"Eat to Live, Not Live to Eat"
John H Weisburger, 2000
This series of occasional articles on nutrition is intended to complement the horticultural information, providing a rationale for why we might bother to grow our own plants instead of buying fruit at commercial outlets. Producing your own fruit has its advantages: the pleasure of growing your own, lower cost, convenience, confidence in pesticide history, superior flavour when the fruit is picked at the right time, and increased likelihood of improved nutrition. Relevant scientific publications are used to provide information on recent advances and understanding.
Obesity, the forerunner of so many health problems, is now a global epidemic. Considering only US data, prevalence has increased by 40% in the last 30 years so that now two in three adults are overweight or obese. Average daily calorie intake has increased by 450kcal/d in this period. The biggest contributors to this increase have been flour and cereal products, followed by added fats and sugars. These have gone up by 40, 49 and 10% respectively in the last 40 years. Particular nutritional concern has centred on processed and refined products in the grain-based foods (eg cakes, biscuits, muffins, pies and pastries, and white rice) that are calorie-dense where valuable nutrients may have been lost or destroyed and/or with addition of unhealthy ingredients, in the added fats where saturated and trans fats are over-consumed rather than the unsaturated types, and in the added sugars category, especially sugar-sweetened beverages (SSBs) such as colas, fruit drinks, teas, milk, yoghurt and sports drinks. These can represent 20-40% of all carbohydrates in the diet and teenagers may consume 50% more than adults when expressed as a fraction of their daily energy intakes. SSBs have nutritional negatives from many perspectives. They are empty calories; in liquid forms they're absorbed very rapidly, causing glucose and insulin spiking, they don't have the same satiation properties as solid foods so there's increased risk of over-consumption, when consumed in excess there's also an increased risk of important micronutrient dilution, and because they're more usually consumed at irregular intervals throughout the day without teeth cleansing, the resulting continual sugar exposure is disastrous for dental caries. The American Heart Association has recommended that adult males and females should not consume more than 150 and 100kcal/d resp from all added sugars, intakes that are exceeded by most adults.
Glucose and fructose are the most common simple sugars (monosaccharides) in Western diets, with other less common ones being mannose and galactose. Table sugar (sucrose) is a 1:1 disaccharide of fructose and glucose that is very rapidly split in two in the body. There's also a range of higher order saccharide molecules (oligosaccharides) made from 3-8 sugar units, and larger ones (polysaccharides) of varied structure and sizes up to thousands of units - in the case of starches and glycogen, consisting entirely of glucose units. Glucose is essential in the body, and if there's an inadequate carbohydrate dietary intake it can be synthesised from various stored compounds such as glycogen. Pure glucose is very rapidly absorbed causing postprandial hyperglycemia and a large spike in insulin, which if repeated habitually can lead to insulin resistance, diabetes and other metabolic abnormalities tied to chronic diseases.
Apart from spermatazoa, fructose is not an essential energy source in the body. It is roughly 50% sweeter than sugar while glucose is 30% less, and it has bi-directional effects, beneficial at normal doses but when very high, adverse outcomes such as increased triglycerides and uric acid, and fatty liver disease. At reasonable doses, it's almost all converted to glucose, lactose and glycogen in the liver. At much higher intakes without a comparable co-ingestion of glucose, it's poorly absorbed and can lead to gut disturbances. It doesn't cause a rapid spike in insulin like glucose, a benefit for diabetics. Usually SSB sweetening is achieved by adding table sugar to various foods but also in the US by high fructose corn syrup which is cheaper. Although called 'high fructose', it contains approximately the same proportion of fructose to glucose (55:45 respectively) as sugar. The seriousness of health problems due to calorie over-consumption, initially in the developed world but now globally, has driven a vast amount of research to improve our understanding. These efforts have led to much controversy regarding the underlying risk factors and what should be done to address them. Fructose has been heavily caught up in the SSB part of the story in the last 10 years, with some calling it 'poison', 'a threat to global health', 'pure, white and deadly' or 'alcohol without the buzz'. Does it deserve these inflammatory labels or has it been falsely accused by association?
