Where does fat digestion start and end?

Fat digestion starts in the mouth and finishes in the small intestine with the absorption of free fatty acids and monoglycerides through the lymphatic system. 

Your saliva and gastric juices contain special enzymes such as lingual and gastric lipase that emulsify the fat present in the meal and make it ready for digestion further down the digestive system. 

Although the actions of lingual and gastric lipase begin the process of fat emulsification and digestion, the majority of fat breakdown happens in the small intestine.

Once the partially digested meal reaches the small intestine, bile acids produced in the liver and released into the small intestine work to further emulsify fats and increase their surface area so that pancreatic lipase can break down fats into free fatty acids and monoglycerides.

For absorption, medium-chain fatty acids (derived from medium chain triglycerides) directly travel to the liver via the hepatic portal vein whereas long-chain free fatty acids (derived from long chain triglycerides) pass through the mucus barrier and into the epithelial cells with the help of bile acids. 

Once in the epithelial cells, fatty acids, monoglycerides, cholesterol, and phospholipids combine to form chylomicrons. Chylomicrons travel through the lymphatic system and enter the bloodstream from the jugular vein in the throat. 

As chylomicrons come in contact with cells across the body, they are broken down into fatty acids which can either be used for energy or stored for later use.

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So, what happens when your fat metabolism isn’t working as well as it should? Or your fat consumption is simply higher than your digestive system can effectively break down?

Ideally, all fat would be broken down and absorbed in the small intestine but when undigested fat reaches the large intestine, it might lead to diarrhea. 

Undigested fat in the large intestine has been associated with steatorrhea which is characterized by fatty loose stools that might float in the toilet and may be difficult to flush down. 

Undigested fat in the gut might either be due to low bile acid production resulting in incomplete fat digestion or due to a lack of fat digesting microbes in the gut. If you’re struggling with floating stools, then it’s advised to consult your healthcare provider to identify any underlying causes for undigested fat.

If you’re at the beginning stages of keto induction, you might consider taking an ox bile supplement to help with insufficient bile acids as your body ratchets up its own bile production.

Keto adaptation can take a few months as all organs adapt to primarily using ketones as the primary source of energy, and as you adapt to the high fat diet, your keto flu symptoms should resolve on their own.

But undigested or malabsorbed fat might not be the only reason behind keto diarrhea.

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Too much MCT

Medium-chain triglycerides (MCTs) are ketogenic and have been associated with increasing tolerability to low-carb diets. While supplementing with MCTs can be a great way to advance your way to keto-adaptation, too much MCT might make that progress difficult.

Once in the small intestine, MCTs can travel directly to the liver through the hepatic portal vein and be preferentially utilized for ketones. But one of the most common side effects of excessive consumption of MCT is diarrhea.

Starting with small amounts of MCT and gradually increasing its dosage might be helpful in decreasing the incidence of diarrhea.

Hidden laxatives in your diet: caffeine and artificial sweeteners

Caffeine has been associated with increasing the rate of ketosis along with MCT. It can also be helpful in the initial period of keto induction as it suppresses appetite and helps with fat burning. But too much caffeine might add to your troubles too.

Coffee might have a mild laxative effect and as such might increase gut motility. When you combine that with the possibility of undigested fats in your gut or MCT in your coffee, it might be adding to your tummy troubles. 

Additionally, caffeine is a mild diuretic - meaning it makes you pee more than usual. If you aren’t keeping up with your hydration, it might add onto an electrolyte imbalance while you’re already working on adapting to a high-fat diet. 

Scaling back on your coffee consumption and altering the amount of butter or MCT oil in your bullet coffee might help ease some of your GI distress at this stage.

Carbohydrate withdrawal and electrolyte supplementation

In a carbohydrate-rich diet, your body secretes more insulin and as a result, retains more water and electrolytes. But as you switch to a low-carb, high-fat diet, your insulin levels drop and as a result, so do your electrolyte levels.

In response to this initial electrolyte dumping and associated symptoms such as fatigue and muscle cramps, keto dieters tend to turn towards mineral supplements to increase their mineral intake. But consumption of high doses of some mineral supplements such as magnesium might lead to diarrhea. 

If you’re taking a magnesium citrate supplement, try switching it with magnesium glycinate or magnesium malate as well as consider adding keto-friendly and mineral-rich whole food sources to your keto diet. 

As your keto diet and digestion stabilizes, you might want to consider adding epsom salt (magnesium sulfate) to your diet or drinks since it's a great way to have additional magnesium. But be mindful that epsom salt can have a laxative effect and it isn’t advisable to take it while you’re dealing with diarrhea induced dehydration.

Some magnesium and potassium-rich foods to add to your diet are avocados, almonds, spinach, nuts, seeds, and citruses such as lime, oranges, grapefruit, etc.  When selecting foods make sure you are getting organic as much as possible, as chemicals in the food supply impact the microbiome and its ability to help you digest foods, including fats.  

Changes in your gut flora

Changes in your diet lead to changes in your gut flora and any initial shifts in the gut microbiome have been associated with GI issues such as diarrhea and constipation. 

Levels of certain carb-dependent species such as Bifidobacterium go down, whereas other beneficial species such as F. prausnitzii stay stable as you begin eating keto.

Enzymes produced by microbes in your gut work together with pancreatic enzymes to digest and absorb your meals. Probiotics containing lactase-positive strains such as B. longum and B. animalis might help relieve symptoms of lactase intolerance related diarrhea. 

Additionally, strains such as B. subtilis, B. longum and L. rhamnosus have been associated with potential lipase - a fat digesting enzyme -  activity in the gut. Microbial lipase in the gut might further help in fat digestion as your gut readjusts to the keto diet. 

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Too much protein and lactose intolerance

Starting the keto diet not only alters how much fat you eat, but it also alters how much and which type of protein you eat. For most keto dieters, their keto protein intake might be more than they’re used to and a sudden shift in their protein intake might lead to diarrhea. 

Not everyone digests protein equally. The source of protein (animal or plant), whether you’re lactose intolerant, and your own capacity for protein digestion (depending on the diversity of your gut microbiome) will determine how well you respond to an increased consumption of protein.

Additionally, if you’re consuming a protein powder to meet your daily protein intake, check the ingredients list to ensure it's not adding to your GI troubles. 

Protein powders might have fillers such as fibers like psyllium husk and coconut flour, dairy protein sources such as WPC (whey protein concentrate) and casein protein, artificial sweeteners, and sweetening agents such as dextrose and milk solids, any of which might add additional stress to your already reactive belly. 

Overall, slowly increasing the consumption of whole foods such as wild game to fulfill your protein needs will be a more sustainable way to improve your own protein digestion capacity as well as give your gut flora a chance to diversify to break down a more fat and protein dominant diet.

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For now, rest. Load up on water and electrolytes - mix up an adrenal cocktail with a whole food source of Vitamin C such as lemons or oranges, a bit of cream of tartar (for potassium) and some sea salt - and give your body a chance to adjust and adapt. Slowly incorporate changes in your diet rather than going all in at once.

When you’re further along on your diet, consider sprinkling your foods with spices such as turmeric and black pepper which help with fat digestion and adding probiotic foods such as sauerkraut and kimchi to enhance your gut microbial diversity. 

Adding dietary supplements such as keto digestive enzymes (lipase and protease) and probiotics such as Sugar Shift formulated with B. subtilis and B. longum might help enhance your keto diet in the long run.

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