Fat Loss Biology
How many calories should I eat

How many calories should I eat a day is a common yet misguided question. For the purpose of weight loss, calories are an almost irrelevant consideration. Here is why.


How many calories should I eat myth

Here is a bit of a nerdy explanation of what calories really are and why counting calories for weight loss is irrelevant: Unimportance of Calories.

(Note: 1 Calorie [capitalized] equals 1,000 calories [not capitalized], aka 1 kilocalorie or kcal.)

To reiterate the main point in that link, calories in foods reflect the maximum potential heat that can be released from them when they are completely incinerated. The numbers of calories in foods, therefore, represent how much heat they can yield if burned up at 100 percent efficiency.

This is what 100 percent efficiency yields for the three food groups:

  • Carbohydrates = 4 Calories per gram
  • Proteins = 4 Calories per gram
  • Fats = 9 Calories per gram

These are NOT dietary yields!

Humans could not possibly digest any food group to 100 percent efficiency. This is obvious, since some foods are completely indigestible – i.e., yield zero percent efficiency. Cellulose is a clear example. Cellulose ‘contains’ the maximum potential calories of a carbohydrate, since that is what it is. Cellulose yields 4 Calories per gram when it is completely incinerated. It’s rating as a food, however, is zero Calories.

This means that digestive efficiency is a scale that ranges from zero to some percentage that is less than 100. How efficient is it for foods that are not zero?

Some Really Geeky Biochemistry

WARNING! … This is REALLY geeky. That’s why I love it.

A few years ago I co-authored a college-level plant biology textbook, and I was faced with explaining one of the typical nightmares that have been foisted on all beginning biology students for decades: i.e., The step by step biochemical process for the breakdown of a molecule of glucose during cellular respiration (aka, ‘digestion’), including where and how all 36 molecules of ATP are generated, from beginning to end.

Students had to be ready to explain it on the midterm exam.

I had to do this as student, and all other beginning biology students had to do it. The big surprise for me, as an author digging into the research behind it, came when I discovered that all the textbooks and all the professors were wrong!

That is why I added a full 2-page essay on this topic to my textbook, which I have duplicated here for all of you serious biochemists (and formerly misinformed biology students) out there:

Efficiency of Respiration

The question you might have is…

Why is this important?

Even if you are challenged by all the biochemistry, here is why this concept is important: It explains why digestion can never, ever be 100 percent efficient.

It also explains that we don’t really know how efficient it can be, although the numbers that I suggest in my essay (20-25 percent) are probably an overestimate.

A Key Missing Variable

Imagine this: to burn a piece of wood, you must first light it, either with a match or with some other form of ‘pre-fire’, to get it started. Biochemists call this the ‘energy of activation’ – the energy required to activate the release of energy (mainly heat) that is available in the otherwise inert wood.

This is the same concept that applies to the digestion of food. The role of enzymes is to lower the energy of activation so that biochemical reactions won’t require so much of it. Otherwise, we would have no such thing as life.

How much is the energy of activation for different foods? We can only estimate it, although it seems to range between 10-30 percent of the maximum potential energy (calories) that are available. The energy of activation is higher for digesting protein than for digesting carbs or fat.

Other Crucial Variables

HOW you metabolize different foods depends on food composition, health and fitness, hormone balance, and lifestyle.

1. Food Composition: Just one example, and one of the most important ones, is the metabolic difference between glucose and fructose. When glucose comes into the body (e.g., from sugar, starchy carbs, or food additives), 20 percent of it is metabolized through the liver and the remaining 80 percent through the rest of the body. In contrast, when fructose comes into the body (e.g., from sugar, high fructose corn syrup, or other food additives), 100 percent of it is metabolized through the liver.

The biochemical steps that break down fructose have a lot in common with those of alcohol, which is why fructose is now considered to be a liver poison. (See comparison table in this post: High Fructose Corn Syrup – Deadliest Food Additives).

Since I have already used up my Geek Quotient for this post, I’ll just say the one of the consequences of digesting excess fructose in the liver is an elevated triglyceride level. You make more fat from fructose that you ever could from glucose!

I think this is ironic because fructose has been recommended as the ‘safer’ sugar due to its lower glycemic index.

The number of Calories per gram for glucose is that same as it is for fructose: 4. In this example, the glucose vs. fructose composition of food, however, is the key for whether you synthesize too much fat (i.e., triglycerides). This has nothing whatsoever to do with the number of calories in either one.

