Can a vegan diet help people with type 1 diabetes save on insulin? A secondary analysis of a 12-Week randomized clinical trial

Abstract

Aims/Hypothesis

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This secondary analysis compared the effect of a vegan to a portion-controlled diet on insulin use and insulin costs in people with type 1 diabetes (T1D).

Methods

Fifty-eight adults with T1D were randomly assigned to a vegan (n = 29) or a portion-controlled group (n = 29) for 12 weeks. Federal Supply Schedule pharmaceutical pricing was used to assess insulin costs.

Results

Total dose of insulin decreased by 12.1 units/day in the vegan group (p = 0.007), compared to no significant change in the portion-controlled group (treatment effect − 10.7 units/day [95% CI, -21.3 to -0.2]; p = 0.046). Total insulin costs decreased by 27% ($1.08/day; p = 0.003) in the vegan group, compared to no significant change in the portion-controlled group (-$0.38/day [95% CI, -$2.13 to +$1.38]; p = 0.66).

Conclusions/Interpretation

This study shows that a low-fat vegan diet could reduce insulin use and insulin costs in people with T1D. Larger trials are needed to confirm these findings.

Trial registration

ClinicalTrials.gov, NCT04944316, registered on June 29, 2021.

Introduction

According to the American Diabetes Association, spending on insulin in the US tripled in the past ten years, reaching $22.3 billion in 2022, due to the increased usage and higher price of insulin. The inflation-adjusted cost of insulin increased by 24% from 2017 to 2022 [1]. Furthermore, the costs associated with type 1 diabetes have been shown to be about 32% higher, compared to those for type 2 diabetes [2]. Despite recent efforts to restrain insulin pricing, novel treatments for type 1 diabetes are urgently needed for both health and financial reasons. Dietary guidance is essential for all individuals with diabetes, and various dietary approaches may differ in their effects on insulin requirements and associated costs. A low-fat vegan diet has been shown to reduce body weight and ectopic fat, which have contributed to increased insulin sensitivity in overweight people [15]. Since about half people with type 1 diabetes are overweight [24], it is plausible to assume that this approach would also help increase insulin sensitivity in this population.

A previous 12-week randomized clinical trial compared the effects of a conventional portion-controlled, carbohydrate-controlled diet with a low-fat vegan diet in people with type 1 diabetes, and demonstrated an improvement in insulin sensitivity and markers of cardiometabolic health with a low-fat vegan diet [3]. This secondary analysis assessed insulin use and insulin costs associated with both dietary interventions.

Methods

The study methods have been described in detail previously [3]. Briefly, this randomized trial took place in 2021–2022 in Washington, DC. This study follows the Consolidated Standards of Reporting Trials (CONSORT) reporting guideline. Adults diagnosed with type 1 diabetes were enrolled. The study protocol was approved by the Chesapeake Institutional Review Board on February 03, 2021. The study participants signed an informed consent.

Dietary interventions

The participants were randomized in a 1:1 ratio to a low-fat vegan diet (n = 29) or a portion-controlled diet (n = 29). The ad libitum low-fat vegan diet (∼ 75% of energy from carbohydrates, 15% protein, and 10% fat) was free of all animal products and consisted of only plant foods, while minimizing added fats, with fat intake up to 30 g a day. The portion-controlled diet emphasized portion control and keeping carbohydrate intake steady. This dietary intervention included individualized diet plans that reduced daily energy intake by 500-1,000 kcal for overweight (body mass index > 25 kg/m²) participants. It derived 60–70% of energy from carbohydrate and monounsaturated fats, 15–20% from protein, < 7% saturated fat, and contained ≤ 200 mg/day of cholesterol/day. Both groups met weekly online and were supported by registered dietitians. The participants tracked all their meals via Cronometer (Cronometer Inc., Revelstoke, Canada). The participants were instructed to keep their physical activity and their medications constant throughout the study. Insulin was modified in response to repeated hypoglycemia. All outcomes were measured at week 0 and 12.

Insulin use and insulin costs

Basal and bolus insulin units injected per day were self-reported daily through the Dexcom app, and an average from three consecutive days (2 weekdays and 1 weekend day) was used at baseline and week 12 of the study. Based on recommendations by the Second Panel on Cost-Effectiveness in Health and Medicine, Federal Supply Schedule (FSS) pharmaceutical pricing was used to assess insulin costs [4]. Costs were expressed in 2024 U.S. dollars. CM and SK, both blinded to the group assignment, linked the FSS insulin prices to the individual basal and bolus insulin doses. The accuracy was checked by WHH, also blinded to the group assignment.

Statistical analysis

The statistician was blinded to the interventions. A repeated-measure ANOVA model was used with between-subject and within-subject factors and interactions to examine group differences. Within each group, paired comparison t-tests were calculated to test whether the changes from baseline to 12 weeks were statistically significant. All results are presented as means with 95% confidence intervals (CIs).

Results

Characteristics of the participants

Of the 377 people who completed an initial screening, 58 met participation criteria and were randomly assigned to the vegan (n = 29) or control (n = 29) groups; Suppl. Figure 1).

Dietary intake, physical activity, anthropometric and laboratory variables, as well as the continuous glucose monitoring data in response to 12 weeks of each diet have been reported previously [3]. Briefly, body weight was reduced by 5.2 kg in the vegan group (p < 0.001), compared with a nonsignificant change in the portion-controlled group (treatment effect − 4.3 kg [-6.1 to -2.4]; p < 0.001). Correspondingly, body mass index was reduced by 1.9 kg/m² in the vegan group (p < 0.001), compared with a nonsignificant change in the portion-controlled group (treatment effect − 1.6 kg/m² [-2.2 to -0.9]; p < 0.001).