Metabolic and nutritional studies have been conducted in laboratory and animal experiments in attempts to dis-entangle the role of different sugars, usually at high doses of individual compounds to facilitate study of mechanisms and effects. From this work we know that high dose fructose can cause a multitude of unwanted biochemical and physiological effects. Caveats are that in human diets fructose is never eaten in isolation and always occurs with other sugars in foods, so studies with isolated fructose are unrealistic. Plus controlled laboratory and animal studies can't be reliably used to predict effects in free-living humans, and also for relevance, we need to focus on doses that are commonly consumed, namely a mean of 133 and 103g/d added sugars for males and females resp according to a 2011-12 study. One third of the population consumes >10% of total energy from added sugars, whereas 12.5% exceed the 25% maximum recommended by the Institute of Medicine. The WHO recommends an even lower intake of added sugars set at 10% energy. On average, 133g of sugars containing roughly 66g of fructose would provide 524kcal of energy and for a normal weight/activity adult male consuming 2700kcal/d, this would be 19% of total energy coming from added sugars. Many of the experimental and animal studies on fructose have used doses up to 70% of energy! At these excessive levels it's unwise to extrapolate back to normal levels as biochemical and physiological processes could be grossly distorted. Randomised clinical trials using realistic doses in real foods are the best way to establish the significance of effects, and as the following study focussing on obesity by European researchers concluded (Nutrition Research Reviews (2014), 27, 119–130), reasonable intakes are no more damaging than glucose. Supporting the case for fructose being falsely accused as a villain in this story, we know that fresh fruits which are the best food sources of natural fructose, are a valuable component of healthy diets.
A causal role of fructose intake in the aetiology of the global obesity epidemic has been proposed in recent years. This proposition, however, rests on controversial interpretations of two distinct lines of research. On one hand, in mechanistic intervention studies, detrimental metabolic effects have been observed after excessive isolated fructose intakes in animals and human subjects. On the other hand, food disappearance data indicate that fructose consumption from added sugars has increased over the past decades and paralleled the increase in obesity. Both lines of research are presently insufficient to demonstrate a causal role of fructose in metabolic diseases, however. Most mechanistic intervention studies were performed on subjects fed large amounts of pure fructose, while fructose is ordinarily ingested together with glucose. The use of food disappearance data does not accurately reflect food consumption, and hence cannot be used as evidence of a causal link between fructose intake and obesity. Based on a thorough review of the literature, we demonstrate that fructose, as commonly consumed in mixed carbohydrate sources, does not exert specific metabolic effects that can account for an increase in body weight. Consequently, public health recommendations and policies aiming at reducing fructose consumption only, without additional diet and lifestyle targets, would be disputable and impractical. Although the available evidence indicates that the consumption of sugar-sweetened beverages is associated with body-weight gain, and it may be that fructose is among the main constituents of these beverages, energy overconsumption is much more important to consider in terms of the obesity epidemic.
The fructose question has been a fiercely controversial one for some years but the scientific community is steadily progressing to a reasonable consensus. However as could be expected until the various facets of the research have been placed in context and then promulgated in public health recommendations, there is still confusion in media reports and public understanding. You might wonder whether all these research findings from elsewhere apply to our lifestyles in WA? Well, yes, they probably do as we're all heading down the same track. This was illustrated, for example, in a 2013 study on the importance of SSBs on weight gain and other metabolic disturbances in 1433 14 to 17-year-old teenagers in the Western Australian Pregnancy Cohort Study. Consumption of SSBs was high, eg when they were divided into SSB intake tertiles, the highest level of 14-year-olds consumed an average of 651g/d. The authors conservatively concluded that increased SSB intake may be an important predictor of cardio-metabolic risk in young people, independent of weight status. At an overview level, numerous societies and organizations including the American Heart Association, the American Academy of Pediatrics, and the US 2010 Dietary Guidelines technical review committee have called for reductions in SSB intake to help prevent obesity and improve overall health.
