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The main reason many nutrition experts take issue with low-carb diets is that avoiding a food group means limiting your range of vitamins, minerals, and other nutrients. (See: Why This Dietitian Is Completely Against the Keto Diet) A recent review funded by the World Health Organization and published in The Lancet gives their argument new merit. Cutting carbs seems to have health implications, especially when it comes to one type in particular: fiber.
First, a quick refresher: Besides helping food pass through your digestive system, fiber can promote healthy gut bacteria and stoke your metabolism.
WHO review spanned 185 prospective studies and 58 clinical trials from 2017 onward that looked at the relationship between carbohydrate quality and health. They looked at three specific quality indicators—amount of fiber, whole grains vs. refined grains, and low glycemic vs. high glycemic—to pinpoint which grouping was most useful in determining a risk of disease or death.
What did they find? The biggest discrepancy in health outcome came from studies comparing high-fiber diets with low-fiber diets.
The participants consuming the highest amount of fiber were 15 to 30 percent less likely than those consuming the lowest amount of fiber to be affected by stroke, heart disease, type 2 diabetes, and colorectal cancer. The high-fiber group also showed lower blood pressure, body weight, and cholesterol. They found that eating between 25 and 29 grams of fiber per day was the sweet spot showing the lowest risk of negative health effects. (Related: Is It Possible to Have Too Much Fiber In Your Diet?)
The review reported a parallel, though weaker, effect when it came to whole grains vs. refined grains. Eating whole grains showed a greater risk reduction for disease vs. eating refined grains, which makes sense considering whole grains are generally higher in fiber.
Finally, the review called into question the efficacy of using the glycemic index as a health indicator, finding that the GI was actually a pretty weak determinant as to whether a carb was "good" or "bad." (BTW, you seriously need to stop thinking of foods as good or bad.)
Evidence that eating carbs lower on the glycemic index will decrease health risks was deemed "low to very low." (The glycemic index ranks foods based on their effect on blood sugar, with a lower index rating being more favorable. However, the list's reliability is controversial.)
Even if you've steered clear of low-carb diets, chances are you're still not getting enough fiber. Most Americans don't, according to the FDA, which has deemed fiber a "nutrient of public health concern." What's more, the FDA's recommendation of 25 grams per day is on the low end of the range that was shown to be optimal in the review.
The good news is that fiber isn't hard to find. Add more plants—fruits, vegetables, whole grains, nuts, and legumes—to your diet to increase your intake. You're better off getting fiber from those natural sources since you'll also receive other nutrients at the same time. (And FYI, the review results apply to natural sources specifically—researchers excluded any studies that involved supplements.)
If you're married to eating low-carb, you can still include foods that pack fiber, like berries, avocados, and leafy greens, instead of going straight-up carnivore.