The diet and lifestyle of domestic cats has changed over the last 20–30 years, and they are increasingly kept indoors and are sedentary. Cats are now typically fed energy-dense food that is moderate to high in fat and carbohydrate content, and commonly fed in excess of their daily energy requirements. In some populations, two of every 3 cats are overweight or obese. The overweight condition in cats is associated with increased risk for the development of diabetes mellitus, among other diseases. As in humans, lifestyle and dietary factors are involved in the development of diabetes.[3, 5] Type 2 diabetes in humans and diabetes in cats are very similar diseases clinicopathologically. In humans, over 50% of patients with type 2 diabetes are undiagnosed, and prediabetes, defined as impaired glucose tolerance or impaired fasting glucose, is 2–4 times more common than diabetes. It is unknown if these statistics are true for cats. The reported prevalence of diabetes in cats has increased, and it is likely there are many cats with undiagnosed prediabetes and diabetes.1,2
There is no consensus in veterinary medicine on the best diet for the prevention of diabetes in cats, with both moderate-carbohydrate, low-fat, high-protein, high-fiber diets recommended to prevent or manage obesity, and low-carbohydrate, moderate-fat, high-protein diets recommended to reduce postprandial hyperglycaemia.[7, 8] Minimizing the increase in glucose concentration after a meal and the subsequent demand on beta-cells to secrete insulin is a primary goal for the management of prediabetic and diabetic human patients. In humans, it is more important (but also more difficult) to normalize postprandial hyperglycemia, as compared to fasting glucose concentrations. Chronic hyperglycemia increases the demand on the beta cells to secrete insulin, and chronic hyperinsulinemia is associated with eventual beta-cell failure and type 2 diabetes. The International Diabetes Federation defines postprandial hyperglycemia in humans as a plasma glucose concentration of greater than 7.7 mmol/L (140 mg/dL), and glucose toxicity can cause impaired beta-cell function at glucose concentrations that are only 1 mmol/L (18 mg/dL) higher than normal.[12-14] Given that the upper cutpoint for normal fasting blood glucose is similar in humans and cats,[5, 15-17] and that cats have a very prolonged postprandial period (≥12 hours), postprandial blood glucose concentrations >7.7 mmol/L (140 mg/dL) might also be detrimental in cats.[18, 19] Diets with moderate to high levels of carbohydrate (6.7–14.5 g carbohydrate/100 kcal; 23–50% metabolizable energy [ME]) result in peak and mean 24-hour postprandial glucose concentrations greater than 8 mmol/L (144 mg/dL) in some cats.[16-18] Of concern was that a diet with 14.5 g/100 kcal (51% ME) resulted in peak glucose concentrations as high as 10.8 mmol/L (194 mg/dL) in lean cats, and 13.4 mmol/L (241 mg/dL) after moderate weight gain (mean body condition score 6.3 out of a 9 point scale), which is considered in the diabetic range for cats. Overweight and obese cats have increased mean concentrations and duration of the postprandial glucose and insulin response.[16, 18] Therefore, reduced carbohydrate diets might be useful in cats that are at risk of diabetes, especially obese, physically inactive cats, European-origin Burmese cats, diabetic cats in remission, and cats with intrinsically low insulin sensitivity. Low-carbohydrate, moderate-fat, high-protein diets decrease postprandial blood glucose concentrations, insulin requirements, or both compared with moderate-carbohydrate, low-fat diets.[7, 21] However, there is still controversy regarding carbohydrate content in feline diets, and in most reported studies comparing diets with differing carbohydrate content, other nutritional differences between the diets confounded comparisons.
The aim of this study was to compare postprandial glucose and insulin concentrations and energy intakes between healthy cats fed diets high in protein, fat, or carbohydrate. Importantly, the knowledge gained from this study informs the debate about the effect of dietary carbohydrate content on postprandial glucose and insulin concentrations, and energy intake. The results are relevant to cats at risk of diabetes, especially those with insulin resistance, impaired fasting glucose, and/or impaired glucose tolerance.
