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Should patients with diabetes follow a low-carb diet?

Before you recommend a low-carbohydrate diet for weight loss, read this review of the effect these diets can have on a patient with type 2 diabetes.

Michael J. Chandler, RD, CNSD, MS; Leslie A. Hildebrandt, PhD, RD

Michael Chandler is clinical dietitian at Appalachian Regional Medical Center, Hazard, Kentucky. Leslie Hildebrandt is Associate Professor of Nutrition and Dietetics, Central Michigan University, Mount Pleasant. The authors have indicated no relationships to disclose relating to the content of this article.

Type 2 diabetes is a heterogeneous disorder characterized by variable plasma insulin levels with associated hyperglycemia and peripheral insulin resistance. Postinsulin-receptor impairments include altered glucose transporter function and enzymatic abnormalities. Insulin resistance manifests as persistent hepatic glucose production and diminished peripheral glucose disposal. These pathophysiologic mechanisms contribute to the development of heart disease, neuropathy, and nephropathy.¹

Insulin resistance alters macronutrient metabolism, resulting in diminished protein synthesis and excessive protein degradation with a loss of lean body mass. Muscle amino acids used to support increased hepatic glucose production contribute to hyperglycemia.¹ Insulin resistance also alters lipid metabolism.

Most patients with type 2 diabetes are overweight (defined as a body mass index [BMI] of more than 25 kg/m²) before their disease is diagnosed; insulin resistance increases with higher body adiposity.¹ Patients are encouraged to decrease body fat in order to decrease insulin resistance, thereby reducing their risk of chronic complications. However, a low-carbohydrate diet may influence macronutrient metabolism differently in patients with type 2 diabetes.^2,3 Carbohydrate-restricted diets may in fact lower lean body mass, thereby increasing percent of body fat. This change in body composition potentially increases insulin resistance.

LOW-CARBOHYDRATE DIETS

Many of the most popular low-carbohydrate diets, also known as ketogenic diets, focus on consuming animal-based foods and suggest limiting intake of fruits, vegetables, and complex carbohydrates, not just simple carbohydrates (see Table 1). Saturated fat content in most cases is more than the current recommendations for disease prevention. And, in view of the limited food choices, adequate micronutrient consumption is often corrected with supplements.

Exogenous carbohydrate restriction lowers glucose levels, resulting in tiredness, headache, and agitation. The body responds by breaking down muscle protein to maintain adequate glucose levels, which depletes muscle mass. In addition, elevated urea production from amino acid catabolism causes dehydration.

This discussion reviews the results of 26 studies that examined the effect of low-carbohydrate diets on body weight. The studies were conducted during the past 40 years. “Table. A review of 26 studies on low-carbohydrate diets” (accessible via the online version of this article) shows a breakdown of the study populations, diet specifics, and weight loss results.

Study logistics Some low-carbohydrate diets have been found to produce significant weight loss. In many cases, however, carbohydrate restriction was accompanied by very low energy intake.^4-13 In contrast, some studies found that carbohydrate restriction (less than 20 g/d) had minimal impact on weight loss.^14-17 Therefore, whether carbohydrate restriction is an important variable for weight reduction is difficult to determine. Important factors that impact weight loss are diet duration, initial BMI, the contribution of carbohydrates to energy intake, total kilocalories (kcal) consumed, and how caloric consumption compares to energy requirements. Consumption of simple versus complex carbohydrates and the amount of dietary fiber consumed also impact weight loss.

Study size The number of participants in these studies ranged from 1 to 119; seven studies included fewer than 12 participants in each treatment group. Therefore, some studies may not demonstrate a qualified evaluation of the impact of carbohydrate restriction on weight loss because an inadequate number of subjects were included in the study. Subjects’ obesity status was based on BMI; however, initial BMI does not influence the effect of low-carbohydrate consumption on weight loss.

Diet duration Depleting levels of water, muscle glycogen, and muscle protein produce a rapid weight loss in the early days of following a diet. Interestingly, a less pronounced effect on weight loss was observed when the study was prolonged. Study lengths ranged from 7 to 365 days. People reducing their body fat over a long period may become disillusioned over time.

CARBOHYDRATE CONSUMPTION

The percent of total energy intake that was dietary carbohydrates compared to the total amount of carbohydrates consumed each day should be considered. In the carbohydrate-restricted groups, 4% to 44% of calories consumed were carbohydrates, compared to 45% to 75% in the control group. In several cases, however, carbohydrate consumption for the control group was not known.

