Diabetes Mellitus, Lipidus Et… Proteinus!

If there had been a simple early clinical test for urine nitrogen or plasma amino acids, the diseases we know as “diabetes mellitus” might instead have been called “diabetes proteinus.” This is because insufficient insulin action impacts all macronutrients, not just carbohydrate. With our present preoccupations with “glycemic control” and the proven risks associated with hyperglycemia and hyperlipidemia, clinicians may indeed be underestimating the importance of altered protein metabolism and of the quality and quantity of dietary protein. An exception to this is diabetic nephropathy, addressed by Wheeler et al. (1) in this issue of Diabetes Care. Uncontrolled diabetes has been recognized for millennia to cause lean tissue loss. Though anticatabolic and anabolic actions of insulin are accepted as dogma, relatively little clinical research is conducted on them. In the present millennium, publications in the American Diabetes Association (ADA) journals have cited “glucose metabolism” 389 times, “lipid metabolism” 196 times, but “protein metabolism” only 30 times! Protein metabolism is not less than one-tenth as important as glucose. For example, though we use HbA_1c as indicator of recent glycemic control, the steady state of glycation of each protein is also a function of the rate of turnover of that protein. The first paper of this millennium in Diabetes Care (2) showed increased whole-body fed-fasted protein turnover in the presence of hyperglycemia; hence, the kinetics of many proteins are increased.

The therapeutic implication of recognizing that protein metabolism is disturbed in diabetes is that dietary protein quality and quantity need to be adapted to individual requirements. Many of the controversies over dietary protein recommendations have been summarized recently by Franz (3). The most recent ADA Clinical Practice Recommendations (4) and corresponding technical papers (5) appeared in Diabetes Care in January 2002. Therein, the level of evidence for establishing recommendations was no higher than B for protein restriction to ≤0.8 g · kg⁻¹ body wt · day⁻¹ (∼10% of daily calories) with the onset of overt nephropathy, C for 0.8−1.0 g · kg⁻¹ body wt · day⁻¹ for individuals with microalbuminuria, and B for others not in optimal control, “protein requirement may be greater than the recommended dietary allowance but not greater than usual intake.” The wording of this last recommendation is subject to the interpretation that each diabetic individual’s “usual” intake is that of the general population and therefore needs no special attention. Furthermore, it does not take account of the variability of glycemic (i.e., metabolic) control among individuals, which affects the metabolism of dietary and endogenous protein (example in 2).

The ADA guidelines (4,5) do not address the different food source(s) of protein, nor the hypothesis that plant protein might have some extra benefit in early nephropathy, tested in the present study by Wheeler et al. (1). Animal studies have shown a variety of physiological effects of protein on renal function and protection from progressive glomerular damage by protein restriction in models of renal disease (6). Mechanisms invoked involve effects on glomerular arteriolar resistance, intraglomerular pressure, glomerular hypertrophy, cytokines, and regulation of genes regulating matrix production. Human studies confirm that animal protein and amino acids (individually and as mixtures) increase glomerular filtration rate (GFR), but egg whites and plant protein do not (7,8). An explanation for some plant proteins being different is their different proportions of indispensable (essential) versus dispensable (nonessential) amino acids. Postulated mechanisms for the protein/amino acid-induced hyperfiltration are via release of hormones (including glucagon, IGF-1, kinins, atrial natriuretic peptide, and renin-angiotensin), intrarenal effects on sodium reabsorption and tubuloglomerular feedback (6), and renal prostaglandin production (7). Compelling clinical, pathophysiological, and mechanistic data on the use of different protein sources in human diabetes have been lacking.

Wheeler et al. address whether microalbuminuria and renal function in type 2 diabetic subjects are improved by 6 weeks exposure to a diet with 100% plant protein compared with a clinically relevant diet with 60% animal/40% plant protein. Given the heterogeneity of the type 2 diabetes population, varied diabetes treatments used, issues of adherence to prescribed diets, challenges of any (mostly) outpatient nutrition/metabolism study, and numbers of end point measures required, this study was done as well as is currently feasible. Why, then, was there no demonstrable difference in GFR, renal plasma flow, albumin excretion rate, lipids, glucose and insulin response to a meal challenge, or fasting serum amino acid profile? Do the data justify the conclusion that there is no advantage to a plant protein-based diet when microalbuminuria accompanies type 2 diabetes?

Several answers are possible. One is that there may indeed be no beneficial effect. Another is that this study may not have been able to show such an effect for the following reasons. The subjects were heterogeneous as to age, body size, and initial level of glycemic control. There was an unequal sex representation. The subjects were treated with different diabetes medications, and half received ACE inhibitors for hypertension. The 6-week duration of the diet periods might have been too short. The design explicitly replicated the typical high U.S. level of protein intake of 1.2 g · kg⁻¹ body wt · day⁻¹. This is a justifiable first step, but the capacity to demonstrate an effect might have been blunted by the high intake, by the high quality of the plant proteins selected, and because of the relatively high contribution of plant proteins in the animal-protein diet. This study was completed before the recommendation for diabetes with microalbuminuria noted above (4) of 0.8–1.0 g · kg⁻¹ body wt · day⁻¹ was published. There is good reason to be circumspect about the extrapolation of this paper’s findings to the lower protein recommendation. Based on the sample menus of the two diets provided by the authors, we have performed an analysis using the Genesis R&D Version 7.01 (Esha Research, Salem, OR) database to estimate the total amounts of individual amino acids for each diet. This permits inferences as to whether there were sufficient differences between the diets to test the hypothesis. The analysis generated similar intakes of both indispensable (except methionine) and dispensable amino acids between the diets (Table 1). All of valine, isoleucine, leucine, lysine, phenylalanine plus tyrosine, threonine, and tryptophan intakes would have been above both World Health Organization recommended intakes and, indeed, above even higher recommendations defined by more recent isotopic studies of leucine, lysine, phenylalanine plus tyrosine, and threonine. The “low” methionine on the plant-protein diet needs to be interpreted in light of the recently demonstrated methionine-sparing effect of cysteine (9). Cysteine content was both in excess of recently suggested safe intakes for healthy men and would be predicted to have reduced the methionine requirement to below that provided by the plant protein diet. Thus, neither diet was restricted in indispensable amino acids.

