Legumes cut cholesterol and blood sugar; the gut benefit went unmeasured in trials

Legumes such as beans, lentils and chickpeas are widely held up as one of the most beneficial food groups: high in fibre and plant-based protein, and naturally low in sodium, saturated fat and sugars. Yet they barely feature in the modern diet, and previous attempts to pool the trial evidence on legumes and cardiometabolic risk have produced inconsistent results. Legumes are also frequently credited with feeding the gut microbiome. This review set out to establish what randomised trials actually show for both the cardiometabolic profile and the gut.

The authors searched EMBASE, OVID Medline and the Cochrane register up to 15 May 2024 for randomised controlled trials of at least six weeks in which legumes were provided to adults, with or without existing conditions such as type 2 diabetes, heart disease or dyslipidaemia. Two reviewers independently screened studies and assessed risk of bias, results were pooled with random effects models, and the certainty of the evidence was formally graded. Thirty papers covering 24 trials and 33 comparisons made the cut, with legumes averaging 86 g a day (range 2 to 251 g) provided to 1,938 participants.

There was moderate-certainty evidence that higher legume intakes improved total cholesterol by 0.23 mmol/L, LDL cholesterol by 0.16 mmol/L and fasting blood glucose by 0.18 mmol/L. Other blood lipid and glucose measures did not improve consistently. Strikingly, not a single eligible study reported on gut microbiome outcomes, so that half of the question could not be answered at all. Most comparisons (70%) used a control food matched for energy, which the authors note may hide benefits that come from legumes being more filling, and results varied with the type and format of legume provided.

This review firms up a previously inconsistent evidence base, giving moderate-certainty backing to legumes' effects on cholesterol and blood glucose. It also exposes a striking gap: the gut microbiome benefit that features heavily in legumes' reputation has no eligible trial evidence of six weeks or longer behind it. The authors call for trials designed to capture microbiome-mediated effects, and for designs that do not mask the satiation benefit of simply eating more legumes.

This is decent evidence that regularly eating legumes, at an average of 86 g a day in these trials, can nudge cholesterol and fasting glucose in the right direction, whether or not you already have a condition such as type 2 diabetes. The improvements were modest rather than dramatic, so legumes look like a helpful habit rather than a cure. And if you eat beans chiefly for your gut microbiome, know that the claim currently rests on reasoning rather than randomised trial evidence.