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- Materials and Methods
Our study showed that in healthy lean cats, consumption of a diet with approximately 50% of energy from carbohydrate (12.9 g/100 kcal) resulted in higher peak and mean blood glucose concentrations for 4–18 hours after eating, compared with consumption of a high protein or a fat diet containing approximately 25% of energy from carbohydrate (4.5–5.4 g/100 kcal). Cats consuming the high-protein diet consumed more energy than cats consuming the high-carbohydrate diet. However, the overall effect was that cats fed the high-carbohydrate diet consumed 25% more carbohydrate than cats fed the high-fat diet, and had 19–25% higher mean and 23% higher peak glucose concentrations. Similarly, cats fed the high-carbohydrate diet consumed an 18% higher carbohydrate load than cats fed the high-protein diet, and had 25–30% higher mean and 30% higher peak glucose concentrations. This finding has important implications for cats predisposed to diabetes, for example obese and European-origin Burmese cats, and cats with insulin resistance. It is also likely important for cats already exhibiting some degree of beta cell failure resulting in impaired glucose tolerance—some obese cats and the majority of diabetic cats in remission.[18, 32] Decreasing the glucose load from a meal has been shown to be important for achieving remission in diabetic cats. Although our study compared a high-carbohydrate diet with a moderate-carbohydrate diet, further decreasing carbohydrate content to ≤12% ME has been shown to increase remission rates in diabetic cats. In fact, the highest remission rates (>80%) have only been reported using diets with approximately ≤6% of energy from carbohydrate.[33, 34] The results of our study highlight the likely mechanism for these observations—decreasing dietary carbohydrate load reduces the postprandial blood glucose elevation, which in cats is prolonged.[17, 18] In our study, blood glucose remained above baseline for 11 to >24 hours in cats fed the high-carbohydrate diet. For all 3 diets tested, some cats had peak blood glucose values more than 1 mmo/L (18 mg/dL) above the upper reference range for cats (<6–6.5 mmol/L; 108–117 mg/dL). Of concern was in some cats fed the high-carbohydrate diet, peak glucose concentration (10.4 mmol/L; 150 mg/dL) was in the range reported for diabetic cats. Even in some cats consuming the high-protein diet, peak glucose concentration was as high as 9.0 mmol/L (162 mg/dL). This finding might explain why, in diabetic cats, a diet with 3.5 g/100 kcal ME (12% ME) from carbohydrate resulted in higher remission rates than one with 7.6 g/100 kcal ME (24% ME). Although the diets in the latter study differed in ingredient sources, fiber content, and micronutrients, which could have accounted for some of the differences observed, the carbohydrate content differed markedly between them, and 2 mechanisms likely explain the findings. Firstly, the higher postprandial glucose concentrations from the higher carbohydrate load make it less likely that a cat with very marginal beta cell function can secrete sufficient insulin to maintain euglycemia. In addition, the ongoing effect of glucose toxicity suppresses insulin secretion and prevents recovery of sufficient beta cell function to maintain euglycemia. The magnitude of the increase in postprandial glucose concentrations would likely be even greater had overweight and obese cats been studied.[16, 18]
The increase in blood glucose concentrations after a carbohydrate load is accentuated in cats compared to dogs and humans.[5, 19, 36] The relative carbohydrate intolerance of cats might be because of a number of unique features associated with glucose metabolism in this species, and could also contribute to their extended postprandial period of 8–15 hours compared to 2–3 hours for humans and 3–6 hours for dogs. The gluconeogenic pathway is almost always permanently “switched on.” Cats have markedly reduced or absent glucokinase concentrations, and rely on low-capacity hexokinase to clear a glucose load. Further, delayed gastric emptying,11 reduced small intestinal disaccharidase activity, and reduced and delayed insulin secretion compared with dogs are some of the factors that result in cats having reduced capacity to decrease glucose concentrations after the ingestion of a high-carbohydrate load.[19, 36]
The lower blood glucose concentrations in our cats fed the lower carbohydrate diets is consistent with previous findings,[16, 40] however, others have not reported differences, largely because of the methodology used,[42-45] or the difference in carbohydrate content was not as great. It was not surprising that we found no differences in glucose tolerance test results between the diets, because the glucose tolerance test is performed after a 24-hour fast, and the glucose load is the same for all cats. For there to be differences, it would imply that there was a physiological adaption to a given dietary carbohydrate load that was not modifiable in the short term for a different carbohydrate load. Therefore, the glucose tolerance test is not a sensitive test for determining the daily impact of diets on glucose and insulin concentrations; its use is to assess glucose tolerance status. Similarly, time 0 glucose concentrations were not different between diets for the meal-feeding test because cats had been fasted for 23.5 hours. In contrast, for the ad libitum feeding test, cats had access to food continuously before time 0, and hence their time 0 glucose concentrations reflected the effect of diet on postprandial glucose concentrations, which were significantly different between diets.
