Net Carbohydrates

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Not all the carbs we eat can be digested, so net carbs is total carbohydrate minus the indigestible carbs. The simplest and most common adjustment is to subtract the amount of Fiber from the total carbohydrate intake, but the adjustments vary by diet.

  • Fiber is not directly digested, so most Low Carbohydrate Diets ignore Fiber in calculating Net Carbohydrates.
  • The Atkins Diet subtracts sugar alcohols from net carbohydrates, but Ketogenic Diets generally do not. Sugar alcohols such as Xylitol are likely to impact ketosis and should probably be considered 50-90% of the impact of a carbohydrate. See below for details.

1 Fiber

Fiber is almost universally ignored in calculating Net Carbs and has a sound physiological basis; Fiber does not provide the body with any carbohydrate. Some Fiber is converted by bacteria to Short Chain Fatty Acids (SCFA) which are then digested. However these SCFA provide some Calories but they don't provide any carbohydrate intake. Not only is Fiber subtracted from Net Carbs, the common recommendation is to try to keep your Fiber intake high as constipation is a common side effect of the Ketogenic Diet.

2 Non-Calorific Artificial Sweeteners

There is little evidence that artificial sweeteners cause an increase insulin production. In rats, artificial sweeteners with a bitter aftertaste, including saccharin, sodium cyclamate (banded in the US, marketed as Sweat N Low in Canada), stevia, and acesulfame-K (marketed as Sunett and Sweet One) increased insulin in isolated rat livers, but Aspartame did not[1]. However, human studies do not show a similar result. A study on sucralose on humans showed no insulin response[2], and a study of a diet soda drink containing sucralose and acesulfame-K did not change the insulin response to subsequent glucose intake[3].

3 Xylitol and Other Sugar Alcohol Sweeteners

Xylitol is a sugar alcohol sweetener that is often recommended for those on a ketogenic diet[4]. Sugar alcohols are types of sugars that are not absorbed in the stomach, but are meet one of three fates[5]:

  • A portion is absorbed in the small intestine. Some sugar alcohols, like Xylitol are then metabolized, where others, such as arabitol and mannitol are excreted in the urine[6].
  • Another portion pass through the small intestine to be fermented by bacteria like Fiber to Short Chain Fatty Acids.
  • The remainder are excreted intact. Because of this excretion, large amounts of Xylitol can produce diarrhea at levels over 0.37 g/kg for males and 0.42 g/kg for females[7], so don't take too much at a time.

If the Xylitol or other calorific sugar alcohols are absorbed in the small intestine they have a Calorie value similar to carbohydrate of around 4 Calories/gram, but if they are fermented by bacteria then they will provide around 2.5 Calories/gram. This is likely to explain the various estimates for the calories from Xylitol. (Estimates of 2.4-3 Calories/gram are commonly used for Xylitol[5].)

3.1 Responses to Xylitol

When 25g of Xylitol is consumed as a drink in a fasted state, Xylitol has been shown to have a number of effects on healthy men[8]:

  • Xylitol only raises blood glucose slightly and has a Glycemic Index of only 7. This suggests a small portion of Xylitol is converted to Glucose.
  • Xylitol does produce a rise in insulin though only about 20-25% of the rise that would be produced by the equivalent amount of glucose.
  • Unlike Glucose, Xylitol slightly reduces carbohydrate metabolism and slightly increases fat metabolism.
  • Xylitol helps prevent tooth decay be inhibiting the harmful bacteria in the mouth[9]. Xylitol also has some broader antibiotic effects[10], but it's not clear if this would have a negative impact on digestive bacteria.

3.2 Sugar Alcohols and Ketosis

Xylitol has been shown to inhibit ketosis in isolated rat livers in a similar fashion to lactate[11], but this does not necessarily mean it will have a similar effect on humans that eat foods containing Xylitol. I would speculate that if a sugar alcohol like Xylitol is absorbed in the small intestine it is in the form of a carbohydrate, but fermentation by bacteria to Short Chain Fatty acids will not act as a carbohydrate. It is not clear to me under what circumstances sugar alcohols are absorbed in the small intestine rather than getting digested by bacteria. One analysis indicates that more than 50% of Xylitol is absorbed by the small intestine[6]. While the Atkins diet does not consider sugar alcohols as carbohydrates[12], a guide to ketogenic diets for epileptic patients does count sugar alcohols[13]. My personal interpretation is that 50-90% of sugar alcohols such as Xylitol should be considered carbohydrates when evaluating a ketogenic diet.

