Can Any Diet Reverse Prediabetes? What the Best Review Found

There is no dietary intervention with moderate- or high-certainty evidence that it reverses prediabetes. That is the uncomfortable conclusion of the most comprehensive review available: 45 comparisons across nine meta-analyses, involving 10,814 participants, and every comparison earned a weak-credibility rating.

The review did find a repeated signal for vitamin D supplementation. It also found strikingly uncertain results for prebiotics and acute sugar-challenge studies. Meanwhile, Mediterranean-style eating, intermittent fasting, high-fiber patterns, ketogenic diets, and calorie restriction did not make the umbrella review's evidence table at all. Their absence does not show that they fail. It shows that researchers have not built enough prediabetes-specific randomized-trial evidence to judge them at this level.

The evidence ranking starts with an awkward fact

Tse et al.'s 2025 umbrella review combined meta-analyses of randomized trials in people with prediabetes. The authors assessed review quality with AMSTAR 2 and judged credibility with NutriGrade. None of the 45 intervention-outcome comparisons reached moderate or high credibility.

That rating matters more than a long list of nominally significant p-values. A pooled estimate can be statistically significant yet still offer a poor estimate of what will happen when a clinician recommends an intervention to a typical patient. Small trials, inconsistent doses, different populations, short follow-up, and heterogeneous outcomes can all make a precise-looking conclusion less trustworthy.

The review therefore cannot identify a proven dietary winner. It can identify where the limited evidence points—and where it becomes too unstable to guide a decision.

Vitamin D has the most consistent signal, but the likely absolute benefit is small

Vitamin D was the only intervention in the umbrella review with statistically significant findings across four reported endpoints. Supplementation reduced fasting blood glucose (standardized mean difference [SMD] −0.377; 95% confidence interval [CI], −0.598 to −0.165) and triglycerides (SMD −0.385; 95% CI, −0.622 to −0.147). It was also associated with a lower risk of progression to type 2 diabetes (risk ratio [RR] 0.897; 95% CI, 0.810 to 0.994) and a greater likelihood of reversion to normoglycemia (RR 1.238; 95% CI, 1.074 to 1.425).

Those are statistically significant pooled results, but all four received weak credibility ratings. The diabetes-progression estimate corresponds to a 10.3% relative risk reduction. Relative reductions can sound larger than they feel in daily life, so baseline risk is the useful next question.

In a 2025 CARDIA cohort study of 974 young adults with prediabetes followed for a mean of 13 years, 34% progressed to type 2 diabetes, 38% returned to normoglycemia, and 28% remained prediabetic. That works out to roughly 2.6% progressing each year. Applying the vitamin D RR from the umbrella review to that annual risk yields an estimated absolute reduction of about 0.27 percentage points per year, or 0.54 points over two years. The resulting number needed to treat is about 186 over two years.

That calculation is an illustration, not a universal prediction. Older adults and people with higher glucose levels may face higher baseline risk and therefore a larger absolute benefit. It also does not establish that vitamin D works equally well in people with normal vitamin D status. The umbrella review could not resolve whether any benefit concentrates among people who begin deficient.

A 2024 meta-analysis in people with established type 2 diabetes reported improvements in glycemic-control markers with vitamin D, but diabetes treatment evidence does not answer a prediabetes-prevention question. Disease stage, baseline risk, and response to treatment may differ.

Vitamin D is the most plausible supplement signal in this review, not proof that a supplement reverses prediabetes. Testing for and correcting deficiency is sensible clinical care; taking extra vitamin D solely to prevent diabetes remains an uncertain bet.

Why the large-looking prebiotic and fructose results do not settle anything

Prebiotic supplementation was associated with lower body fat in pooled analysis (SMD −1.271; 95% CI, −2.326 to −0.216). But the interval spans more than two SMD units, signaling substantial inconsistency across the included agents, doses, and populations. The review reported no corresponding glycemic improvement for prebiotics. Less body fat can matter clinically, but this result does not demonstrate reversal of prediabetes.

