I get asked a lot of questions about gut bacteria these days, and for good reason; over the last decade, research on the importance of the intestinal microbiome for fitness and overall health has exploded.

Take, for example, one particularly persuasive study, which took fecal bacteria samples from pairs of identical twins in which one twin was lean and one was obese, and transplanted the samples into the intestines of germ-free mice. Lo and behold, the mice with transplanted microbiota from the lean twins stayed lean themselves, while the mice with obese twin microbiota quickly piled on weight.

Similar microbiota transplants between humans are already being used very successfully to fight deadly infections like C. difficile colitis, and are being researched for conditions ranging from multiple sclerosis to Parkinson’s disease.

Which leads to the obvious question: will poop transplants for weight loss be the next big fitness craze?

In short, I hope not. While I strongly suspect that managing our microbiome will be an important part of health in the decades to come, at the moment, we just don’t really know what we’re talking about. Clinical data is still scarce, and possible complications are immense. Even if getting microbiota from your skinniest friend did turn out to be a great diet plan, we still have no idea about all of the other effects of that same bacteria down the line.

And, based on historical record, there’s good reason to be concerned. In the 1950’s, for example, doctors began prescribing transplanted Human Growth Hormone to smaller children deficient in HGH. While the treatment proved effective for spurring growth, it wasn’t until decades later that hundreds of cases of the rare and fatal neurodegenerative Creutzfeldt–Jakob disease (colloquially “mad cow disease”) began to crop up in those HGH recipients. Scientists quickly discovered that prions (the cause of CJD) had inadvertently come along for the ride with the transplanted hormone.

So, in short, the microbiome is something we should be keeping an eye on.

And it probably would be wise to start doing the common-sense things that research has begun to show as likely to help your microbiome: eat a whole food diet, and include some pre-biotic (raw garlic, onions, etc.) and pro-biotic (pickled stuff, yogurt, etc.) foods; avoid unnecessary antibiotics; get a dog (seriously!); exercise; manage your stress.

But when it comes to more invasive ideas – whether fecal transplant or even just probiotic supplements (which are currently a bit of a wild west), I’d hold off for now. Whatever the short-term upsides, from my perspective, at least, the long-term unknown risks are just too great.

My parents are in their late 60s, but they remain in very good shape. They’re avid travelers, which regularly requires them to walk 10-15 miles in a day, with stairs and hills climbed, bags toted, etc.

Primarily, they’ve kept fit with ‘cardio’ workouts in their living room, using Leslie Sansone’s solid and much-loved Walk at Home DVDs (or, as my brother calls it, ‘frumping to the oldies.’)

However, recent research has made clear that also focusing on strength training is particularly important as we age. As one recent review paper put it:

Strength-training exercises have the ability to combat weakness and frailty and their debilitating consequences. Done regularly [it] builds muscle strength and muscle mass, and preserves bone density, independence, and vitality with age. In addition, strength training also has the ability to reduce the risk of osteoporosis and the signs and symptoms of numerous chronic diseases such as heart disease, arthritis, and type 2 diabetes, while also improving sleep and reducing depression.

In short, strength training is powerful stuff. And as further research has shown, those benefits are specific to lifting weights; it’s not sufficient to simply maintain a high level of physical activity in general.

So I suggested that my parents also consider hitting the gym once or twice a week. To which my mother replied that they do currently use dumbbells in those Sansone workouts. While that’s great, I clarified that she needed to go to the gym to focus on progressive overload. The health improvements of strength training come from consistently increasing the weight used over time; thus, if you’re using the same ten-pound dumbbells month after month, you’re no longer reaping the same benefits.

To illustrate, here’s an amazing pair of before and after MRI scans showing the increase in leg muscle mass after just twelve weeks of weight training, in a 92-year-old subject. (!!!)

If you want to live longer, healthier, then staying active (in a general, ‘use it or lose it’ sort of way) is hugely important. But adding in weight training, too, is an extremely powerful tool. And, as the scans show, it’s never too late to start.