Does the paleo diet work? The trials say yes, then not so much

Does the paleo diet work? In the short term, yes — trials that ask people to eat lean meat, fish, vegetables, fruit and nuts while dropping grains, dairy, added sugar and processed food do show weight loss and better risk markers [s1]. The honest caveat is that this is largely what any move to whole, unprocessed food achieves, and the paleo edge over an ordinary healthy diet shrinks as the months pass and the strict rules become hard to keep [s2].

The short-term case

A 2019 meta-analysis pooled eight randomised controlled trials of a Palaeolithic diet [s1]. Compared with control diets, it reduced body weight by a weighted mean of 2.17 kg (95% CI −3.48 to −0.87), waist circumference by 2.90 cm, body-mass index by 1.15, and body-fat percentage by 1.38% [s1]. Blood pressure fell too — systolic by 4.24 mm Hg and diastolic by 2.95 mm Hg — alongside small drops in total cholesterol, triglycerides, LDL cholesterol and C-reactive protein, and a slight rise in HDL cholesterol [s1].

Those are real, consistent improvements. But the authors flagged an important weakness: in sensitivity analysis, the effects on lipids, blood pressure and inflammation "were significantly influenced by removing some studies," so the results "must be interpreted with caution" [s1]. When a pooled result depends on which trials are left in, the underlying signal is fragile. Eight short trials are a promising sign, not a settled verdict, and most ran for weeks to a few months — long enough to catch the early win, too short to see whether it holds.

There is also a comparison problem baked into short trials. The paleo diet drops grains, dairy, processed food, added sugar and salt at a stroke [s1] — so it is being measured against people's previous, often poorer, eating rather than against an equally disciplined healthy diet. Much of the early benefit is what any shift to whole food and fewer calories delivers, and a well-run control diet tends to close the gap the longer a trial runs.

The two-year test

The most informative single trial ran far longer. It randomised 70 obese postmenopausal women to either an ad libitum Palaeolithic diet or a diet built on the Nordic Nutrition Recommendations — an ordinary healthy-eating pattern — and followed them for two years [s2].

Both groups lost fat, and early on the paleo group lost more: total fat mass fell 6.5 kg versus 2.6 kg at six months, a gap that was highly significant (P<0.001) [s2]. By 24 months the picture had converged: 4.6 kg versus 2.9 kg, a difference that no longer reached significance (P=0.095) [s2]. Waist circumference and sagittal diameter followed the same arc — a clear paleo advantage at six months that narrowed by two years [s2]. The one marker that stayed ahead throughout was triglycerides, which fell more on the paleo diet at both six and 24 months [s2].

Why the edge fades

The trial also exposed the mechanism behind the fade: adherence. The researchers reported that "adherence to protein intake was poor in the PD group," and nitrogen excretion — an objective marker of how much protein people actually ate — did not differ between the diets by the end [s2]. In plain terms, the paleo group drifted back toward normal eating. A diet only works while it is followed, and the more rules it imposes, the harder that becomes.

What this means

The reasonable read is that the paleo diet is one legitimate way to eat better, not a uniquely powerful one. Its early results look impressive largely because it replaces refined and processed food with whole food and cuts calories — the same move behind almost every diet that shows short-term gains. Where it has been tested head-to-head against another sensible pattern for two years, most of the extra benefit disappeared, and the deciding factor was whether people could stick with it [s2].

That pattern — adherence mattering more than the diet's name — recurs across the nutrition literature, from low-carb versus low-fat trials to the question of how much protein you actually need. This is a description of trial findings, not dietary advice; what suits any individual is a question for a clinician or dietitian.

Sources

• Effects of a Paleolithic Diet on Cardiovascular Disease Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials — Advances in Nutrition, 2019-07-01
• Long-term effects of a Palaeolithic-type diet in obese postmenopausal women: a 2-year randomized trial — European Journal of Clinical Nutrition, 2014-03-01