Nakamura et al. (7) have suggested that GFR is increased by alanine, glycine, and arginine and that different magnitudes of increases in their plasma levels after challenges with different protein sources alone account in part for their different effects on GFR. To the extent that the postprandial plasma levels of these amino acids are a function of their contents in ingested protein, one would not have predicted differences between the two diets. The absence of differences in fasting amino acid profiles in the study does not exclude the possibility of differences having occurred postprandially, however. After mixed meals, their carbohydrate contents may have a greater influence on plasma alanine responses, in particular, but on other amino acids as well. Furthermore, especially in suboptimally controlled diabetes (that affects many circulating amino acids), attribution of a mechanistic role for amino acids would require painstaking studies. The foregoing calculations were based on intakes at 1.2 g · kg⁻¹ body wt · day⁻¹ and refer to recommended intakes for normal adults. Could the recommended intake of 0.8 g · kg⁻¹ body wt · day⁻¹ lead to insufficient indispensable amino acid intake for obese, hyperglycemic, type 2 diabetic individuals with microalbuminuria, while at the same time protecting the kidneys by a lower intake of those amino acids that may increase GFR? The plant-protein diet of the study would still have provided sufficient indispensable amino acids by either requirement criterion (for normal individuals) and no important differences for alanine, arginine, or glycine.

However, the potential of dietary proteins for renoprotective effects depends on many factors in addition to protein quality. The first is the relative inefficiency of utilization of protein due to obesity itself. This is exacerbated in type 2 diabetes in a dose-response manner by the magnitude of deterioration of metabolic (glycemic) control. More of both endogenous and dietary protein is oxidized directly and/or becomes substrate for gluconeogenesis as hyperglycemia increases. This imposes an increased requirement for high-quality dietary protein for body protein equilibrium, but the precise quantities remain to be defined (3–5). The cost of such equilibrium during chronic hyperglycemia is ongoing acceleration of protein turnover (2), whose short- and long-term consequences also have yet to be identified. Furthermore, during the hypoenergetic state required for fat loss, higher protein intakes are required for maintenance of protein balance. Because weight reduction (as fat) is the primary goal in obese individuals with or without diabetes, until this results in substantial metabolic improvement, restriction of dietary protein intake could result in greater contribution of lean tissue to the weight loss. Therefore, each of these factors must be controlled for in the additional studies required to define the optimal conditions for protection of renal function.

In testing plant versus animal protein in type 2 diabetes, two other variables require control. The first was carefully addressed by Wheeler et al., in that soy protein contributed substantially to the total in the plant protein diet. If less “refined” sources are used, the amount of food required for equivalent protein intakes increases considerably. For example, whereas 97 g chicken breast or 111 g pork roast yield 30 g protein, one would have to consume 131 g cod fillet, 5 eggs, 180 g soy beans as mature seeds, or 458 g tofu to achieve the same protein intake. In short, less total protein might be consumed despite a larger weight and volume of food in a diet prescription that emphasizes plant protein sources. The second variable is that many other constituents of diets based on plant protein could well affect renal end points. Whereas many were controlled for in the Wheeler et al. study (e.g., fiber, sodium, calcium, phosphorus, the kinds and proportions of triglycerides, and cholesterol), they are a major challenge when “real food” is consumed.

The “diabetes lipidus” component of this study merits comment. Although soy protein reportedly has a cholesterol-lowering effect, a 9% decrease in total cholesterol occurred with both diets. The animal protein diet had no soy protein, but both diets conformed to current recommendations for fat and cholesterol intakes. This suggests no additive benefit from soy protein beyond that from the recommended diet in this study population. It is reassuring that the rigorous adherence to a diet that follows standard recommendations had exactly the desired effects on not only total cholesterol but also on HbA_1c and diastolic blood pressure, along with modest weight loss.

Diabetes is truly “diabetes proteinus,” and this aspect of disturbed metabolism can no longer be considered metabolically or clinically unimportant. Interventions that test for effects of protein quantity and/or quality must take account of the fact that protein metabolism is affected by obesity and incrementally by worsening of metabolic (glycemic) control in diabetes. The studies of dietary protein restriction in established nephropathy have not been addressed, but many reviews (5,6) and meta-analyses are available. Notably, few have controlled for level of diabetes control, which could well have obscured more subtle effects, including renal function end points. Clearly, there is an opportunity for a major thrust in research on protein in obesity, diabetes, and nephropathy. Our conclusion regarding the study in this issue of Diabetes Care is that at the high intakes of high-quality protein used, although there was not an advantage to the high soy all plant-protein diet, the challenge is now to determine whether this applies to lower intakes and to plant sources that might have both different amino acid compositions and other potentially “protective” nutrients.