7 Signs You’re Not Eating Enough Fiber, According to MDs

Brain Fog

Beyond keeping your energy stable, fiber’s influence on blood sugar can also directly impact your mental clarity. “Fiber helps regulate blood sugar levels by slowing glucose/carbohydrate absorption,” says Benjamin Barlow, MD, a board-certified emergency medicine physician and the chief medical officer of American Family Care. “Without sufficient fiber in your diet, you can experience blood sugar spikes and crashes that can lead to dips in concentration.”

A hazy, unfocused feeling (i.e. brain fog) after meals can absolutely point to low fiber intake, but it’s not the only factor. To know for sure, monitor how much fiber you’re eating, and work to boost your intake to recommended levels. If brain fog persists even after these changes, it may point to another underlying cause—like poor sleep, dehydration, or a medical condition—that’s worth discussing with your doctor.

Rarely Feeling Full

If you find yourself finishing a meal only to feel hungry again an hour later, fiber may be what’s missing. “Fiber takes a while to break down, slowing your digestion, which leaves you feeling fuller for longer,” says Barlow, who adds that high-fiber foods also take longer to chew.

Because fiber adds bulk and slows the pace of digestion, it helps stretch out the feeling of fullness in a way refined carbs and low-fiber foods can’t. You can tell fiber is part of the equation if your meals are generally heavy in white bread, pasta, or processed snacks, while lacking sufficient fiber sources like vegetables, fruits, beans, or whole grains.

Subtle Immune Sensitivities

Fiber isn’t just about improving your digestion—it’s one of the body’s key defenses against inflammation. “When fiber intake drops, gut barrier integrity and microbiome balance suffer. That leads to chronic, low-level inflammation that can manifest as skin irritation, allergic reactions, or unexplained sensitivity,” says Jorge Green, MD, a board-certified bariatric surgeon at Renew Bariatrics, who adds that these are symptoms that many people simply shrug off. 

But how can you tell fiber is the culprit? “If someone is dealing with subtle immune-type flareups like random rashes, the key clue that it’s fiber deficiency is the pattern,” says Green. “True food sensitivities are sharp and reproducible. You eat the food, and symptoms appear within hours. Fiber deficiency, on the other hand, shows up as vague, chronic, shifting issues such as constipation one week, fatigue the next, skin irritation later, and these don’t track to a single trigger food.”

‌Mini Energy Crashes Between Meals

You won’t always suffer from classic hunger pangs if you don’t eat enough fiber, but Green says you might experience a noticeable energy dip, especially midday. “Fiber helps slow digestion and stabilize glucose levels, so when it’s missing, energy crashes become frequent,” he says. One pilot study Green referenced found that low dietary fiber correlates with insulin resistance, unstable blood glucose, and higher post-meal fatigue risk.

As with many of the symptoms on this list, mini energy crashes between meals can have lots of causes. But according to Green, fiber deficiency has a telltale sign: The crashes happen one to two hours after eating refined carbs like white bread or sugary snacks, and you feel both hungry and sluggish at once.

“That’s a blood sugar spike-and-crash cycle, he says. “When patients bump their fiber intake, say by swapping white rice for lentils or adding chia seeds to breakfast, the crashes often flatten out within days because fiber slows glucose absorption and keeps insulin curves steady.” So if the post-meal fatigue improves quickly with added fiber, that’s your confirmation.

Constipation

Constipation is one of the clearest signs your body may not be getting enough fiber. “People who do not eat enough fiber are more likely to end up with symptoms of constipation, such as hard stools and straining during bowel movements,” says Cynthia Odogwu, MD, a board-certified family medicine physician.

While it can be hard to tell if constipation is caused by low fiber intake over other causes, certain signs may increase the likelihood, says Odogwu. “The first thing that is recommended to do when a person has constipation is to increase fiber intake. If the constipation does not improve after adequate fiber intake, see a doctor.”

Bad Breath

One surprising place fiber’s influence can show up is in your breath. “A lack of fiber can slow digestion and disrupt the balance of gut bacteria, which may lead to constipation,” says Barlow. “When waste lingers too long, it can release odor-causing compounds that can contribute to bad breath.”

Unfortunately, you can’t fix all instances of bad breath just by eating more vegetables. The key distinction is that bad breath linked to fiber usually appears alongside other digestive changes, like constipation or bloating, rather than in isolation. That connection can help you determine when upping your fiber might be part of the solution.

High Cholesterol

Another less obvious sign of low fiber is its impact on cholesterol levels and heart health. “The latest research shows that not eating enough fiber can lead to high cholesterol,” says Odogwu. Alternatively, a high fiber diet reduces LDL-C, which is often referred to as “bad cholesterol.”

“If a person has elevated cholesterol, and their diet is high in food that contains saturated fats, such as, red meat, fish, chicken, butter, coconut oil, eggs, etc., and low in fiber-rich plant foods, then the high cholesterol is likely due to a lack of fiber in the diet,” says Odogwu. To increase your fiber intake, she suggests eating more foods like beans, leafy green vegetables, broccoli, cauliflower, nuts, and seeds.