This is a guest post from Chris Kresser of ChrisKresser.com  and author of the new book Your Personal Paleo Code . Join me, Chris and numerous other presenters and Primal enthusiasts from around the world at the 5th annual PrimalCon Oxnard  in 2014.
Every week it seems there?s a new bestselling diet book promising the ?secret? to losing weight, building muscle, or even curing chronic disease. Over the last few decades we?ve seen books that make a wide range of claims:
- Fat is the enemy, and we should all eat a low-fat diet
- Carbs make us fat and sick, and we should all eat a low-carb diet
- Protein is the key to weight loss and health, and we should all eat a high-protein diet
- We should all eat 40 percent of calories from carbohydrate, 30 percent from fat, and 30 percent from protein
- Animal products are to blame, and we should all follow a vegetarian or vegan diet
- Cooked food is the source of our ills, and we should all eat a raw food diet
- We can eat whatever we want and still lose weight?as long as we don?t eat anything at all two days out of each week
It?s enough to make your head spin, isn?t it? And if you?re like most of my patients and readers, you?ve probably tried several of these approaches. Maybe some of them even helped?at least for a little while. But eventually you gained that extra weight back, or you developed some new symptoms, or perhaps you never experienced much improvement in the first place.
Why do we continue to see one diet after another make a splash on the bestseller lists, only to fade from the popular consciousness a few years later? Do these approaches really offer the ?secrets? to success they claim to? If so, why don?t they last?
One reason is that these dietary approaches aren?t consistent with what evolutionary biology has taught us about optimal human nutrition. Studies of the fossil record as well as contemporary hunter-gatherers following their traditional ways have revealed the following facts:
- Human beings evolved on a diet of meat, fish, vegetables, fruits, nuts and seeds, and starchy tubers.
- Humans can tolerate (and even thrive on) a wide variety of macronutrient ratios.
- Animal products made up a significant percentage of our ancestors? diet, and we know of no traditional hunter-gatherer cultures that followed a vegetarian or vegan diet.
This is, of course, why a Paleo diet makes so much sense. But I?ve come to understand that while Paleo is a great starting place, it?s just that?a starting place. This brings us to the second reason that most bestselling diets?including Paleo in some cases?fail in the long run: they aren?t personalized to meet your unique needs as an individual.
We share a lot in common as human beings, but we also have important differences. We have different genes, gene expression, lifestyles, activity levels, health status, and goals. Each of these factors will determine what is optimal for a given individual. What?s more, they can change over time, which means that what?s optimal for you now may not be in a year or a decade.
Let?s take a closer look at how each of these factors might affect individual differences in diet:
- Constitution (genetics, physiology, biology). Modern studies have shown  that some people have genes that predispose them to problems metabolizing glucose (sugar), while others have genes that make it more likely they will have problems burning fat. There is still much we don?t understand about the contribution of genetics to diet and the relationship between genes and environmental factors.
- Season. During the summer, your body will naturally crave different foods than it does during the winter . It?s true that our ancestors had access to certain foods only at certain times of the year. If they lived in Northern Europe, they weren?t eating mangoes from Thailand in the winter.
- Geography/climate. If you?ve been to the tropics, you probably found yourself craving lighter foods with higher water content, like fruits and vegetables, more than you did at home. Likewise, in cold climates, you probably gravitate toward eating more protein and fat-rich foods, like meat stews. There?s a reason for this.
- Health status. Have you ever noticed that you crave different foods when you?re coming down with a cold or the flu? The body has different needs in different physiological states. Women often crave more carbohydrates during pregnancy because the developing fetus has a need for glucose, and women naturally become somewhat insulin resistant as a result. People with thyroid problems may suffer on very low-carb diets, because insulin is required for proper thyroid-hormone conversion. As people age and become less active, they often find that they need less food, or perhaps less of a particular macronutrient, than they did when they were younger.
- Activity level. A construction worker doing manual labor for eight hours a day or a high-level athlete in training will have different dietary and macronutrient needs than someone who works at a desk. This should go without saying, but amazingly, it is often ignored in the discussion about macronutrients.
