Glycemic Index, High Carbohydrate Diets, Syndrome X and CVD

Syndrome X is a combination of metabolic problems that may encompass adverse changes in blood lipids (i.e. elevated triglycerides and decreased HDL-C), insulin resistance, hypertension and hyperinsulinemia [1]. Most of the people who have Syndrome X have a relatively high waist to hip ratio and many have or will develop non-insulin dependent diabetes mellitus (NIDDM). This month we will take a closer look at the design and protocols of studies purported to demonstrate that diets higher in carbohydrate (CHO) aggravate the metabolic changes associated with Syndrome X which some suggest may increase morbidity and mortality from cardiovascular disease.Many studies have shown that high-CHO diets do lead to higher blood sugar levels, higher insulin levels and/or adverse effects on blood lipids compared to diets higher in unsaturated fat [2] [3] [4]. However, the studies that have shown presumable adverse metabolic effects on the higher CHO-diets required the subjects to consume the same number of calories (and/or maintain the same body weight) on both the high-fat and high-CHO diets. In several such studies the authors stated that they had difficulty getting people to eat as many calories on the high-CHO diet as they ate willingly on the higher fat diet. However, in another study, when a high-CHO diet was compared to a high monounsaturated fat diet but fed both ad libitum as well as isocalorically with the high fat diet then the negative metabolic effects associated with the high-CHO diet largely disappeared. When fed ad libitum, the high-CHO diet appeared to be superior to the higher monounsaturated fat diet for reducing CVD disease risk factors [5]. So it appears that it is more the consumption of calories in excess of what is needed to maintain a healthy body weight rather than the macronutrient mix itself which is primarily responsible for the development of metabolic problems referred to as Syndrome X.Pima Indians living in Mexico who consume a high-CHO diet rarely develop metabolic problems that plague their Arizona ?cousins? who eat a higher fat diet. Since these metabolic problems appear to be the same as seen in Syndrome X, this epidemiological research suggests that high-CHO diets may be preferable to higher fat diets. Looking only at this type of epidemiological data one might ask if it is higher fat diets that often lead to excessive caloric intake and metabolic problems like Syndrome X rather than diets higher in CHO (at least when consumed ad libitum). Of course, the Mexican Pimas are also more active than those in Arizona so this complicates matters. Increased activity reduces the risk of obesity, insulin resistance, NIDDM and cardiovascular disease. Nevertheless, if high-fat diets promote increased caloric intake and obesity relative to high-CHO diets then one could claim that Syndrome X is more likely to occur on a high-fat rather than a high-CHO diet.So are High-Fat Diets Fattening?When the fat content of foods was manipulated covertly and palatability was held constant, human subjects were shown to increase their ad libitum caloric intake and gain weight as dietary fat increased [6]. However, high-fat diets do not invariably lead to increased calorie intake compared to higher CHO diets if the energy density (ED) of the two diets is the same [7].? It has been shown that on both a 20% and 40% fat diet that it is primarily variation in the ED of the diet that leads to differences in ad libitum caloric intake [8] and? [9]. Of course, the addition of fat to foods invariably increases their ED and most naturally occurring high-fat foods have a high ED so it is very difficult to consume a high-fat, low ED diet. By contrast, fresh fruits, vegetables, many whole grain foods, non-fat dairy products and very lean animal protein foods have a very low to moderate ED. It is precisely because diets higher in fat are generally more calorie dense that they are also more likely to lead to excessive caloric intake, weight gain and metabolic problems, including Syndrome X when compared to diets higher in CHO.A study that compared a low ED diet consisting primarily of minimally processed plant foods that are high in fiber and low in fat compared to a typical high-fat, low-fiber American diet found that the ad libitum caloric intake was dramatically lower on the low ED diet (3000 vs. 1570 Cal/day) [10]. In this study the satiety ratings of the high and low ED diets were similar as was food acceptance. In this study, the high ED diet had 681 cal/lb and the low ED diet had only 318 cal/lb. As we have discussed in the past, it is largely a high ED diet coupled with a sedentary lifestyle that promotes obesity and leads to insulin resistance and other metabolic problems. If you are interested in reading about this further, see the June 1998 issue of Communicating Food for Health Newsletter article, ?Calorie Density vs. % Fat? for a more detailed explanation.It is a challenge to plan acceptable meals that average about 300-400 calories per pound and are high in fiber. Such meal plans require more nutrition knowledge than most Americans possess. They also require more culinary skill than most Americans possess to make them palatable. This is because the palatability of foods tends to decrease with decreasing ED even though ED is the primary determinant of ad libitum caloric intake [11]. Nevertheless, a high-fiber and low ED meal plan is probably