Netherlands Fiber Study Surprises: High-Protein Diet Warning

A Study from the Netherlands Raises Important Questions About Fiber and High-Protein Diets

A study from the Netherlands — conducted at Maastricht University Medical Center, one of Europe's leading metabolic research institutions — delivers a surprising finding that challenges a widely held assumption in diabetes nutrition: that adding fiber supplements to your diet is always beneficial. For the millions of Americans with prediabetes or type 2 diabetes who follow high-protein eating plans, this research deserves careful attention.

What the Researchers Did

Scientists at Maastricht's NUTRIM Institute ran a 12-week randomized, placebo-controlled trial — the gold standard of medical research — involving adults with overweight or obesity. Participants either received a daily supplement combining potato fiber and sugar beet pectin or a placebo (maltodextrin), both layered on top of a high-protein diet providing 25% of daily calories from protein, with roughly 45% of that protein coming from plant sources.

The theory behind the study was sound: dietary fiber feeds gut bacteria, which produce short-chain fatty acids (SCFAs) — compounds linked to better blood sugar control and heart health. Specifically, the researchers used fibers designed to ferment slowly and reach the lower (distal) colon, where SCFA production is thought to be most beneficial for metabolic health.

What They Found — And Why It's Surprising

Rather than improving insulin sensitivity, the fiber supplement group actually showed significantly decreased whole-body insulin sensitivity (p = 0.034) compared to placebo. In practical terms, this means the body became less efficient at responding to insulin — the opposite of what patients with diabetes or prediabetes need.

Additional findings included:

• Reduced metabolic flexibility: The fiber group showed less ability to switch between burning carbohydrates and fats for energy — a key marker of metabolic health.
• Increased gut permeability: The fiber supplement appeared to compromise the gut's protective barrier, sometimes called "leaky gut" (p = 0.046).
• Higher inflammation marker: Blood levels of interleukin-6, a protein associated with inflammation, rose significantly in the fiber group (p = 0.025).
• Lower protein oxidation: The body burned less protein for energy, which may indicate reduced absorption of amino acids when fiber and protein are consumed together.

Converting the Numbers for US Patients

While this study measured insulin sensitivity rather than blood glucose directly, for context: the standard US target for fasting blood glucose is under 100 mg/dL (5.6 mmol/L), with prediabetes defined as 100–125 mg/dL (5.6–6.9 mmol/L) and diabetes at 126 mg/dL (7.0 mmol/L) or above per ADA guidelines.

Why This Matters for US Patients

High-protein diets are extremely popular in the United States — from Atkins to keto to general low-carb eating plans — and are frequently recommended for people managing blood sugar. Simultaneously, fiber supplements like psyllium husk and pectin-based products are marketed heavily to Americans with diabetes. This Dutch research suggests that combining these two strategies may actually work against each other, potentially worsening insulin resistance and gut health rather than improving them.

The Dutch healthcare context is valuable here: the Netherlands has robust nutritional research infrastructure and a population with different dietary baselines than Americans, giving researchers a cleaner window into how specific dietary components interact biologically.

Importantly, this was a small study (40 participants total), and more research is needed before changing any personal dietary plan. Always consult your diabetes care team before modifying your diet or supplement routine.

Original Study Citation

van Kalkeren, van Deuren, Coenjaerds, et al. "Effects of a slowly fermentable fiber mixture against the background of a high-protein diet on insulin sensitivity and metabolic health in individuals with overweight: a randomized, placebo-controlled trial." Gut Microbes, 2026. DOI: 10.1080/19490976.2025.2606473. PMID: 41459804.