As mentioned in the introductory comments, high carbohydrate and added sugar intakes are not the only worrying component of unhealthy diets. Excessive fat and protein, and insufficient fibre and micronutrients also cause their own problems. An unbalanced physiology ensues, especially when combined with a lack of physical activity, leading to increased risk of future health problems.
The date palm originates from Middle Eastern countries and today most of the world production comes from that region and North Africa. It’s one of the oldest domesticated fruit trees with a history going back millennia, and is ideally suited to hot and dry arid conditions where temperatures can soar to 50°C. In these regions dates form an important part of the economy, their culture and diet, with daily consumption in some countries sufficient for it be regarded as a staple food. There is great diversity in the species and many thousands of varieties have been named, with multiple names given in different countries and even in different regions within countries. Much of this confusing state of affairs regarding how many distinct varieties there really are should be clarified in coming years with the recent sequencing of the genome; this will also speed up further development of plants with even better properties.
The dried and pitted black dates we all know in the West contain 70-85% simple sugars. These are mainly fructose and glucose in roughly equal proportions and sucrose usually decreases to low levels with ripening. As a consequence, they’re a high-calorie food, 270-330kcal/100g, very sweet and have good storage properties. But can they be good for you at such high sugar levels? There’s so much of it that they can easily be used as sugar substitutes in prepared foods. For the sweet tooths amongst us, the hope would be that at least they’re not 99+% pure sucrose, and surely the other 15-30% of the fruit must have something of nutritional value in there? The answer is positive, although nutritional features can vary more than ten-fold across varieties, even amongst those of superior commercial-grade. Generally they have 4-11% fibre, the normal low fruit levels of protein (1-3%) and fats ( 0.2-1.5%, almost no starch and some vitamins. More than 80 volatile chemicals have been identified that contribute to their aroma. Consuming 100g can supply 5-7.5, 5-13, 19-25, 33 and 3-4.5% of daily recommended intakes for Fe, Mn, Mg, K and Zn resp.
Dates are one of the few plant-based foods that are an excellent source of selenium, and if grown in non-deficient soils, 100g can provide several times the daily requirement. Most importantly, they have very good levels of healthy antioxidant phytochemicals, again varying markedly between varieties. A 2005 US study compared the total (free plus bound) polyphenol content of dates with 5 other common fruits (apricots, cranberries, figs, green grapes and plums, all dried to <20% moisture) and found that dates were the highest at 1959 mg/100g catechin equivalents, with the others being 402, 870, 320, 551 and 788 respectively. For these 6 dried fruits, dates were 3rd highest in fibre and 5th highest in K, Ca and Fe. The normal sun-drying approach for marketable product causes significant loss (up to 50% and more) of antioxidants, anthocyanins and carotenoids.
The glycemic index (GI) is used to classify foods nutritionally in terms of blood glucose response following consumption. They can be broadly divided into those called low (GI <56), medium (56-69) and high (>69). The goal for all of us should be to try and eat slow burn foods that are lower in these categories. This avoids the rapid and more extensive elevations in glucose and insulin from high GI foods that very quickly lead to craving unhealthy snacks between meals, over-consumption, insulin resistance, metabolic syndrome, obesity etc. But the GI is not a perfect index of carbohydrate effects for all foods in the body. For example, whole cucumbers have such low available carbohydrate content (1.4g/100g) that to measure the GI, 3.5kg would have to be consumed, an amount so far in excess of the average serving size that the result would be pointless. Plus, such low energy foods might have other positive features such as vitamins, minerals and fibre that may outweigh the carbohydrate considerations. Then there’s the effect of other foods which may be eaten together with the test food and this can also have major impact. Other foods like spices and herbs are eaten in such small quantities as flavouring agents (ie < 3-5g) that a high GI is not necessarily going to be the end of the day. Introduction of the glycemic load that combines GI with serving size was an attempt to address this quality/quantity question. However despite these confounding issues, low GI diets have been shown in many clinical studies to be associated with reduced risk of chronic disease.
