I’m trying to lose weight and I keep stumbling upon the keto diet and other low carb options for weight loss. What’s your opinion on diets that limit or completely eliminate carbohydrates? Would it be ok for me to do a 4-week crash keto diet, then slowly reintroduce carbs? -Keri

Dr. Chutkan: Keri, growing a good gut garden (aka, protecting and enhancing your microbiome) is my #1 goal with my patients, so I’m always skeptical when I hear about strict diets that restrict or completely eliminate carbohydrates. Here’s why: carbohydrates, including vegetables, fruits, legumes, beans, gluten-free grains like brown rice, rolled oats, millet, amaranth, and quinoa, are the building blocks for a rich and diverse gut microbiome because they’re high in fiber – and act as food for beneficial gut bacteria. And a rich and diverse microbiome, as you probably know by now, is one of the key foundations for overall health. 

Some studies have highlighted the benefits of a keto diet showing improvements in brain function, body weight, metabolic markers, blood glucose levels, and even increasing beneficial gut bacteria. So it’s no wonder many of us are considering jumping on the bandwagon. But there’s also a lot of additional scientific evidence that supports avoiding very low carb diets and recent studies have illustrated the negative implications of low carb and keto-like diets on the gut microbiome (including the possibility of these diets may actually raise our mortality risk).

Carbohydrates are the most fiber-rich foods, so eliminating them makes it very difficult to consume enough fiber, which can result in bacterial species die-off. A 2018 study found that low fiber diets lead to gut bacteria extinction and have lasting effects on gut microbes, even in subsequent generations. Studies also show that in the absence of fiber, your gut bacteria feed on your intestinal lining, eating away at the mucosa and compromising the intestinal barrier that helps control whole body inflammation. 

A recent study also found that when a balanced diet is swapped for a no-carb, high fat diet, strains of bacteria that metabolize fatty acids are increased and beneficial strains that metabolize carbs and proteins (Bacteroides, Clostridium, and Roseburia) are decreased. These changes result in reduced production of short chain fatty acids (SCFAs) and antioxidants, compounds that fight against DNA damage and aging.

Instead of jumping all in and adopting a low-carb or keto diet that’s difficult to sustain and can lead to yo-yo dieting, focus instead on eating the right type of carbs. I recommend first switching your carbohydrate intake to 100% whole food sources, focusing primarily on vegetables, and supplementing with fruits, legumes, and gluten-free whole grains like brown rice and oats. Second, if you’re trying to lose weight, eat more starchy carbohydrates (sweet potatoes and squash are two good ones) when you need energy, for instance in the first half of the day and before exercise. Lastly, think about eating for health instead of for weight loss, and you’ll usually find you end up settling at an optimal weight. If you’re focusing on feeding your body (and your microbes!) whole, fresh, plant-based foods, it’s difficult to go down the wrong path, regardless of how many grams of carbs you’re eating daily.

Tip: Instead of counting carbs, try my simple 1,2,3 rule to maximize fiber intake: 1 vegetable at breakfast, 2 at lunch and 3 at dinner.  

The information on this website is for informational or educational purposes only, and is not intended as a substitute for the advice of your healthcare professional or physician. All readers should consult with their healthcare provider before beginning any new medical, dietary, or lifestyle programs.