Transcript Priority Issues in Mineral Research

ASSESSING HEALTH REQUIREMENTS
AND
SETTING DIETARY STANDARDS
FOR MINERALS
NUTRITIONAL CONCERNS
Evaluating Individual Need
Evaluating Individual Status
Setting Standards for optima
Population Approach
Balance Studies
Experimental
Biomarkers
Functional tests
Assessing Mineral Status
Major Categories to Consider and
Attack
Body stores of the mineral
Functional indices
Response to increase intake
Caution: Seldom can one mineral be evaluated by all three. Choice of
most favorable will depend on the mineral being assessed.
Cumulative Risk
1.0
EAR: Estimated Average Requirement
UL: upper limit
RNI: Recommended nutrient intake
LOAEL: Lowest observed adverse effect level
NOAEL: No observed adverse effect level
0.5
Acceptable range of intake
LOAEL
Risk of
deficit
Risk of
excess
NOAEL
EAR RNI (97.5%)
(50%)
UL (0%)
Intake
How are data that generates this curve obtained?
Question: How does one assess mineral status?
Question: How do we estimate safe mineral intake?
Approaches
1. Balance studies
2. Clinical observations
3. Optimal intake
4. Evidence-based approaches
5. Sub-clinical evaluations
Balance Approach (keeping the status quo)
Matching input with excretion
Assess the ability of the system to:
Maintain body stores
Recover loss due to metabolic turnover
Delay appearance of clinical signs
Problems:
Choice of endpoint dubious
Choice of time may be critical
Overlooks prior exposure and system adaptation
Balance
Matching what goes in with what goes out
Absorption
A
Excretion
Kout
Kin
B
K1
D
K-1
C
Absorption-Excretion
Retention
Retention
Kin
=
Kout
Balance
Kin
>
Kout
Positive Balance (growth)
Kin
<
Kout
Negative Balance (wasting)
K1
=
K-1
Balance
K1
>
K-1
Positive Balance
K1
<
K-1
Negative Balance
Problems With Balance as a Criterion of Amount
1. End Point is in doubt
2. Excretion is episodic not continuous
3. Multiple connecting pools of the same mineral
4. System adaptation
Clinical Approach:
Look for or create a disease state
Look for a structural or functional abnormality
Link these with known biochemical or functional changes
Parameters:
Growth rate
Physical appearance (skin, bone, hair, etc.)
Physiological impairments (gastrointestinal, immune and nervous system)
Cognitive functions
Biochemical impairments (stress factors, enzymes and metabolic factors)
Medical (proneness towards disease)
Optimal Intake Approach for Assessing Requirement
Specific nutrient requirement of minerals is that which:
1. Optimize physical or mental performance
2. Thwarts or reverses disease
3. Increase longevity
Compare with an accepted standard
Example:
1. Optimal mineral nutrition of milk should
duplicate the composition of human milk.
Problems:
1. Too general
2. Defies population-based approaches
Evidence Based Approach
Evaluate studies pro and con and weigh strengths,
weaknesses, design differences, etc. in arriving at a
set of numbers
More a propos to setting dietary risk standards than
determining requirements:
Sub-clinical (Functional) Approach
Biomarkers of adequacy
and deficiency
An ideal biomarker is some
reliable internal factor that
responds directly,
specifically, and
quantitatively to changes in a
mineral’s homeostasis
Its function is to signal a
disturbance in the
functional stores of a
mineral
The most common
signal elicited is a
depression in blood or
tissue levels of the
mineral
Body stores of the mineral
Applies mainly to iron
Circulating levels of ferritin are a measure of iron tissue stores
Total iron binding capacity (TIBC)
Transferrin saturation
Hematocrit
Hemoglobin levels
Red cell morphology
Functional indices
Inadequate intake of some minerals causes major perturbations in
biochemistry. This is most noted by depressed levels of enzymes,
homones, or altered tissue morphology.
Its imperative that a direct connection exist between the mineral in
question and the functional component.
Examples
Suspect
Measure
Iodine deficiency
circulating thyroid hormone
Copper deficiency
plasma ceruloplasmin
Selenium deficiency
glutathione peroxidase
Organs and Systems that Play a Major Role in
Mineral Homeostasis
Gastrointestinal tract
Mineral
Iron
Absorption
Endogneous excretion
---
---
Substantial
Major (liver)
---
Manganese
Major
Major (liver)
---
Zinc
Major
Major
---
Copper
Single major site
Kidneys
Iodide
Selenium
Major
(After Hambidge, 2003)
Major
Chromium
Major
Molybdenum
Major
Assessment of
Iron
4-5 grams of iron
Erythrocytes
2.5 grams
Twothirds
10 -14 mg ingested
Absorption
0.5 -2 mg
Plasma - 4 mg
(0.08-0.1%)
Excretion
1-2 mg
Ferritin
Storage
1,500 mg
Enzymes
5 mg
Liver, spleen, bone marrow
one-third
Myoglobin
200 mg
Myoglobin,
mitochondria -one third
Measurement of Iron status
• Transferrin saturation
– TIBC (total iron binding capacity)
– UIBC (unsaturated iron binding capacity)
– 33% saturated normally
– 47% in the morning (after fasting)
– 13% at night
• TIBC = 400 ug/dl …..deficient
–
= 200 ug/dl…..inflammation
Test is performed when there is concern for anemia, iron
deficiency, or iron deficiency anemia
Meaning of Terms
TIBC = Iron binding capacity of a volume of serum (dL)
Expressed as ug of iron needed to fill all Tf molecules in that volume
Determined spectrophotometrically
UIBC = total iron in a volume of serum / TIBC for that volume
Expressed as percentage of saturated transferrin
Determined by atomic absorption analysis of iron in serum
UIBC first determines how much iron is present in a serum
sample. 99% of that iron is bound to transferrin. TIBC says how
much iron is needed to fill all vacancies on the transferrin in that
volume of serum. Comparing how much is there to how much is
need to fill vacancies measures percent saturation
Measuring TIBC
Draw Blood
Serum
Fe3+
Centrifuge
Blood Cells
Change in color due
to more Tf becoming
saturated with iron
Fe-Tf complex
Saturation
Fe
The amount of Fe3+
required to reach saturation
of all the transferrin present
is a measure of the total iron
binding capacity (TIBC)
Sample Problem
(Note: Knowing the exact amount of Tf is unnecessary)
An clinical lab draws 5 ml of blood to determine the TIBC and
UIBC of a patient. 2 ml of the serum from the blood requires 12
ug of iron to reach saturation. That same serum contains 4.7 ug
of iron per ml. Calculate the patients TIBC? UIBC? What can
you conclude about the patients iron status?
