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Why Diets Fail & How To Determine The Perfect Diet For You

There isn't a one size fits all, standard approach to nutrition.

Convenient as it might sound to find an optimal “human species diet,” the fact is that we all have individual biological responses to food.

Although we all share common traits, things like our genetic makeup, lifestyle, gut microbiome, environment, and level of activity vary widely. Regardless of sensational claims and marketing tactics exploiting whatever happens to be trendy, there’s a simple and much more effective approach to nutrition than blindly following the latest diet book that worked for your neighbor.


As humans, we're highly adaptable and have thrived on a wide spectrum of foods. In the past, diet was determined based on whatever was provided by the immediate environment. A big part of your individualized response to food is based on your personal genetic background. For example, some people's ancestors came from Northern Europe, other people descend from sub-Saharan Africa and others are Southeast Asian. Depending on your genetic makeup, you create different levels of things like amylases for digesting sugars, hydrochloric acid in the stomach for digesting protein, and different amounts of bile in the gallbladder for digesting fats. This means that your capacity to extract energy from foods is largely influenced by what your ancestors ate and how that affected your current gene expression.

There are certainly some general principles when it comes to a healthy diet. Just about everybody benefits from a diet composed mostly of whole foods, including lean protein and plenty of fresh vegetables. But with the choices presented by our modern environment and so much conflicting information, how do you cut through the complexity to discover an optimal strategy for you on an individual basis? This article will outline different ways you can get started with customizing your diet.

Personalized Nutrition Projects

Some of the most compelling recent research in support of a personalized diet approach comes out of the Weizmann Institute of Science in Rehovot, Israel. From the study abstract: “Here, we continuously monitored week-long glucose levels in an 800-person cohort, measured responses to 46,898 meals, and found high variability in the response to identical meals, suggesting that universal dietary recommendations may have limited utility.” Their study confirmed a high variation in glucose responses to identical meals. Things like cookies, bananas, sushi and whole-grain bread produced sharp increases in blood sugar for some participants, while others showed only a flat or moderate response. Their findings indicate that reliance on something such as the glycemic index, which gives fixed numbers for each food item, may be relatively useless when compared to looking at individualized blood sugar response.

Applying this information with the right approach can be extremely valuable. After showing that each of us is unique in the way we absorb and metabolize nutrients, the scientists in Israel were able to use machine learning to develop an algorithm for accurately predicting personalized responses to meals. The algorithm integrates a number of factors like blood parameters, family history, physical activity, and gut microbiome to prescribe highly tailored dietary advice. With the deeper research being done on links between DNA and nutrition, tailoring nutritional needs based on quantification seems to be an effective way forward.

Another recent study, entitled “Food4Me,“ investigated 1,500 participants in seven European countries over a six month period who were randomly given personalized dietary advice based on their genetic data, or instead told to follow standard dietary prescriptions such as eating lots of fruits and vegetables, lean meats and whole grains. Those who were in the personalized diet cohort had much better health-related outcomes than those in the one-size-fits-all diet group, making the researchers pretty confident that personalized diets are the way forward.

When it comes to personalizing nutrition, it’s important to recognize that there will be certain nutrient, vitamin and mineral deficiencies that need to be addressed based on different factors such as age, religious or ethical preferences, or sporting demands.

For example:

  • Aerobic and anaerobic athletes will have different needs, and while anaerobic athletes can restrict excess sugar and starches while still maintaining adequate glycogen levels for explosive or intense activities, aerobic sports will require a different approach.
  • With vegan or vegetarian preferences, there are notorious deficits such as creatine, vitamin B12, DHA, taurine and amino acids.
  • On the ends of the age spectrum, seniors and children will have far different needs to address. This also applies to gender differences- male and females will have unique aspects to address.

Genetic Testing For Personalized Nutrition Recommendations

One of the most useful and important ways to customize your diet is to understand your personal genetic background using DNA testing. This can be performed using a service such as 23andMe with a simple and inexpensive salivary measurement. Literally, you just spit into a tube, mail it off and get the results back and it tells you where you came from as well as what your genes do in response to certain foods. DNAFit is another service that will give you diet and fitness recommendations based upon your genetics.

