Unfortunately there are not a lot of “long term studies” into the effect of low carb diets on T2D. Most are 3-6 month studies. Longer term studies are generally regarded as 12 months or more.
Let’s look at the two most recent (2021) systematic reviews/meta-analyses by Goldenberg et al published in the BMJ earlier this year, and Nicholas et al published last month in Journal of Nutritional Science.
Goldenberg and colleagues state that “Three studies reported on T2D remission at 12 months.(Refs 35, 39, 41) When remission was defined independently of medication use, low carb diets (LCDs) increased remission”, and that "At 12 months, eight studies reported on HbA1c levels, showing that the effect size had decreased by around half (mean difference –0.23%, –0.46% to 0.00%; n=489; GRADE=moderate)”.
Goldenberg JZ, Day A, Brinkworth GD, et al. Efficacy and safety of low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis of published and unpublished randomized trial data. BMJ. 2021;372:m4743. doi:10.1136/bmj.m4743
Nicholas and colleagues state that “ over the past 5 years, ten meta-analyses, based on nearly fifty randomised controlled trials (RCTs), have aimed to address the question of whether diets low in carbohydrates produce greater improvements in weight and glycaemic control compared with higher carbohydrate control diets(21–30). The majority of these meta-analyses have found a beneficial effect from carbohydrate restriction(21–27), and none have favoured higher carbohydrate comparators, although several studies have found no difference between diets(28–30). Most recently, Goldenberg et al. (27) performed a comprehensive meta-analysis of the effect of LCD (<130 g/d) on T2D remission. They identified higher rates of diabetes remission among LCDs compared with low-fat diets at 6 months, an effect which diminished at 12 months.
From the Nicholas review, the paper looking at the effect of low carb diet on T2D which has the longest follow up ( 24 months) is the paper by Athinarayanan et al (2019) which shows a change in HbA1c less than after 1 year but still significant (graph below).
Athinarayanan, SJ, Adams, RN, Hallberg, SJ, et al. Long-term effects of a novel continuous remote care intervention including nutritional ketosis for the management of type 2 diabetes: a 2-year nonrandomized clinical trial. Front Endocrinol 2019;10:348
Nicholas AP, Soto-Mota A, Lambert H, Collins AL. Restricting carbohydrates and calories in the treatment of type 2 diabetes: a systematic review of the effectiveness of ‘low-carbohydrate’ interventions with differing energy levels. J Nutr Sci 2021;10. doi:10.1017/jns.2021.67