With high levels of sugars in dates it might reasonably be expected that the GI would be high and especially unsuitable for diabetics. This question is important globally but even more so for those countries where considerable daily intake is enshrined in their national cuisine and the incidence of diabetes is high. For example, daily consumption in Oman of predominantly dried fruits is 55-164g/d, and in the neighbouring United Arab Emirates the prevalence of diabetes is the second highest in the world, with age-standardized rates (diagnosed and undiagnosed) and pre-diabetes among 30-64 year olds of 29% and 24% respectively. The following study (Nutrition Journal 2011, 10:59) found that even diabetics who have to watch their blood glucose carefully could eat the studied date varieties as they were low GI and didn’t magnify the glucose response seen in healthy individuals.
This study was designed to determine the GIs of five commonly used varieties of dates in healthy subjects and their effects on postprandial glucose excursions in individuals with type 2 diabetes mellitus. Composition analysis was carried out for five types of dates (Tamar stage, fully ripe and dried). The weights of the flesh of the dates equivalent to 50g of available carbohydrates were calculated. The study subjects were thirteen healthy volunteers with a mean age of 40 years and ten participants with type 2 diabetes mellitus (controlled on lifestyle measures and/or metformin) with a mean HbA1c (glycosylated hemoglobin) of 6.6% and a mean age of 41 years. Each subject was tested on eight separate days with 50g of glucose (on 3 occasions) and 50g equivalent of available carbohydrates from the 5 varieties of date (each on one occasion). Capillary glucose was measured in the healthy subjects at 0, 15, 30, 45, 60, 90 and 120min and for the diabetics at 0, 30, 60, 90, 120, 150 and 180min. The GIs were determined as ratios of the incremental areas under the response curves for the dates compared to glucose. Statistical analyses were performed using the Mann-Whitney U test and repeated measures analysis of variance. Mean GIs of the dates for the healthy individuals were 54, 53, 46, 49 and 55 for Fara’d, Lulu, Bo ma’an, Dabbas and Khalas, respectively. Corresponding values for those with type 2 diabetes were very similar (46, 44, 52, 50 and 53). There were no statistically significant differences in the GIs between the control and the diabetic groups for the five types of dates, nor were there statistically significant differences among the GIs of the dates. The results show low GIs for the five types of dates included in the study and that their consumption by diabetic individuals does not result in significant postprandial glucose excursions. These findings point to the potential benefits of dates for diabetic subjects when used in a healthy balanced diet.
The GI of different dates can vary considerably depending on variety, harvest time, degree of ripening and dehydration, soils, cultivation conditions, seasons, storage conditions and so on. Some studies have reported higher GIs than that above, so the findings are not an unequivocal recommendation that all dates can be consumed with gay abandon. Desirable features that lead to a reduction in the magnitude and duration of blood glucose and insulin response from what might be expected with such high sugar concentrations are (i) about half of these are fructose and this has very much less effect on glucose response, and (ii) the presence of considerable fibre reduces the rate and extent of absorption through reduced gut transit time and increased viscosity of the food matrix. Both of these are nutritionally positive with habitual consumption. However, two points should be kept in mind with dried dates. First as a high calorie food, eating 100g can represent 10-15% of the average adult daily energy requirement, so they should only ever be eaten in moderation. Second, like all dried fruits their sticky/chewy nature means particulate matter can persist on teeth after eating and cause dental caries if oral hygiene is poor.
The overview is that dates shouldn’t be summarily dismissed as lollies growing on trees, consisting almost entirely of sugar and not much else. They can make a valuable contribution to a healthy diet when eaten in proper balance with other plant-based foods - fruits, vegetables, nuts, seeds, cereals and legumes.