The raw data gathered from these genetic tests can then be exported to a service like Promethease where you get this incredibly detailed report about everything you’d ever want know about how your genes influence your function. Testing like this allows you to get super specific about what your ancestors ate and what you should eat to optimize your nutritional status.

Scientists have now linked at least 38 different genes to nutrient metabolism. Variants of these genes dictate whether you should consume more or less folate, choline, vitamin C, fatty acids, starches, caffeine and beyond. A host of other such genes exists, including MTHFR (folate, vitamin B metabolism), FTO (body weight and fat composition), TCF7L2 (blood sugar regulation), APOE ε4 (cholesterol) and FADS1 (fatty acid metabolism).

In addition to genetic testing, there are various other ways to get started with customizing your diet plan.

Testing Your Blood Sugar Response To Food

Although standardized tools like the glycemic index of foods attempt to determine the health effect of foods, the fact is that your body likely breaks down and utilizes the energy from something like a banana or a potato differently than somebody else. For instance, you may eat a banana and get a smooth dose of energy from it along with only a slight rise in blood glucose, whereas your spouse may get a huge blood sugar spike and an energy crash from that same amount of banana. You can either painstakingly keep a subjective food log to determine this, or you can buy a glucometer and some test strips from your local pharmacy (for around $20) to get an accurate depiction of your body's response to food.

For at least a week, note and observe your blood sugar response to everything you put in your body. You’ll want to start by measuring your blood glucose in the morning when your body is in a fasted state. According to the National Institute of Health, a fasting glucose level between 70 and 100 mg/dL (3.9 and 5.6 mmol/L) is considered normal. With that as a baseline, you’ll want to test immediately after each meal as well as two hours after each meal (less than 180 mg/dL is considered normal by the NIH two hours after a meal, however in the podcast with Robb Wolf a 90 to 115 mg/dL range is recommended).

You’re looking to find what spikes your blood sugar and what keeps it high for longer periods. With a week or so of this detailed logging, you'll be able to take a quick glance at something like a banana, a slice of pizza, or a handful of almonds and be able to gauge your response.

Blood Testing To Determine Nutritional Deficiencies

A good blood test can determine vitamin and mineral deficiencies that need to be filled from a supplement or food standpoint, such as low vitamin D status (vitamin D can be quite toxic if you’re simply taking it because you heard you should when in fact, your levels are actually already adequate), blood sugar response to certain foods or high blood sugar in general, mineral status, thyroid status, cholesterol status, red blood cell and white blood cell levels, vitamin B, acidity, alkalinity and beyond. Some blood tests can even be used to determine food allergies and intolerances, although most of these are notoriously inaccurate, and will simply give you an exhausting “false positive” list of foods you shouldn’t eat.

Stool Testing

A stool panel allows you to determine presence or absence of certain types of bacteria, yeast, fungus, parasites and digestive inflammation, all of which can then be used to determine the need for certain probiotics, cleansing compounds to eliminate things like small intestinal bacteria growth (SIBO) and Candida, the need or non-need to avoid fermentable substances such as simple sugars and starches, and more. Because poor digestion and malabsorption can lead to immune dysfunction, nutritional insufficiencies, and various disease states – along with food allergies and other toxicities – a stool panel provides you with a valuable idea of how your body is processing the foods you’re putting into it and potential things that need to be eliminated.

Metabolic Testing

A Resting Metabolic Rate test (RMR) determines the amount of energy (calories) your body is using at rest. This measurement is made by analyzing the amount of oxygen your body uses and the amount of carbon dioxide your body produces. Almost all of the energy your body produces is created through aerobic (oxygen utilizing) metabolism. The oxygen is then combined with carbohydrates and fats to make energy in your body’s tissues. When carbohydrates and fats are broken down to make energy carbon dioxide is produced. The percentage of the energy created from carbohydrates and fats – and how many calories you are burning at rest – can be determined by measuring the carbon dioxide your body produces. When paired with an exercise metabolic test, you can find out the same data relevant to exercise (e.g. how many calories you burn at any given heart rate). Armed with this information, you can then know precisely how many calories to eat.

Nutrition Intuition – How To Listen To Your Body When You Eat

This may not necessarily tell you what nutrients you’re lacking, but tracking what you eat and how it makes you feel will give you a sense of your ideal meals, if you’re eating enough, specific cravings you may have, food sensitivities, and a whole host of other personal data depending on how detailed you get. And you can get started right away, for free.

