction in salt intake in order to prevent hypertension must reconsider their approach. Any health education programme that is geared simply to reduce discretionary salt use in the home, will have little impact on the total amount of NaCl ingested by the population.