2. Health and Fitness: Foods are inflammatory. During digestion, the inflammatory load of food byproducts induces your body to calm down the flames, so to speak. Carbohydrates are more inflammatory, as a group, than are proteins and fats. If you consume a certain amount of food, dominated by carbs, your body will be a 5-alarm fire regarding inflammation. Not so with the same amount of protein and fat. This is even more problematic with carbs from modern wheat, regardless of whether it is whole grain. Even if you restrict your intake to starvation levels and are hungry all the time, a carb-heavy diet will be inflammatory. Inflammation is like super-glue for fat.

The way your body responds to inflammation is an indication of overall health and fitness. The most obvious indicator is fat: if you have too much of it, then your body’s inflammation response is being overwhelmed. If so, you are not healthy and fit. Your food has become your enemy.

3. Hormone Balance: Good health and metabolism depends on a balance of hundreds of hormones. If you carry too much fat, you are essentially carrying an estrogen factory, so your hormones are already out of balance. If you are aging, like me, into your 60s and beyond, your growth hormone levels have dropped off a cliff. (You can still do something about that, though.) If you have become insulin resistant (diabetic or pre-diabetic) or leptin resistant, no matter how little you eat you will stay fat.

Counting calories will not help you until you address hormone resistance and hormone imbalance. As far as eating is concerned, this means WHAT you eat and WHEN you eat are much more important then how much you eat. By the way, hormone balance and eating patterns are core topics that I explain in my Belly Fat Book, in case you have not got your copy yet.

4. Lifestyle: Oh, this is a huge topic. The most important influence on weight loss or weight gain in the ‘Lifestyle’ category is stress, including sleep deprivation. Americans have a culture of losing sleep and building stress.

My advice is: STOP IT!

Again, this is an influence on your long-term health and weight management that far exceeds any effort you make toward counting and limiting your caloric intake. In fact, just worrying about calories is a source of stress that you don’t need.

Stop Worrying About Calories!

Worrying about how many calories you consume is a nearly useless endeavor. If anything, just trust your body to know how much to eat, when you eat the right foods at the right times. Eat slowly and enjoy what you eat. When you feel satisfied (not stuffed), stop eating. It is as simple as that.

Leave the calorie counting up to the biochemists.

All the best in weight loss,

Dr. D

9 Comments so far »

  1. by Kyle


    I am learning more and more about how losing weight or fat is a mind thing because you have to know what to eat, when to eat and how much to eat. You have to be in tune with your stomach so you know when you are hungry and when you are satisfied and can stop eating before you are stuffed. Thank you for all the work you have put into all your blogs.

  2. by Bryan


    The opening line in this article is true indeed. This is a nerdy, highly technical explanation of how many calories a person should intake on a daily basis. I think I will stick to doing it instinctively. I go by how hungry I feel and I try not to over eat. And as long as I eat healthy stuff and I burn calories with workouts, I’m good to go.

  3. by Sara


    There are obviously a variety of online sources are available that can help you determine how many calories you need each day. Besides this informative article, the American Heart Association provides a basic chart outlining caloric needs based only on age, gender and activity level. I myself used to weigh all of my food amounts, but that gets to be tedious after a while.

  4. by Barbara


    I just hope people out there learn through websites like this one, not to severely restrict their calories. My friend has a 16 year old daughter who started looking skinny to a point of looking somewhat unhealthy, all because her boyfriend told her to lose some weight. She thinks the way to do it is by starving herself without exercising. I hope she learns.

  5. by Stephen


    Now that the U.S. is famous for having the fattest people on earth and obesity is considered a new disease, the weight loss industry is doing it’s best business ever. Leading the way for profits which are in the billions per year are the big pharmaceutical drug companies, which have yet to manufacture a drug that can successfully and safely help anyone lose weight long-term and keep it off. Fascinating isn’t it?

  6. by Terri


    How much of a roll does stress play on weight loss? I have been trying to lose weight for almost a year now and I have had no luck at all, but I will admit that my sleep pattern sucks and I am under a lot of stress. I understand that will affect it but what I am wondering is how much will it effect it?

  7. by Josephine


    I am glad to read that it doesn’t matter about the calories as long as you are watching what you eat. If you are eating lettuce in salads a lot then you probably don’t have to watch the calories of that as much depending on the dressing. Thank you for posting this it is helpful to know that I don’t have to always count calories.

  8. by Richard


    Certainly not all low carb diets are created equal; that is, a garden salad topped with salmon is much healthier than a steak cooked in butter. But in my opinion, getting carbs right is about both quantity and quality. Carbohydrates are the most efficient source of fuel for your body’s cells, which is probably why they’re so abundant in nature (grains, beans, fruits, veggies).

  9. by Dr. Dennis Clark


    Hey, Bryan: Yup, I am a self-professed nerd. However, your approach is the best, regardless of all the verbiage that I and my fellow diet geeks have to offer. Keep up the good work!

    All the best,

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