There were also 2 trends worth noting: firstly, postprandial insulin concentrations were consistently higher after the high-carbohydrate diet was consumed compared to the high-protein diet, but were similar to the high-fat diet. The small group size (n = 8), higher CV for the insulin assay compared to glucose assay, and the variability in the actual amount of food consumed by each cat likely contributed to the variability in insulin results and lack of statistical significance. Secondly, glucose to insulin ratio was highest for the high-carbohydrate diet and lowest for the high-fat diet. The finding of a trend to the highest glucose to insulin ratio for the high-carbohydrate diet might reflect the higher glucose concentrations, secondary to the high carbohydrate load coupled with reduced clearance of blood glucose in cats.[19, 36] An additional contributing factor was that insulin concentrations in cats fed the high-fat diet were similarly high to those fed the high-carbohydrate diet. This finding might be related to stimulation of incretin secretion by free fatty acids in the high-fat diet, which increases insulin secretion.[47, 48]
In our study, dietary protein did not appear to be a substantial driver of postprandial insulin concentrations in cats compared to dietary carbohydrate. This is despite amino acids being secretagogues for insulin in dogs, cats and humans.[49, 50] However, this finding may have been because of the postprandial amino acid concentrations being insufficient to fully stimulate insulin secretion. Therefore, to reduce the postprandial increase in glucose and the subsequent demand for postprandial insulin secretion, restricting carbohydrate content appears most effective in cats. Our results are consistent with results from studies in humans and dogs, which indicate that carbohydrates are the principal nutrients involved in determining the magnitude of the postprandial changes in plasma glucose and insulin.[52, 53] Based on the collective effects in our study of diet on peak and mean glucose and insulin concentrations, substitution of protein for carbohydrate would be expected to lead to lower postprandial glucose and insulin concentrations compared to substitution with fat.
Obesity is a recognized risk factor for development of diabetes,[4, 54] and in our study, cats fed the high-protein and high-fat diets consumed more energy than the high-carbohydrate diet. Cats consuming the high-protein diet consumed 9% more energy than cats eating the high-fat diet, but the difference did not reach statistical significance. Although previous studies have reported high-fat diets fed ad libitum are associated with obesity in cats,[44, 54, 55] the energy consumption of the high-protein diet was less expected. However, in a recent study, when cats were fed diets of the same energy density and similar fat content, those eating a low-carbohydrate, moderate-fat, high-protein diet gained more weight than those on a high carbohydrate, moderate-fat, low-protein diet, so it was unknown which macronutrient had the greatest effect on weight gain. Weight gain was reported to be caused by the higher energy efficiency of the low-carbohydrate, moderate-fat, high-protein diet. The finding in our study that cats consuming the high-carbohydrate diet had the lowest energy intake was expected because cats are reported to limit their total energy intake when consuming a high-carbohydrate diet. Cats have a ceiling for carbohydrate intake, which limits ingestion when consuming high carbohydrate foods and constrains them to deficits in protein and fat intake, relative to their targets for these macronutrients. The authors suggested this “carbohydrate ceiling” can reflect the cats' adaption to a carnivorous diet. In the same study, cats overshot protein intake relative to their protein target, presumably to gain energy (a limiting resource). The similar energy intakes observed in our study with the high protein and high-fat diets, and lower intake with the high-carbohydrate diet, might have simply reflected that cats fed the high-protein and high-fat diets were eating to achieve a similar perceived required energy intake, and cats fed the high-carbohydrate diet had lower energy intake because of the need to limit carbohydrate intake to a ceiling, which was reported to be 75 kcal/cat/day (approximately 30% of cats' maintenance energy requirements) for cats fed a high-carbohydrate diet for a week. Our cats consumed on average 129 kcal/cat/day (approximately 53% of the study cats' maintenance energy requirements) of carbohydrate, after being fed the high-carbohydrate diet for 5 weeks, and we have previously reported that cats fed a high-carbohydrate diet for longer periods have even higher levels of carbohydrate intake. For example, cats fed for 4 weeks at maintenance energy requirements and 8 weeks ad libitum, consumed 150 kcal/cat/day of carbohydrate (approximately 88% of the study cats' maintenance energy requirements), double the carbohydrate ceiling reported in cats fed a high-carbohydrate diet for 1 week. This suggests that over time, the carbohydrate ceiling is higher if cats are more adapted to the diet.
