Summary
When starting a ketogenic diet, some people experience diarrhea as a side effect. Dr. Berg explains that the most common cause is an increase in saturated fat intake, which stimulates excess bile production and leads to a lubricated — and overactive — colon. The solution is typically simple dietary adjustments rather than stopping the diet entirely.
Key Takeaways
- Increased saturated fat is the primary trigger for diarrhea when starting keto
- Bile is produced in the liver, stored in the gallbladder, and is essential for breaking down dietary fats
- Saturated fats are a stronger bile trigger than other fat types
- Too much bile acts as a colon lubricant, causing loose stools or diarrhea
- The body adjusts over time — bile production will regulate as you adapt to higher fat intake
- Reducing fat intake temporarily allows the body to acclimate and resolves diarrhea
- Undigested greens (e.g., kale) passing through whole indicate a separate digestive issue
- If specific vegetables are causing problems, swap to more easily digestible options in the short term
Details
Cause 1: Excess Bile from Saturated Fat Intake
When transitioning to a keto diet, most people dramatically increase their intake of saturated fats — often for the first time. This spike in saturated fat consumption is a strong stimulus for bile secretion. Bile functions as a lubricant in the colon, which is normally beneficial for digestion. However, when bile is produced in excess relative to what the body is used to, it over-lubricates the colon, resulting in loose stools or diarrhea.
Recommended fix:
- Temporarily reduce the amount of fat consumed per meal
- Allow the body time to adapt — bile output will naturally regulate itself
- Gradually reintroduce higher fat levels as digestion normalizes
Cause 2: Undigested Vegetables Passing Through
A second, distinct cause of diarrhea on keto is the inability to digest certain raw or fibrous vegetables, such as kale (commonly consumed in green smoothies or shakes). If vegetables are appearing visibly undigested in stool, the digestive system is not breaking them down properly.
Recommended fix:
- Identify which vegetables are causing the issue
- Avoid those specific foods temporarily
- Switch to easier-to-digest vegetables until digestion improves
- Experiment with cooking methods that break down tough plant fibers
General Adaptation Note
Both issues are typically temporary and associated with the initial transition period. The body adjusts to new macronutrient ratios over time, and symptoms generally resolve without needing to abandon the diet altogether.