The fructose comparison looked even more dramatic: replacing glucose with fructose produced an SMD of −8.275 for glucose-related outcomes (95% CI, −12.663 to −3.887). A confidence interval nearly nine SMD units wide is a warning label, not a nutrition prescription. These were acute postprandial challenge paradigms measuring glucose area under the curve after single sugar loads. They do not test whether changing habitual fructose intake improves prediabetes over months or years.

Acute physiology can be useful, but it cannot substitute for a chronic clinical outcome. The body's response to one beverage or meal is not a verdict on a long-term dietary pattern.

The most popular diet advice has a prediabetes-specific evidence gap

The umbrella review contained no qualifying meta-analyses for Mediterranean diets, intermittent fasting, high-fiber dietary patterns, ketogenic approaches, caloric restriction, chromium, or cinnamon in prediabetes-specific randomized trials. Its inclusion standard required meta-analyses containing at least three such trials per intervention.

This creates a selection problem. The interventions evaluated most often—primarily vitamin D and prebiotics—are not automatically the interventions most likely to help. They are simply the interventions with enough narrowly defined trial literature to pool. Conversely, an intervention missing from the review has not been cleared or condemned; it has not passed through the review's evidence pipeline.

Time-restricted eating illustrates the distinction. A 2026 umbrella meta-analysis found favorable cardiometabolic and anthropometric effects of time-restricted eating in adults with type 2 diabetes. That is relevant context, but it cannot be imported wholesale into prediabetes. The population is different, and the prevention question remains under-tested.

The same restraint applies to probiotics and turmeric. A GRADE-assessed 2022 review of probiotics and synbiotics found improved glycemic-control parameters across participants with prediabetes and type 2 diabetes, while a 2025 curcumin/turmeric meta-analysis found improvements in inflammatory and oxidative-stress markers in prediabetes and diabetes. Neither result received the umbrella review's prediabetes-specific credibility assessment. Inflammation markers and glycemic markers are also not the same endpoint as preventing diabetes.

Exercise has a better claim on your attention

If the goal is better glycemic control and insulin sensitivity in prediabetes, exercise has stronger direct evidence than any isolated dietary intervention assessed by the umbrella review. A 2022 network meta-analysis of exercise interventions in people with prediabetes found meaningful improvements in glycemic control and insulin resistance across exercise approaches.

That does not mean food quality is irrelevant. The CARDIA cohort linked higher Life's Essential 8 cardiovascular-health scores—which include diet quality among several behaviors—to lower risk of diabetes progression. But that is an association involving a composite lifestyle profile, not a trial that assigns credit to diet alone.

Large lifestyle programs commonly combine dietary change, physical activity, and weight management. The PREVIEW trial analysis followed 2,178 people with ADA-defined prediabetes through an eight-week low-energy diet and showed how much conclusions can depend on the chosen biomarker: only 33% met HbA1c-defined prediabetes criteria, despite all participants meeting eligibility through impaired fasting glucose and/or impaired glucose tolerance. Prediabetes is not one uniform biological state, and a single lab measure does not capture every response.

What to do with this evidence

Do not buy a supplement because a review reports a significant biomarker change. Ask whether the finding applies to people with prediabetes, whether it measures a clinical outcome, how large the absolute effect is, and whether the evidence survived a credibility assessment.

For now, structured physical activity deserves priority because it has stronger prediabetes-specific evidence. Dietary quality belongs in a broader lifestyle plan, but the evidence does not justify declaring one named diet proven to reverse prediabetes. Vitamin D deserves a measured discussion, especially when deficiency is present, yet its pooled prevention signal remains weak-certainty and modest in absolute terms at the CARDIA baseline risk.

Nutrition science has not produced a magic dietary reversal protocol for prediabetes. It has produced a clear research agenda: larger, longer randomized trials that test real eating patterns, define prediabetes consistently, report clinically meaningful outcomes, and separate transient biomarker movement from durable diabetes prevention.