Our finding of the highest energy consumption with the high-protein diet and lowest with the high-carbohydrate diet might explain why there is no clear evidence from epidemiological studies of an effect of dietary composition on the predisposition to diabetes. Higher consumption of the high-protein and high-fat diets leading long term to obesity might counter the beneficial effect of lower glucose concentrations. Indeed, findings from a recent study in cats suggested that feeding a diet with 50% of energy from carbohydrate produced similar adverse effects on mean postprandial blood glucose and insulin concentrations as short-term moderate weight gain, although other dietary differences might have confounded results. One epidemiological study has reported a dietary effect on risk of diabetes. In a study among feline patients in the United Kingdom, consumption of a mix of wet and dry foods was associated with a lower risk for diabetes, compared to only dry diets (typically high carbohydrate, moderate fat, and protein), or only wet diets (typically lower in carbohydrate and higher in fat and protein). In that study, cats fed wet diets were 3 times more likely to develop diabetes than cats fed mixed diets; cats fed dry diets had 2 times the risk, suggesting the adverse effect of obesity was greater than the adverse effect of increased postprandial glycemia on risk of diabetes. Although wet foods have on average lower carbohydrate content than dry foods, the sauces may contain simple sugars, which would be expected to exacerbate postprandial glycemia. Well-designed studies are urgently required to better understand dietary risk factors for feline diabetes. In our study, cats were fed ad libitum for only 48 hours, and intake measured in the last 12 hours. The effects of fat, protein and carbohydrate on energy intake and risk of diabetes need to be evaluated over a longer time to determine if this finding is true over longer periods.
An important factor that differentiates our study from others is that all diets used contained the same ingredients and did not differ in carbohydrate source because this may cause some variation in blood glucose concentration. The carbohydrate source used in our study (corn/sorghum) produces a lower glycemic response in cats than rice, which is commonly used as a starch source in feline diets. Fiber sources were also the same between diets (mostly soluble fiber from beet pulp). Fiber content was similar between all test diets (0.3–0.5 g/100 kcal), and comparable in amount to a low-fiber, low-carbohydrate diet (0.1 g/100 kcal) associated with increased remission rates in diabetic cats, when compared to a high fiber, moderate-carbohydrate diet (3.1 g/100 kcal). There are almost no data in the literature on the effect of fiber on postprandial glucose concentrations in cats. One study reported that a high-fiber diet (3.9 g/100 kcal) promoted better glycemic control in diabetic cats than a diet lower in fiber (0.5 g/100 kcal). However, results were confounded by differences in carbohydrate content between diets, as the low-fiber diet had higher carbohydrate content (38% DM) than the high-fiber diet (27% DM). In our study, the carbohydrate load may not be responsible for the entirety of the differences seen between the diets used in our study, and other unidentified effects of protein and fat may have influenced the results, including incretin effects and effects of fat and carbohydrate on insulin sensitivity. However, it is not possible to test just 1 effect in isolation from the others, because changing 1 macronutrient inevitably changes others—lowering carbohydrate content must result in increases in either fat, protein, or both. It is clear that further studies are needed in this area of feline nutrition to gain better understanding of the effects of dietary macronutrients on energy intake and postprandial glucose concentrations, and the subsequent risk for obesity and diabetes.
In conclusion, our study shows that a diet with approximately 50% of energy (12.8 g/100 kcal) from carbohydrate results in significantly higher postprandial blood glucose concentrations for a median of 24 hours after eating compared to diets with approximately 25% of energy from carbohydrate (7.1–7.3 g/100 kcal) and 50% from either fat or protein. However, even diets with 25% of energy from carbohydrate result in peak blood glucose concentrations as high as 9 mmol/L (162 mg/dL) after eating in lean cats with normal glucose tolerance. Given the frequency of impaired glucose tolerance in obese cats, diets with carbohydrate content <25% ME are recommended to minimize postprandial glucose and insulin response, and to avoid potential glucose toxic damage to beta cells. However, the high-protein diet was associated with the highest energy intake, and so cats at risk of diabetes should be fed at maintenance energy requirements to avoid obesity. Further research is required to determine the optimum level of dietary carbohydrate, fat, and protein for cats.