About Dietary Reference Intake (DRI)



The Dietary Reference Intake can be defined  a system of nutrition recommendations that estimates the amounts of nutrients required to prevent deficiencies and maintain optimal Health.


The DRI includes some concepts that sometimes are used as equivalents, when they are not. In other cases, the same concept receive different names, depending of the source of the article you are reading.

DRIs consists of four dietary reference standards:


1.- Estimated Average Requirement (EAR): Average daily nutrient intake level estimated to meet the requirement of one half of the healthy individuals in a specific age and gender group.


 2.- Recommended Dietary Allowance (RDA): Average daily Dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97-98%) individuals (EAR+2DS) on each life state and gender group. It is approximately 20 % higher than the EAR. RDA are printed in food labels in the United States and Canada.  (The definition of RDA is similar to the definition of  Recommended Nutrient Intake (RNI)  used by the World Health Organization and other international organizations).


 3.- Adequate Intake (AI): Estimation of nutrient intake in healthy people (used when EAR and RDA are not known)


 4.- Tolerable Upper Intake Level (UL): Highest average daily intake level from food, water and supplements that is likely to pose no risk of adverse health effect from excess in almost all apparently healthy individuals in an age and gender specific population group. (This concept is important mainly in relation to nutritional supplements, since the ingestion of natural foods is regulated in the body through mechanisms of absorption and excretion), In absence of observations of known adverse effects, it is used a default value of 10 times de Recommended Dietary Allowance.


More information about these reference standards can be found at:


Dietary Reference Intake

Reference dialy Intake 

 Interactive DRI for health care professionals

 Yates, A.A.: Dietary Reference Intakes –What is new and how to use them (Power point presentation)

Who Experts Committee:

Vitamin and Mineral requirements in human nutrition

World Health Organization, 2004


Today FDA warns to stop using Hydroxycut products


The Food and Drug Administration have warned consumers today to stop immediately the use of Hydroxycut products, since the use of these products have been related to some cases  of severe liver injuries.


More information in the FDA site:


FDA Warns Consumers to Stop Using Hydroxycut Products


Hydroxycut products.

Solution to the Puzzle “Find seven hydrosoluble vitamins”





N + + + + F + + + + B + + E +
+ I + + O + + + + + I + + N +
+ + V L + + + + + + O + + I +
+ + A A + + + + + + T + + X +
+ T + + L + + + + + I + + O +
O + + + + F + + N + N + + D +
N + + + + + O I + + + + + I +
+ I + + + + M B + + + + + R +
+ + A + + A + + I + + + + Y +
+ + + C L + + + + R + + + P +
+ + + A I T H I A M I N E + +
+ + B + + N + + + + + + + + +
+ O + + + + + + + + + + + + +
C + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + +


When Tyrosine becomes an essential amino acid

Answer to Question AM-06


Short answer: (g)


In patients with PKU, Tyrosine becomes essential, since it is formed from Phenylalanine in the reaction that is impaired in Phenylketonuria.


Additional information:


Most of the textbooks classify amino acids from the nutritional point of view, in two groups: essential or not essential.  Essential amino acids are considered those amino acids that can not be synthesized by an organism and so should be consumed in the diet; non essential amino acids are those amino acids that can be synthesized.  This classification is not related to the importance of the amino acids, but with the fact of them being required in the diet or not.


According to this classification, the essential amino acids are:


Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine


















Non essential amino acids are:

Alanine, Asparagine, Aspartate, Cysteine, Glutamate, Glutamine, Glycine, Proline, Serine, Tyrosine


Like most facts in biology, this “black and white” classification is not 100 % accurate. Actually, some amino acids are conditionally essential or partially essential, since some “essential” amino acids, like arginine,  can be synthesized by the body. Arginine is synthesized in the urea cycle, for example, but it is considered essential since the quantity of Arginine that is synthesized is not enough during the growing process.


 Tyrosine is an amino acid that is synthesized in the body from Phenylalanine, that is an essential amino acid. This reaction is catalyzed by the enzyme Phenylalanine Hydroxylase, that use as cofactor reduced tetrahydobiopterine.

If Phenylalanine is deficient in the diet, then the body requires tyrosine in the diet.


In Phenylketonuria there is an excess of Phenylalanine, since the body can not metabolize it, but Phenylketonuria is a consequence of a deficit of Phenylalanine Hydroxylase (Classic Phenylketonuria) or a deficit of Tetrahydrobiopterin Reductase. In both cases, the organism is not able to synthesize Tyrosine from Phenylalanine, so even when there is an accumulation of Phe in these patients, it can not be used to synthesize Tyrosine.


