COMMENT:

The paleo diet seems to have fallen out of fashion these days, in favour of the more extreme "keto" — short for ketogenic — diet. Some of those who started as paleo people eating meat, vegetables and sweet potato are now avoiding the sweet potato, loading up on fat and testing their pee every day.

Whether keto is ultimately healthy is a conversation for another day. But what keto and paleo dieters alike may want to contemplate is that common phenomenon: unintended consequences.

In the early days of the paleo diet, experts sounded a note of caution, not just because the diet seemed to emphasise unhealthy amounts of meat, but also because of what it eliminated: grains, legumes and dairy. The speculation then was that cutting these things out might cause changes — not necessarily positive — to the gut flora, which could cause consequences which were at that time not researched or known.

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Now it seems we might be getting an inkling of what those consequences are. Researchers at Perth's Edith Cowan University have just completed the first study of the paleo diet's impact on gut bacteria, and the outcome was not good for fans of the caveman way.

The researchers compared 44 people on the paleo diet with 47 following a traditional Australian diet. They measured the amount of trimethylamine-n-oxide (TMAO) in the participants' blood. High levels of TMAO, an organic compound produced in the gut, are associated with an increased risk of cardiovascular disease.

They found more than twice the amount of TMAO in the paleo people compared to the regular eaters. They suggest that excluding whole grains — which contain resistant starch and other fermentable fibres known to be good for gut bacteria — might change the bacteria population in a way that enables higher production of TMAO. Potentially larger amounts of meat also creates precursor compounds to TMAO.

The research is yet to be published, so no doubt there's more to learn here. But what it points to is the idea that when we restrict what we eat, it might have effects beyond what we intend. Going on any diet — paleo and keto included — may well cause weight loss, and may also improve some health markers such as blood pressure, cholesterol levels and blood sugar. They can be healthy ways to eat, especially if people are shifting from a high-processed-food diet.

But we don't always know what the long-term effects are of cutting out whole food groups. And we might not know that for a while. Will the young women avoiding dairy now, for example, have bone-density problems when they're in their 60s? Will the keto eaters' kidneys pack up? Will paleo people be dropping like flies from heart disease or bowel cancer?

What we do know about any kind of extreme diet is that they are hard to stick to long term. And that can lead to harmful yo-yo weight loss and regain, which is bad for body and mind. It's unsexy, but moderation — for a lifetime — has its benefits.

Niki Bezzant is editor-at-large for Healthy Food Guide www.healthyfood.co.nz