Insulin use and insulin costs

Insulin use and insulin costs are shown in Table 1. On the vegan diet, total daily dose of insulin decreased by 28%, i.e. by 12.1 units/day (p = 0.007), while it did not change on the portion-controlled diet (treatment effect − 10.7 units/day [95% CI, -21.3 to -0.2]; p = 0.046). No increase in hypoglycemia or hyperglycemia was noted in either group. Total daily insulin costs expressed in year 2024 prices decreased by 27% ($1.08/day; p = 0.003) in the vegan group, compared to no significant change in the portion-controlled group (-$0.70/day; p = 0.38; treatment effect -$0.38/day [95% CI, -$2.13 to +$1.38]; p = 0.66).

Table 1 Insulin use and costs in the vegan and Portion-Controlled group at baseline and at 12 weeks

The dose of basal insulin decreased in both groups: by 7.7 units/day; p = 0.009 on the vegan diet and by 4.5 units/day; p = 0.03 on the portion-controlled diet (treatment effect − 3.2 [95% CI, -9.9 to + 3.5]; p = 0.34). Basal insulin cost decreased on the vegan diet by 34% ($0.73/day) (p = 0.003), compared to no significant change on the portion-controlled diet (between-group difference p = 0.69). The dose of bolus insulin and its cost did not change significantly in either group. As reported earlier [3], despite these major reductions in insulin use, HbA1c values were reduced by 0.8% point on the vegan diet and by 0.6% point on the portion-controlled diet (between-group difference p = 0.34), and there was no increase in hypo- or hyperglycemic events in either group.

Discussion

In this study, compared to a portion-controlled diet, a low-fat vegan diet, with no limits on calories or carbohydrate intake, reduced the total daily insulin use by 28% in people with type 1 diabetes, with concomitant improvements in glycemic control. Total daily insulin costs decreased by 27% in the vegan group, compared to no significant change in the portion-controlled group.

The reductions in insulin use in the vegan group likely reflect improved insulin sensitivity, resulting from the reduction in liver and muscle fat [5]. This lower insulin use on the low-fat vegan diet reflects a marker of improved cardiometabolic health [6] and of reduced treatment costs, in addition to the likelihood of lower food costs on a vegan diet, as previously reported [7].

In a retrospective study of United Healthcare patients (2015–2017) that included 12,700 people with type 1 diabetes, the average yearly medical costs to the payer totaled $18,800 per person with type 1 diabetes, $4,600 higher than the cost of treating a person with type 2 diabetes, and far higher than for those without diabetes. Of the costs associated with type 1 diabetes, the average prescription costs were $10,200 (±$11,000) per person per year [2]. In another retrospective study using 2011–2016 IBM MarketScan and electronic medical record data, the average cost per person with type 1 diabetes per year was approximately $9,600. The majority of these costs (55%-61%) were due to pharmacy costs [8].

Insulin prices have been increasing steadily and are projected to increase even further, despite efforts to rein in such costs. Between 2014 and 2019, the average price of insulin in the US increased by 55%, from $3,819 to $5,917 per year [9]. The out-of-pocket costs of insulin differ greatly depending on the population and the type of insurance. In a 2018 IBM Marketscan Database study of 65,000 people with type 1 diabetes covered by private insurance, the average annual out-of-pocket cost per person using insulin varied between $472 (±$625) and $1,037 (±$1,039) [10]. The yearly out-of-pocket insulin costs for Medicare Part D recipients increased from approximately $588 in 2014 to $696 in 2019 [9]. These studies highlight the need for interventions that will help people with type 1 diabetes save on the cost of insulin. A low-fat vegan diet appears to be a promising option, partly due to its ability to reduce body weight and ectopic fat, and to increase insulin sensitivity.

The strengths of this study include the randomized parallel design, in which all participants started simultaneously, allowing the investigators to rule out possible effects of seasonal fluctuations in the diet. The study duration was reasonably long, providing sufficient time for adaptation to the diet. A vegan diet is healthy and nutritionally adequate, and provides all the needed nutrients and macronutrients, except for vitamin B12, which needs to be supplemented [11]. Given that the participants were living at home and preparing their own meals or eating at restaurants, our results are applicable outside the research setting, in free-living conditions.

The study also has important limitations. Dietary intake was calculated based on self-reported diet records, which have well-known limitations [12], but this was partly offset by the repeated collection of the dietary data by each participant. That means that participants served as their own controls for the dietary data collected at different time points, which minimized the impact of inter-individual differences. The same applies to the self-reported insulin data. While the 3-day average only represents a snapshot of the overall insulin use, it has been found to be a fairly accurate representation, which has allowed people with diabetes to achieve comparable glycemic control as with physician-directed titration [13]. Furthermore, the study requirements included careful meal and blood glucose monitoring for research purposes, beyond that required for self-care, which contributed to a substantial attrition rate. The small sample size was modest, and the study duration was limited. Prices for short-acting and long-acting insulins vary over a wide range, which may explain why the between-group difference in insulin price did not reach the statistical significance, although the groups differed in total insulin dose. The expenses associated with dietetic instruction and may or may not be covered by insurance. However, all individuals with type 1 diabetes receive dietetic instruction, and these costs would be expected to be similar, regardless of diet type. Some may supplement instruction with free resources, such as books and pre-recorded videos. Finally, our participants were health-conscious volunteers and may not be representative of the general population, but they are likely representative of individuals seeking clinical care.

Conclusions

In conclusion, this study has demonstrated that a low-fat vegan diet, without limits on calories or carbohydrate intake, significantly reduced insulin use and insulin costs in people with type 1 diabetes. The study suggests the potential health and financial benefits of a low-fat vegan diet in type 1 diabetes management. Larger trials with longer follow-up are needed to confirm these findings.