Fat is the fuel of choice in the animal kingdom for feats of extreme endurance.
Fat fuels epic feats from dogs pulling sleds 1,000 miles across the arctic [1], to birds migrating across continents without eating [2,3]. Fat is the universal fuel for long distance travel because it is the lightest form of energy storage, fitting 9kcal a gram and 3,000kcal in a pound [4].
Why Eat Fat While Backpacking?
 A ketogenic diet has two key advantages for long distance backpacking.
Eating a ketogenic diet improves one’s ability to burn fat.  This means an efficient use of both body fat and ingested fat for a constant energy supply during long, moderate to low intensity endurance activity such as hiking. Even lean humans are likely to have 10lbs of fat, 30,000kcal, which is enough to fuel 110 miles of hiking [7]. In comparison, carbohydrate storage of even the most elite athletes is only 2,000kcal and will be depleted within hours or days depending on the intensity of the activity [5].
Ketogenic meals can be half the weight of conventional backpacking meals. This is in addition to lightening your load by burning fat stored on your body.
Why Avoid Carbs and Keep Protein Moderate?
Many people on long distance hikes such as the Appalachian trail, carry pounds of fat on their body and don’t effectively burn it. Others hike the same number of miles then gorge themselves on junk food and remain lean and muscular. Why?
Part of the answer lies in genetics. Some people burn body fat easily on any type of diet, while others struggle from meal to meal feeling hungry with hundreds of thousands of calories of energy stored on their bodies as fat that they aren’t able to burn [8].
The other part of the answer lies in diet. Carbohydrates impair fat burning for several days because they stimulate the secretion of insulin [9,10]. To a lesser extent, the same is true of protein as protein also results in an insulin response [11]. By keeping carbohydrates low, protein adequate but not excessive, and combining them with enough fat to provide sufficient energy for the activities at hand, one ensures maximal efficiency in burning fat.
How do you know if you are burning fat?
Fat burning can be measured directly via a RER test at a lab such as DEXAfit. The production of ketones is a good indicator of fat metabolism and can be easily measured with a finger prick test strip. 
You can evaluate the likelihood of any given meal in promoting nutritional ketosis with the following formula named after Dr Russell Wilder who pioneered the ketogenic diet in 1924. The Wilder Ratio = (grams of fat/(grams of carbhydrate + grams of protein) [12]. Protein is included because it can be converted to carbohydrates. Meals with a Wilder Ratio of 3 or greater are considered highly ketogenic [12].
Adequate daily protein intake is 1 gram of protein per kilogram of body weight [13]. More protein will be required if you are not eating sufficient calories to replace what you are burning [13].
A ketogenic diet is a major dietary shift for everyone but the artic inuit. Because the liver does not begin to convert fat into ketone bodies to take the place of carbohydrates in fueling the brain until carbohydrate stores are very low, it may take 3-6 weeks to fully adapt to a ketogenic diet [5]. Meal size will need to be kept moderate at first to reduce gastrointestinal upset. Sodium intake will need to be maintained above 3,000mg/day for most people [14]. I recommend further reading before embarking on a ketogenic diet. The Art and Science of Low Carbohydrate Performance by Stephen Phinney and Jeff Volek is an excellent place to start. Consult a doctor familiar with low carb diets if you have any health concerns.
What Do You Eat on a Ketogenic Backpacking Trip?
Footnotes and References.
 A recent analysis of Iditerod sled dog diets found they were 50% fat, 15% carbohydrate and 35% protein. The dogs lost weight over the course of the race as they were unable to consume the 8,995kcal/day required to maintain body weight. 2014 Loftus- Energy requirements for racing endurance sled dogs. J Nutr Sci.
This study investigated the postexercise metabolism of six species of free-living, night-migrating passerine birds (European robin, pied flycatcher, wheatear, redstart, blackcap, and garden warbler). High blood ketone levels were detected during the resting state, as the birds where metabolizing fat to save their limited carbohydrate and protein stores. 2001 Jenni-Eiermann- Postexercise ketosis in night-migrating passerine birds. Physiol Biochem Zool.
The Art and Science of Low Carbohydrate Performance by Stephen Phinney and Jeff Volek.
In this landmark study on elite cyclists, fat burning while pedaling on a stationary bike increased from a baseline range of 90-540kcal/hr toand 666-1008kcal/hr after four weeks on a diet of <10g of carbohydrates per day.
 Ten pounds of human adipose would provide enough energy for a 155lb person carrying a 21-42lb pack to hike for 55 hours and cover 110 miles at 2mph. “Backpacking” is 7.8 MET in the Compendium of Physical Activity 2011.
1985 Venables- Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. Journal of Applied Physiology.
Insulin inhibits the enzyme called hormone sensitive lipase which is involved in breaking the storage form of fat called a triglyceride into its usable forms of glycerol and fatty acids. The Art and Science of Low Carbohydrate Performance by Stephen Phinney and Jeff Volek.
Insulin levels rose to 2 micro units per /ml at one hour after 500g of lean ground beef or chicken liver. Floyd- Insulin Secretion in Response to Protein Ingestion.
2009 Wirrell- Ketogenic Ratio Calories and Fluids: Do They Matter?
When carbohydrates are not present, more of your energy comes from burning protein and thus more protein is required. 2004 Tarnopolsky- Protein requirements for endurance athletes Nutrition.