4 See Also

5 References

  1. Willy J. Malaisse, Anne Vanonderbergen, Karim Louchami, Hassan Jijakli, Francine Malaisse-Lagae, Effects of Artificial Sweeteners on Insulin Release and Cationic Fluxes in Rat Pancreatic Islets, Cellular Signalling, volume 10, issue 10, 1998, pages 727–733, ISSN 08986568, doi 10.1016/S0898-6568(98)00017-5
  2. J. Ma, M. Bellon, JM. Wishart, R. Young, LA. Blackshaw, KL. Jones, M. Horowitz, CK. Rayner, Effect of the artificial sweetener, sucralose, on gastric emptying and incretin hormone release in healthy subjects., Am J Physiol Gastrointest Liver Physiol, volume 296, issue 4, pages G735-9, Apr 2009, doi 10.1152/ajpgi.90708.2008, PMID 19221011
  3. RJ. Brown, M. Walter, KI. Rother, Ingestion of diet soda before a glucose load augments glucagon-like peptide-1 secretion., Diabetes Care, volume 32, issue 12, pages 2184-6, Dec 2009, doi 10.2337/dc09-1185, PMID 19808921
  4. The Art and Science of Low Carbohydrate Living: An Expert Guide to Making the Life-saving Benefits of Carbohydrate Restriction Sustainable and Enjoyable, Authors Stephen D. Phinney MD, Rd Jeff S. Volek Phd, Publisher Beyond Obesity LLC, 2011, ISBN 0983490708, 9780983490708 page 444
  5. 5.0 5.1 Tsuneyuki Oku, Sadako Nakamura, Digestion, absorption, fermentation, and metabolism of functional sugar substitutes and their available energy, Pure and Applied Chemistry, volume 74, issue 7, 2002, pages 1253–1261, ISSN 0033-4545, doi 10.1351/pac200274071253
  6. 6.0 6.1 G. Livesey, The energy values of dietary fibre and sugar alcohols for man., Nutr Res Rev, volume 5, issue 1, pages 61-84, Jan 1992, doi 10.1079/NRR19920007, PMID 19094313
  7. T. Oku, S. Nakamura, Threshold for transitory diarrhea induced by ingestion of xylitol and lactitol in young male and female adults., J Nutr Sci Vitaminol (Tokyo), volume 53, issue 1, pages 13-20, Feb 2007, PMID 17484374
  8. SS. Natah, KR. Hussien, JA. Tuominen, VA. Koivisto, Metabolic response to lactitol and xylitol in healthy men., Am J Clin Nutr, volume 65, issue 4, pages 947-50, Apr 1997, PMID 9094877
  9. H. Lynch, P. Milgrom, Xylitol and dental caries: an overview for clinicians., J Calif Dent Assoc, volume 31, issue 3, pages 205-9, Mar 2003, PMID 12693818
  10. Marjo Renko, Päivi Valkonen, Terhi Tapiainen, Tero Kontiokari, Pauli Mattila, Matti Knuuttila, Martti Svanberg, Maija Leinonen, Riitta Karttunen, Matti Uhari, Xylitol-supplemented nutrition enhances bacterial killing and prolongs survival of rats in experimental pneumococcal sepsis, BMC Microbiology, volume 8, issue 1, 2008, pages 45, ISSN 1471-2180, doi 10.1186/1471-2180-8-45
  11. HF. Woods, HA. Krebs, Xylitol metabolism in the isolated perfused rat liver., Biochem J, volume 134, issue 2, pages 437-43, Jun 1973, PMID 16742803
  12. Dr. Eric C. Westman, Dr. Stephen D. Phinney, Dr. Jeff S. Volek, The New Atkins for a New You: The Ultimate Diet for Shedding Weight and Feeling Great, Accessed on 4 July 2013, date 2 March 2010, publisher Touchstone, isbn 978-1-4391-9028-9
  13. James Rubenstein, Ketogenic Diets: Treatments for Epilepsy and Other Disorders, Accessed on 4 July 2013, date 17 June 2011, publisher Demos Medical Publishing, isbn 978-1-936303-10-6