In fact, some of the signs and symptoms of Phenylketonuria, like mental retardation and other neurological symptoms, have been related to the unavailability of tyrosine for the synthesis of the neurotransmitters that derive from tyrosine.



The lack of pigmentation of PKU patients, has been related also to the lack of tyrosine, since Tyrosine is a precursor of melanine also.


It is obvious that if Tyrosine is formed in normal persons from phenylalanine through the reaction cited above, in case that this reaction can not be produced, like in PKU, it is necessary to supplement the patient with Tyrosine, since the patient can not synthesize it, so Tyrosine becomes an essential amino acid for these patients.



A: About Protein Classification


Original Question


Answer: (e) a complete protein.




Proteins can be classified according to different criteria. There are very complex ways of doing it. Anyway, simple and common ways of classifying proteins in Biochemistry textbooks are the following:


1.- Based on the shape or role of the proteins:


a)   Globular proteins (spheroproteins):


These proteins do not aggregate. Major backbone conformation include the helix, the strand, and the turn. These proteins do metabolic work: catalysis, transport, regulation, protection…These functions require solubility in blood and other aqueous media of cells and tissues. All globular proteins are constructed with a defined interior and exterior. In aqueous solutions, hydrophobic amino acids usually are in the interior of the globular proteins, while hydrophilic ones are interacting with water.

Ex. Myoglobin and Hemoglobin.


b)   Fibrous proteins (scleroproteins) :


These proteins are water insoluble proteins with elongated shapes.  

Aggregate tightly into fibers or sheet. Most play a structural and/or mechanical role. Tend to form elongated, curvilinear structures of high regularity. The regular structure comes from high primary sequence regularity.  Usually present modified amino acids. Ex. keratin and collagen.


2.- Based on the composition:


a)     Simple Proteins: Formed only by amino acids linked through peptide bonds (polypeptide chains)


b)     Conjugate proteins: Formed by amino acids and a different compound.

In conjugate proteins, the polypeptide portion is called apoprotein and the nonpolypeptide moiety is called prosthetic groups. According to the kind of prosthetic group, conjugates proteins can be nucleoproteins, glycoproteins, hemoproteins, flavoproteins, etc.


3.- According to the Nutritional Value, proteins can be classified as:


a)   Complete: A protein that contains all the essential amino acids. Generally comes from animals.


b)   Incomplete: A protein insufficient in one or more essential amino acids. Generally comes from plants.


The only information in the question that allows us to classify the protein X is the fact that it has all the essential amino acids. It allow us to classify it as a complete protein (the lack of some non essential amino acid is not significant from the nutritional point of view, since we can synthesize non essential amino acids)


We lack information about the conformation of the protein, its function,  so we can not classify it as a fibrous or globular protein, a simple or conjugate protein, or according to its function.


Is it possible to follow a vegetarian diet and obtain all the essential amino acids in the diet?


Yes, it is possible. Besides the fact that Soy protein in fact contains the essential amino acids, the combination of incomplete proteins in the diet allow to compensate for the lack of specific amino acids in any of them, so if we combine two proteins with low nutritional value, we can obtain a mixture with a higher value that the two original proteins.This is called the  supplementary action of the proteins.


In other words, if Protein A lacks lysine and protein B lacks methionine, the combination of Proteins A+B will have a higher nutritional value than the two separate proteins. Nowadays it is considered that it is not necessary to combine incomplete proteins in the same meal.



More information:








Answer to Question N-03 about Hypervitaminosis D



Answer to Biochemistry Question N-03 : ( c ) Vitamin D



Vitamin D is synthesized from a derivative of cholesterol, the 7 dehydrocholesterol.  By the action of hv light (290-315 nm) in the skin, it becomes cholecalciferol or Vitamin D3 (inactive).


                                   Cholecalciferol structure


Cholecalciferol is hydroxylated in the liver by  25 hydroxylase, and in the kidney by a 1 hydroxylase,  becoming 1,25 dihydroxycholecalciferol or calcitriol, the active form of vitamin D3.


                                    Calcitriol structure


Vitamin D participates in the regulation of multiple processes, being the more obvious the regulation, together with Parathormone, of Ca++ homeostasis, regulating Ca++ in blood and the balance between Ca++ deposition and Ca++ mobilization from bone.


Hypocalcemia stimulates releasing of PTH, that through cAMP activates 1 hydroxylase  in the conversion of VitD to calcitriol.


Calcitriol interacts with a vitamin D receptor (VDR)  and initiates a cascade of molecular changes that stimulates the transcription of specific genes, including  the synthesis of an intestinal Ca++ binding protein, essential for uptake of dietary Ca++. Together with PTH, Vitamin D  facilitates Calcium reabsortion by the kidneys and mobilizes calcium from bone.