The quantity of carbohydrates consumed by many of the treatment subjects was much less than current recommendations (55% to 65% kcal from carbohydrates). Carbohydrate levels in 10 studies were 10% of recommended kcal or less, which can substantially reduce lean body mass. Yet carbohydrate consumption levels in the treatment groups of two studies were very close to current recommendations, and the diets these groups followed may not be considered low-carbohydrate.^18,19

The type of carbohydrate (simple versus complex) and the amount of dietary fiber consumed also are important factors. Eight studies measured dietary fiber content. On average, the participants eating a low-carbohydrate diet consumed slightly less dietary fiber than the control participants (18.2 g/d vs. 20.4 g/d).^10,15,20-25 Only one study measured the type of carbohydrates consumed; however, the contribution to total calories was unknown.¹⁵ Future studies are needed to assess the role of starches, simple carbohydrates, and dietary fiber content in weight loss.

To promote a safe rate of weight loss that preserves metabolically active tissue, most clinicians recommend consuming more than 1,200 kcal/d.¹⁹ The caloric intake of the low- carbohydrate treatment groups ranged from 500 to 2,300 kcal/d. One study provided participants with caloric intake that maintained their body weight; another study did not place any restrictions on caloric intake.^14,26 Thirty-three percent of the studies required caloric intakes that were below the current recommendations for safe weight loss. Weight losses reported in these studies could have been from fluid and muscle depletion. The caloric intakes of the control groups were sometimes quite low, often matching those of the treatment group. In some cases, caloric intake by the control group was not known; therefore comparisons between the groups were not possible. Participants consuming comparable amounts of total kcal exhibited similar amounts of weight loss.^{11,12,15,21,24,25,27,28} However, initial and final body weight values were unavailable. The missing data prevent investigators from comparing percent weight change, which renders the study results inconclusive.^26,29-32

Caloric intake should be compared to the amount of energy expended when evaluating the impact of carbohydrate-restricted diets on weight loss. Larger or more active people require more energy than smaller or more sedentary people. Unfortunately, 27% of the studies did not determine energy requirements; therefore energy balance could not be calculated. In 80% of those studies, carbohydrate-restricted participants had a negative energy balance because they consumed only 30% to 88% of recommended kcal requirements. As expected, weight loss was more pronounced in participants for whom kcal were restricted. The composition of that loss is not known because muscle mass and water are assumed to be lost during rapid weight loss.^33-35 Results of two studies showed carbohydrate-restricted participants lost significantly more weight than the control group even though the treatment group had a more positive or equal energy balance.^10,20 However, the sample size in one study was small and dietary consumption was determined through self-reported food records.²⁰ Future studies need to include larger populations, longer diet durations, and complete energy-balance information.

An important goal for patients with type 2 diabetes is to maintain their weight loss. Two studies examined the long-term effects of carbohydrate restriction.^24,36 After 1 year, weight loss achieved by consuming 40% to 43% kcal from carbohydrates was not significant compared to weight loss achieved by consuming 52% to 54% kcal from carbohydrates. The effect of low-carbohydrate diets on maintaining weight loss needs further investigation.

Total weight loss needs to be distinguished from changes in body fat. Depletion of metabolically active muscle tissue is an undesirable result. Nine studies measured body composition; however, the results were inconsistent because a variety of methods were used.^{10,12,17,20,22,25,29,30,37} More studies on the impact of carbohydrate restriction on body fat loss and lean body mass preservation are needed.

HYDRATION STATUS

None of the studies assessed total fluid consumption and fluid balance. If high protein consumption and gluconeogenesis secondary to carbohydrate restriction resulted in enhanced urea production, the participants may have become dehydrated. Water loss could have contributed to the weight that was lost. Interestingly, 51% of the carbohydrate-restricted participants (5% kcal from carbohydrates) in one study developed a headache that could have been linked to fluid depletion.²⁹ In another study, 60% of the participants consuming 8% kcal from carbohydrates developed a headache and 35% of the participants developed muscle cramps.¹⁷ In future studies, fluid status should be monitored to evaluate weight loss composition.