61%
39%
Solution
TIBC is expressed in deciliters (dL) of blood. One deciliter = 100 mL
If 2 ml requires 12 ugm of iron, 100 mL requires 50 x 12 = 600 ugm
TIBC = 600 ugm/dL
UIBC is expressed as percent of saturation
If 4.7 ugm is present in 2 mL, 100 mL contains 50 x 4.7 = 235 ugm
UIBC = 235/600 x 100 = 39% saturated
Within NORMAL range
Ferritin (major iron storage protein)
1. Most sensitive indicator of iron stores
Adults
1 g ferritin/L plasma = 8 mg stored iron (Bothwell et al, 1979)
<12 ng ferritin/L plasma = no iron stores
Children
1 g ferritin/L plasma = 14 mg stored
National Surveys
Percentile
Men (19-30)
Women (19-30)
Children
( g ferritin/L plasma)
5th
36
7
6
50th
112
36
23
394
212
116
99th
Problems with Ferritin
An Acute phase protein that is elevated in:
inflammations
infections
disease (neoplasms especially of the colon,
cardiovascular)
ethanol consumption
hyperglycemia
body mass index
New Frontiers with Iron
Soluble transferrin receptor concentration of the plasma
(a good index of early iron deficiency)
Basis of analysis
Cells requiring iron express transferrin receptors
Extracellular domain or the receptors is subject to proteolytic cleavage
and released into plasma
Number of receptors is in proportion to the number that were expressed
on the cell surface…which measures iron requirement
Assessment of
Zn
Clinical Evaluation of Zinc
Adequacy
Before and After Zn Therapy
Problem: Assume you are a physician who specializes in
mineral deficiencies. A patient is referred to you by his
physician who suspects a zinc deficiency. How can you
confirm the diagnosis?
Give the pros and cons for each of the following
1. Measure serum zinc levels
2. Measure 24 hr urine zinc excretion
3. Measure fecal zinc excretion
4. Measure lymphocyte zinc content
5. Measure zinc in hair
The above scenario also
applies to a veterinarian who
is attempting to determine
the reason for stunted
growth in livestock and
suspects zinc deficiency as
the cause
6. Measure liver zinc
7. Measure activity of a zinc-dependent enzyme
8. Measure metallothionein levels of blood or mRNA in monocytes
9. Measure body pools of zinc with radioactive tracer
Zinc (a major trouble mineral for biomarkers)
Problems with Zinc:
1. Zn deficiency can be manifested by slight decrements in tissue
Zn. An impairment can occur before a change is detected.
2. Internal Zn changes are immediately corrected by a very effective
homeostatic mechanism. Thus a biomarker may never be turned on
3. Zinc’s usage is so wide-spread and interconnected that to focus
on any one factor is not always practical.
4. Plasma contains only about 0.1% of the body Zn, questioning whether
plasma Zn changes reflect total body Zn. Nonetheless, plasma Zn is
currently the most widely used and accepted biomarker of Zn status.
5. Plasma Zn is depressed whenever an acute phase response is
triggered
6. Plasma Zn tends to be down in hypoalbuminemia
IS PLASMA ZINC THE BEST INDICATOR OF ZINC STATUS
Experimental Observations:
1. No correlation was found between intake/absorption and plasma Zn
levels over a wide range of dietary Zn in adult women (Sian et al, 1996).
But, dietary and absorbed Zn was found to correlate with dynamic Zn pool
size (Sian et al, 1996; Krebs et al, 2000).
Comment: Evidence for a rapid self-correcting homeostatic mechanism?
2. Young children with a Zn-limiting growth impairment have plasma Zn
levels within the normal range (Wada et al, 1985)
3. Low plasma Zn levels (<70 ug/L) are a good predictor of growth
response to oral Zn supplementation.
OTHER ZINC INDICATORS
1. Cellular components (carbonic anhydrase). The major erythrocyte
pool of zinc. But,
1. Does not turnover
2. Membrane-bound Zn also a factor
3. applies also to monocyte, neutrophil, and platelet Zn
2. Hair Zn. Good marker. But,
1. More relevant to chronic as opposed to transient Zn status
3. Zinc Excretion (urine and feces)
1. Requires tracers and metabolic collection
2. More applicable to research than clinic screening
4. Tissue stores
1. Also requires tracer and sample collection over many days
2. Metallothionein mRNA (PCR) in monocytes promising
OTHER ZINC (cont.)
5. Zinc-dependent enzymes. Suppression of the following is inconsistent
1. Alkaline phosphatase
2. CuZn superoxide dismutase
3. lymphocyte 5’-nucleotidase
6. Response to Zn supplementation
1. Dietary levels was also be taken into account
2. Response to Zn must be specific and non-pharmacological