- Goals. If you?re training for the next Mr. Olympia competition, you will very likely eat different foods than an obese person trying to lose weight.
Within a basic ?template? of Paleo or Primal foods, all of the factors above will influence what specific ways each person should tweak their approach in order to achieve the best results. I?d like to share two case studies from my clinical practice to show you how this works.
The first case was a patient I?ll call Samantha. She was 42, and came to see me after she was diagnosed with Metabolic Syndrome. She was 40 pounds overweight and had several markers of poor metabolic function, including insulin resistance, abdominal obesity, high triglycerides and low HDL, and high blood sugar. Her doctor had prescribed a cocktail of medications, which Samantha was advised she?d have to take for the rest of her life. Samantha came to see me for a second opinion.
I put her on a low-carb (15 percent of calories), high protein (25 percent of calories) Paleo diet to promote fat loss, stabilize her blood sugar and normalize her metabolic and cardiovascular function. I also suggested that she eat all of her meals between 11am and 7pm each day. This strategy, called ?intermittent fasting ?, has been shown to improve metabolic function and accelerate weight loss in some patients. I also counseled her to reduce her sedentary time, add high-intensity interval and strength training to her exercise routine, sleep for at least eight hours a night, consume fermentable fibers and probiotic foods to improve her gut health, and practice regular stress management. Within three months Samantha had lost 36 pounds, and her metabolic and lipid markers were back in a normal range.
The second case was a patient I?ll call Marissa. She was 43 and came to see me complaining of persistent fatigue. She was a mother of two young children (the youngest was two years old), and she worked full-time outside of the house. Prior to having children she was energetic, enthusiastic, and an early riser?she woke up looking forward to the day. By the time her youngest was two, she could barely get out of bed in the morning and felt exhausted throughout the day. She also had cold hands and feet, hair loss, constipation, and severe brain fog. After running some tests, I determined that Marissa was suffering from Adrenal Fatigue Syndrome and hypothyroidism.
Marissa had been following a very low-carb diet (less than 10 percent of calories from carbohydrate) prior to coming to see me. She started this after her first child was born to lose the weight she gained during pregnancy. This did help with weight loss, but I?ve found that most patients with hypothyroidism and Adrenal Fatigue Syndrome don?t do well with very low-carb diets. If I had suggested the same approach for Marissa that I used with Samantha?a low-carb diet with intermittent fasting?she almost certainly would have continued to get worse. Instead, I recommended a moderate carbohydrate intake (approximately 20?25 percent of calories from carbohydrate) primarily from fruit and starchy plants like sweet potatoes, plantains, taro, and yuca. I suggested that she eat every two to three hours, and start the day with a high-protein (e.g. 40?50 grams) breakfast. Both of these strategies help to stabilize blood sugar and adrenals. I also suggested that she stop or cut back on her intense CrossFit workouts, and instead favor lower-intensity activity like walking, cycling, swimming and yoga. Finally, I made several recommendations for sleep and stress management, and I prescribed supplements for her adrenals and thyroid. Because Marissa?s condition was so advanced, it took about six weeks for her to notice significant changes, and about six months to fully recover and feel like herself again.
These two cases illustrate the importance of a personalized approach to a Paleo diet and lifestyle. They aren?t unique; in fact, I use a similar approach with every single patient that walks through my door. My experience?both personally, and as a clinician?has taught me that personalizing your Paleo/Primal program to meet your unique needs is the single most important factor in determining your long-term success.
While this does require a little bit more time and effort up front, it pays off in spades. You won?t feel confused anymore by the conflicting advice you read on the internet. You won?t have to rely on strangers on forums or even experts to tell you what you should eat. Instead, you?ll know exactly what works for you based on your own experience. Even better, you?ll be able to change your approach on the fly as your circumstances and needs change.
For more information on how to personalize a Paleo diet to meet your specific needs, check out my new book Your Personal Paleo Code .