GLUCOSE AND INSULIN

Glycemic response reflects the body’s uptake and metabolism of carbohydrates. In addition to consumption of simple versus complex carbohydrates and total carbohydrate consumption, dietary fat intake plays a pivotal role. Fats delay gastric emptying, thereby delaying glucose disposal. Adherence to carbohydrate-restricted diets rich in fat can alter the glycemic response.

Fasting glucose levels were comparatively measured in 11 studies. Participants in six of these studies were euglycemic throughout the study period.^{10,12,22,25,37,38} Preprandial and postprandial glucose levels were recorded in only one study.²⁵ Most of the studies were of short duration (4 to 12 weeks) and contained a small number of participants; only four included patients with diagnosed type 2 diabetes.^21,23,24,39 In many cases, the type of carbohydrates consumed and the amount of dietary fiber were not disclosed. Both of these factors can have a significant effect on fasting glucose levels. When glucose levels in the treatment and control groups were compared, low-carbohydrate intake produced significantly lower circulating glucose levels. After 30 days of consuming 7% kcal from carbohydrates, circulating glucose levels were lowered from 116 mg/dL to 76 mg/dL.¹⁵

In four studies that did not find significantly different glucose levels between groups, carbohydrate restriction was not extensive (40% to 44% kcal from carbohydrates)^12,25,36,39 or dietary fiber intake was higher in the control group.¹² Interestingly, in three of these studies, high glucose levels were maintained both before and after dietary interventions.^25,36,39 Therefore, as expected, exogenous carbohydrate restriction results in decreased fasting glucose levels in normoglycemic people. However, this response was observed in only one of the studies that included patients with type 2 diabetes.²¹

Altering fasting glucose levels also may change insulin response. Insulin regulates tissue glucose uptake and macronutrient metabolism, with secretion occurring only when circulating glucose and select amino acids are present.²⁶ Insulin resistance contributes to the pathophysiology of type 2 diabetes and, as tissues fail to respond to circulating insulin, insulin production rises above normal levels. Hyperinsulinemia may be observed when carbohydrate consumption, energy balance, and body fat are high. However, circulating insulin concentrations may not reflect insulin response by target tissues.

Thirteen studies measured fasting insulin levels.^{10,12,15,
20-22,24,25,32,36-39} Circulating insulin was normal in 10 studies,^{10,12,15,20-22,25,36,38,39} and two studies^25,37 included people with hyperinsulinemia. In 8 of the 13 studies, insulin levels decreased significantly when carbohydrate consumption was decreased.^{10,12,15,20,22,37-39} One study did not provide data for the control group, thereby limiting comparative analysis.³² Circulating insulin levels increased in normoinsulinemic subjects during carbohydrate restriction in one study, possibly because carbohydrate-restricted subjects had a higher dietary fiber intake (25 g/d).²⁵ The percent drop in circulating insulin was greater for the treatment group compared to the control group. The six studies that illustrated the greatest change in plasma insulin did not indicate the type of carbohydrates consumed.^{12,20,22,37-39} Dietary fiber and fat composition, as well as the level of physical activity, also were not monitored in some studies.^12,37,39 The majority of the studies demonstrated that circulating insulin concentrations dropped more often in the low-carbohydrate groups.

Patients with type 2 diabetes produce insulin, but their cells are resistant to its mechanism of action. A decrease in circulating insulin may be beneficial for patients with type 2 diabetes; however, only four studies included participants with diabetes.^21,24,36,39 In addition, a drop in insulin level does not necessarily translate into reduced insulin resistance. If carbohydrate restriction lowers glucose and insulin levels, circulating lipids also may be altered. Further investigation is needed; new studies should have larger populations that include more patients with diabetes, be conducted for longer durations, and include more comprehensive monitoring of glucose and insulin.

LIPOPROTEINS

The risk of developing cardiovascular disease (CVD) is 2 to 4 times higher for the diabetic population. People with diabetes exhibit multiple coronary heart disease (CHD) risk factors including elevated total plasma cholesterol, increased LDL cholesterol, lower HDL cholesterol, and elevated triglycerides. Total plasma cholesterol, although not as indicative as lipoprotein cholesterol levels, is positively related to CHD. Fifteen studies compared total cholesterol levels, with seven of them including participants with normal plasma cholesterol levels prior to dietary intervention.^{21,24,28,37-40} Two studies included people with diabetes.^21,39 In 10 of the 15 studies, no significant difference in total cholesterol levels were found following dietary intervention between the treatment and control groups. However, the low-carbohydrate groups of five studies had significantly more circulating total cholesterol compared to the control groups.^{13,14,24,38,39} The increased plasma cholesterol levels following carbohydrate restriction may be a result of elevated LDL-C levels.