All these actions help bring back blood calcium levels within the normal range.


Intoxication with Vitamin D is usually a consequence of a prolonged ingestion of an overdose of supplements or as a consequence of accidents in the formulation of pills or in the misuse of some oils or containers.


The intoxication requires the prolonged ingestion of very excessive amounts of Vitamin D (around 100 times the Recommended Diary allowance) during a period of months, or an acute overdose of 10 000 times the RDA.


Vitamin D intoxication is characterized by hypercalcemia, hypercalciuria, gastrointestinal disorders, weak bones, deposits of calcium in soft tissue that can be revealed by imagen studies, renal calcium stones and kidney failure.

25 hydroxycolecalciferol is tipically elevated in serum.


General information about Vitamin D can be found in:


Linus Pauling Institute. Micronutrient Information Center. Vitamin D



Dusso, A.S. et al: Vitamin D

Am J Physiol Renal Physiol 289:F8-F28, 2005





Information about Vitamin D requirements and safety levels:


Vieth, R.:

Vitamin D Supplementation, 25-hydroxyvitamin D concentrations and safety

Amer J Clin Nut 69:5, 842-856, 1999


Muskiet, F.A.J. et al:

Do we really need > 100 ug vitamin D/d and is it safe for all of us?

Amer J Clin Nutr 74: 6, 862-863, 2001


More about Vitamin D intoxication:


Klontz, K.C. and Acheson, D.W.: Dietary Supplement-induced vitamin D intoxication

NEJM, 357(3) 308-309,  2007


Vitamin D Intoxication Associated with an Over-the-Counter Supplement

NEJM, 345 (1)  66-67, 2001


Marriot, B.M.: Vitamin D supplementation: a word of caution.

Annals of Internal Medicine, 127(3)  231-233






Calculating calories in meals


Answer to Biochemistry Question N-02: (e) 500 Calories



                                      from zamber on flickr



Since usually we use large amounts of energy compared to the calorie unit used in Physics, the calorie unit used in Nutrition is different to the classical calorie definition used in Physics. While in Physics 1 calorie is defined as the quantity of energy necessary for increasing the temperature of one gram of water 1 Celsius degree at 1 atmosphere of pressure, the nutritional calorie correspond to 1000 this value (in a more accurate way, it correspond to what is called a large calorie, in contrast to the  classical “small” calorie of Physics, and should be abbreviated as Cal). It means, that 1 Nutritional calorie is equivalent to 1 kilocalorie in physics.


In the SI unit 1 physics’ calorie is equivalent to 4.184 joules, so a Nutritional calorie is equivalent to 4 184 Joules.



The body needs energy for:


1.- Mantaining Resting Metabolic Rate (RMR, former BMR)


2.- Heat production  (An important factor in heat production is the thermic effect of food, or diet induced thermogenesis, that may amount 5-10 %  of total energy expenditure/day)


3.- Physical activity



The human body obtain the energy it needs from the foods.


The major dietary energy sources:



4 cal/g  (nutritional calorie)


9 cal/g (nutritional calorie)


4 cal/g (nutritional calorie)



As you can see, fat is the most concentrated source of energy – weight for weight it provides just over twice as much as either protein or carbohydrate.


Alcohol provides almost as much energy as fat: 1 gram of alcohol can supply 7 cal/g . For some people alcoholic drinks form a large part of their energy intake. This can be harmful to health since a high alcohol consumption is a risk factor for several diseases.


The energy content of a food or drink depends on how many grams of carbohydrate, fat, protein and/or alcohol are present.


Since the source of energy in the diet has been implicated as a risk factor in certain diseases, it has been described a recommended distribution of calories in diet. The Acceptable Macronutrient Distribution Range (AMDR) has been defined as the range of intake for a particular energy source that is associated with reduced risk of chronic disease while providing intakes of essential nutrients (Dietary Guideline for Americans, Glossary . The caloric composition of diet should be approximately, 45-65 % from carbohydrates, 20-35 percent from fats and 10-35 % from proteins, but there are recommendations for specific groups.


Different websites show the caloric value of thousands of foods. This is one of those sites with several related links:  http://www.caloriecountercharts.com/



Anyway, it is important that you know the general principles described above.


USMLE released sample questions, that exemplify contents of the examination, have included questions about caloric calculation in past years (Ex: Question 45 in 2005 USMLE Step I Content Description and Sample Test, that also appears as Question 35 in the 2006 edition). Unfortunately, copyright issues do not allow us to reproduce that question here.



More information can be found in:


A classic:

Merrill, A.L.; Watts, B.K. :Energy values of food: Basis and derivations


Food composition and Nutrition Links from the USDA