All you need to do is record what you eat and how it makes you feel. You can set specific times after eating to record and decide what to assess, such as energy level, mental clarity, etc.

If you want to more thoroughly assess specific food sensitivities or triggers, then you can begin by following a brief elimination diet. You remove all potential immune system inflammation triggers for a minimum of 30 days to give your body a chance to reduce any chronic inflammatory response to foods so that you get a good baseline for improvements. Then, when you reintroduce different foods, your body will have a much higher sensitivity. After the thirty days, you’ll methodically reintroduce specific foods and note whether you have a poor reaction to them.

There are several different elimination diet plans out there, but the most common foods that cause negative reactions and are eliminated are gluten, dairy, eggs, nuts, soy, corn, and alcohol.


You’ll be surprised at how much you can learn about yourself with a little bit of relatively simple testing and tracking. Once you’ve established and dialed in this understanding, unless you’re someone who likes to quantify and track every little detail, you can rely on your intuition with a much more sophisticated relationship to the food you put in your body.

If you want to tackle everything mentioned above, here are recommendations for how often you should test:

-Blood test once yearly

-Gut test once yearly

-Microbiome test once yearly

-Metabolic test once in a lifetime or when your body composition or fitness dramatically changes

-DNA test once in a lifetime

28 thoughts on “Why Diets Fail & How To Determine The Perfect Diet For You

  1. Hi Ben,
    I was surprised to see you recommending 23andMe after your recent interview with Dr. Karim Dhanani. That seemed like a game changer and he wasn’t big on the quality of the info from 23andMe. Are you going with Dr. Dhanani’s recommendations? I’m seriously considering his test instead.

    1. Hi Chris, Thanks for the comment! I would recommend posting it to the podcast page though. Ben doesn’t typically review Kion article comments.

    2. Hi Chris,

      My wife and I did the test from Dr. Dhanani’s clinic, the results came as promised, very interesting. However, his company was so overwhelmed by the orders, thanks in part to Ben’s podcast, so my review of the results with the doctor is 5 months away. If you are okay crunching the results on your own, which is what I am doing, I’d say go for it, if need a review and want to subscribe to the supplements customly formulated based on your test, expect a few months wait. Overall, we were very pleased with the level of detail in the test results, and many surprising discoveries. Good luck!

  2. If you have the raw data from 23andme, to find out diet recommendations- promethase has you go to Is that correct? Seams odd!

    1. be aware that Viome have stopped taking samples for the time being as they are restructuring their labs. i did go through the effort of sending off for the, taking the sample , returning it to have them cancel the process and refund my money. That said i’ll have another go when they ‘re-open’

    1. Dale, I would suggest 23andMe, then uploading your raw data to a service like Promethease. There are a few others out there as well that will interpret your raw data for free or a very low cost.

  3. Great stuff as always Ben.

    But I need to echo Dale’s question, would you say that Viome is not the ideal choice for the most comprehensive diet customization information ?

  4. This article has left me a bit confused. The recommendation at the bottom is for an annual test of both the gut and the microbiome, but the microbiome isn’t mentioned at all in the article. What is the difference between a gut and a microbiome test? I had thought that they were the same thing. Also, the link for the gut test takes me to a page full of tests, none of which are either a gut or a microbiome test. Is there a trusted provider of either of these tests?

  5. ben, what do you reckon could be good foundational tests one could take / purchase in order to suss out basic dietary and lifestyle suggestions tailored to their DNA, on a tighter budget? i leaned toward viome being a worthwhile investment, alas it’s not available in europe (as of yet, anyway). 23andme seems a ubiquitous reference, but i am worried how to best interpret what i understand will be an overwhelming amount of data. thank you.

    1. Lukas, we recommend 23andMe and then uploading your raw data to a site like Promethease for more a more in-depth analysis. DNAFit is also a good option. It is an overwhelming amount of data, but luckily more and more companies that can help you interpret it are popping up.

  6. Great read! I recently had my DNA tested to see how I react to certain elements of my nutrition. Seems that there is loads more that I can do to determine the nutrition that’s right for me.

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