Patients who have diabetes and CHD with elevated LDL-C are 2 to 3 times more likely to have recurrent cardiac events than are patients who have CHD and similar LDL-C but do not have diabetes. Eleven studies measured LDL-C;^{10,13,14,17,21,24,25,37-40} in three of these studies, LDL-C was elevated before dietary intervention.^17,25,40 People with diabetes were included in two of these studies.^21,24 In seven studies, LDL-C increased significantly after carbohydrate restriction.^{13,14,17,21,24,38,40} Many of the carbohydrate-restricted diets resulted in high levels of total fats (all of the studies) and saturated fats (57% of the studies), which may explain the increased LDL-C. None of the reviewed studies found a significant decrease in LDL-C following carbohydrate restriction. Therefore, low carbohydrate diets should not be recommended because resultant increased LDL-C may accelerate mortality and morbidity in patients with diabetes.

HDL transports cholesterol and other lipids back to the liver, which reduces CHD risk. Fourteen studies measured HDL-C;^{10,12-14,17,21,22,24,25,28,37-40} people with diabetes were included in three of them.^21,24,39 HDL was normal prior to dietary intervention in 50% of the studies.^{10,14,17,28,38-40} The impact of dietary intervention on HDL-C was minimal in the majority of cases. However, three studies found significantly increased HDL-C during carbohydrate restriction.^14,17,38 Unfortunately, environmental factors such as exercise and alcohol consumption were not measured; therefore, the effects of these factors were not a part of the study results.

Hypertriglyceridemia reflects insulin resistance and the constellation of metabolic abnormalities that predispose patients with diabetes to CHD. Fasting triglyceride levels reflect endogenous fat synthesis. Fat production occurs when food consumption creates a positive energy balance. Alcohol and simple carbohydrates also increase fasting triglyceride levels. Circulating triglyceride levels were compared in 15 studies.^{10,12-14,17,21,22,24,25,28,36-39} Preintervention triglyceride levels were normal in seven of these studies,^{10,14,22,28,36,38,40} and people with diabetes were included in three of them.^21,24,39 No significant difference in triglyceride levels was observed following carbohydrate restriction in five studies.^{12,28,36,37,39} However, nine studies found circulating triglyceride levels decreased during carbohydrate restriction.^{10,14,17,21,22,24,25,38,40} Severe caloric restriction, which occurred in the majority of the carbohydrate-restricted groups, may have contributed to the decrease in plasma triglycerides. A negative energy balance of the magnitude observed can lead to muscle loss and lower basal energy requirements. This observation is difficult to interpret when alcohol and simple carbohydrate consumption for the control groups is unavailable, which occurred often.

The risk of CVD is a major concern for patients with type 2 diabetes. Low-carbohydrate diets also increase circulating homocysteine and fibrinogen levels, which are positively associated with CVD incidence.¹³ These diets are also low in the micronutrients and phycochemicals that serve as antioxidants and can reduce CVD risk.^10,18,41 Inadequate antioxidant consumption promotes more free radical damage to arterial walls and increases oxidized LDL-C, thereby increasing the incidence of stroke and MI in patients with diabetes.

CONCLUSION

Research has identified key factors that deem carbohydrate-restricted diets to be inappropriate for patients with type 2 diabetes (see Table 2). Many patients with type 2 diabetes follow low-carbohydrate diets in an attempt to lose body fat; however, adhering to low-carbohydrate diets can increase the risk of glycogen depletion, dehydration, and a loss of metabolically active muscle tissue. Furthermore, body fat levels may rise beyond what they were prior to carbohydrate restriction when the diet is discontinued. Also, low- carbohydrate diets may elevate circulating LDL-C and, in turn, increase risk of CVD.

Low-carbohydrate, high-protein diets have other effects on systemic function beyond the scope of this review. Limiting carbohydrate consumption also can impair renal function,^42-44 diminish bone density,^45,46 cause constipation,^17,29,47 and alter thyroid function.^19,27,48 Ketosis created during carbohydrate restriction can impact neurologic function.^17,29 In view of these risk factors, clinicians should carefully consider the appropriateness of recommending a low-carbohydrate, high-protein diet to their patients with type 2 diabetes.

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