more comparable to what our ancient ancestors consumed and what the human body is biologically designed to handle [12]. Returning to a less processed and refined diet with far less concentrated calories and increasing daily activity may be necessary for most people to lose weight and keep it off without chronic hunger.The claim that high GI foods cause increased insulin output and this hyperinsulinemia leads to atherosclerosis may also be too simplistic. A recent study found that only the ratio of insulin area to glucose area predicted athersclerotic progression and restenosis [13]. It was only when the insulin output was proportionately much greater than the rise in blood sugar that atherosclerosis was promoted. This enhanced secretion of insulin occurs in Syndrome X and NIDDM because of insulin resistance. This study found that insulin resistance is a risk factor for ishemic heart disease (IHD) but higher insulin levels alone were not associated with an increased risk.Last month evidence was sited which demonstrates that both dietary fat and protein enhance insulin output relative to the rise in blood sugar. Could this enhanced insulin secretion be one reason why Ornish observed significant progression of atherosclerosis on a Step 2 diet even though most patients were taking cholesterol-lowering medication for four years while those on a very low fat diet experienced regression even though none were taking cholesterol-lowering drugs [14]?? If so, this may also be part of the reason why populations that customarily eat a very low-fat diet also have a very low incidence of occlusive artery disease and why patients on a very low-fat diet often experience regression, instead of progression, of atherosclerosis. Clearly more research is needed in this area but claims or suggestions that very low-fat diets may be dangerous because they cause adverse lipid changes that promote atherosclerosis [15] are not supported by credible clinical research using hard clinical end points (e.g. angiograms, heart attacks, overall mortality). Indeed, both heart attacks and overall mortality were found to drop dramatically over 12 years in patients at high risk for IHD when on a very low-fat diet compared to those who followed a more typical Western diet [16].Refining and Processing Foods Usually increases both GI and EDIn general, more processed and refined foods have both a higher GI and ED than foods in their natural whole state and also provide less satiety [17]. But is it the higher insulin response or the greater ED of such foods that is primarily responsible for their lower satiety value? Recently, some fad diet books have claimed that foods that trigger greater blood sugar elevations lead to a greater insulin release which then somehow triggers overeating. But does insulin itself promote hunger and increased food intake? In animals insulin administered directly into the brain actually suppresses food intake [18]. Insulin injected into primates with enough CHO to prevent hypoglycemia also suppresses food intake [19]. This is consistent with the observation that the potato which causes a relatively high glycemic response and insulin score or output compared to many other foods, is also a very high satiety food [20]. If insulin is a satiety hormone, then foods that produce more insulin per calorie may actually be preferable for weight loss. In any case, a food?s ED appears to be far more important than its GI in determining its satiety index. Since foods with a high satiety index are likely to lead to lower total calorie intake when consumed ad libitum, clinicians should focus more on a food?s ED than its GI.The Bottom LineClaims that the key to good health and treating obesity, NIDDM and even CVD is a diet that is higher in protein and fat and consisting of only low GI is not supported by scientific evidence. However, the GI of foods may be of some limited value, particularly in planning meals for diabetics [21]. Even if weight is not lost on a high-CHO diet, one study found that a high-CHO diet that consisted largely of natural foods with a high fiber content compared to one with more refined and processed foods led to a drop in serum triglycerides (rather than a rise which is typically seen) and concluded that there appear to be important cardiovascular benefits from choosing a plant-based diet over a convenience-food based diet for meeting national dietary guidelines? [22].? Had this more natural plant-based high-CHO diet been fed ad libitum (rather than isocalorically) the subject?s caloric intake would likely have been dramatically reduced and the metabolic benefits would be predictably even greater.A diet high in minimally processed whole grain foods like pasta, hot cereals, corn and brown rice and generous amounts of fruits, vegetables and starchy foods like potatoes, beans, lentils and peas with modest amounts of nonfat dairy products and a little seafood or very lean poultry or meat appears to be the best for preventing and treating obesity and NIDDM as well as preventing IHD and many types of cancer.By James J. Kenney, PHD, RD, FACN.1. Reaven GM Syndrome X Clinical Diabetes 1994;March/April:32-62. Garg A et al JAMA 1994;271:1421-83. Coulston AM et al Diabetes Care 1989;12:94-1014. Morgan SA et al JADA 1997;97:151-65. Schaefer E et al JAMA 1995;274:1450-56. Lissner L et al Am J Clin Nutr 1987;46:886-927. Bell EA Am J Clin Nutr 1998;67:1332-98. 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