One major question regarding carotenoids is whether there is a threshold effect, or whether there is a continuous dose-response effect up through at least the concentrations commonly seen in populations. Within these sixty-two studies there were 40 that had multivariate regression models that gave odds ratios across the range of carotenoids within the study populations (see , Benefit, Fraction column). From these models it is apparent which fractions of the population are significantly different from the reference fraction, usually the lowest fraction. So, of these 40 studies there were 24 studies in which only the top tertile or top one or two quartiles or quintiles had a statistically different positive health outcome from the lowest reference fraction. There were 16 studies in which only the lowest fraction had a statistically different health outcome from the rest of the population.
Whether there is a threshold or dose-response may depend on the particular outcome. In all of the studies that reported all-cause mortality only the lowest partition of the population seemed to be at risk due to low levels of carotenoids. There seemed to be a threshold for this particular outcome. However, for metabolic syndrome or dysglycemia only the upper tertile or top two quartiles had a significant benefit compared to the lowest tertile or quartile. For cancer outcomes the results are mixed, but overall favor a significant benefit only in the upper concentrations of carotenoids.
Perhaps there is a triage effect for carotenoids, as proposed for other nutrients by Bruce Ames. McCann and Ames demonstrated such an effect for vitamin K [83
]. The triage concept is that nutrients will go to the systems of the body that are most critical for short-term survival first. Then if there is a sufficient quantity of the micronutrient all of the other systems of the body will also receive a beneficial amount to prevent subtler dysfunction and age-related disease. The amounts of carotenoids needed to avoid mortality from any cause may be less than the amount to help prevent dysglycemia, diabetes, and some forms of cancer.
The mechanism or mechanisms by which the triage effect might take place with carotenoids is not certain. However, it is known that carotenoids have functions beyond their antioxidant properties. In vitro
studies with AGS gastric cells showed that β-carotene and lutein both showed anti-inflammatory action. Not only did β-carotene and lutein cause a reduction in the levels of reactive oxygen species when the gastric cells were exposed to H2
, indicating antioxidant activity, but there was also an inhibition of the activation of NF-κB and the subsequent expression of IL-8, indicating anti-inflammatory activity [84
]. In a study of preterm infants supplemented with β-carotene, lutein, and lycopene compared to unsupplemented preterm infants and full-term infants fed human milk, Rubin and coworkers [85
] found that the supplemented infants had decreased concentrations of C-reactive protein, a marker of systemic inflammation. The supplemented group also had greater rod photoreceptor sensitivity, indicating that lutein had protective effects for the retina of pre-term infants as well. More beneficial effects of lutein for retinal health were found in a mouse model of endotoxin-induced uveitis. Along with a reduction in reactive oxygen species in the retina neural cells, the negative effects of inflammation (a decrease in rhodopsin expression, shortening of outer segments of outer photoreceptor cells, levels of STAT3 activation and downstream inflammatory cytokines), were all prevented by lutein treatment given before and concurrently with the endotoxin to induce inflammation in the mice [86
]. Together these studies indicate that carotenoids can suppress the inflammatory cascade and normalize cellular function.
Carotenoids have been shown to have specific anti-cancer properties as well. Lycopene induced apoptosis in LNCaP human prostate cancer cells at 0.3 to 3.0 µmol/L [87
] or at 1 and 5 µmol/L [88
]. Kotake-Nara and coworkers [89
] studied the effect of 15 different carotenoids on the growth of three different prostate cancer cell lines. At 20 µmol/L neoxanthin from spinach and fucoxanthin from brown algae decreased cell viability at least 85% in all three cell lines via apoptotic mechanisms. Significant effects were also seen for phytofluene, ζ-carotene, and lycopene. Lycopene, and to a smaller extent β-carotene, enhanced gap junction communication in KB-1 cells derived from human oral cancer [90
]. In 10T1/2 cells Zhang and coworkers [91
] found that β-carotene, canthaxanthin, lutein, lycopene, and α-carotene all increased gap junctional communication, which was not correlated with the carotenoids’ pro-vitamin A activity or ability to quench lipid peroxidation. Physiological concentrations of β-carotene inhibited cell growth in several human colon adenocarcinoma cell lines by inducing cell cycle arrest in G2
/M phase and apoptosis [92
]. Palozza and coworkers [93
] also found that tomatoes digested in vitro
were able to stop the growth of colon adenocarcinoma cell lines HT-29 and HCT-116 and induce apoptosis. Similar results were also seen in tomatoes genetically modified to express large amounts of β-carotene [94
]. Induction of apoptosis by β-carotene via the caveolin-1 pathway, an intracellular signaling pathway usually deregulated in cancer cells, showed yet another mechanism for carotenoids to act [95
]. Carotenoids also have antimetastatic activity. Lycopene inhibited adhesion, migration, and invasion of SK-Hep1 human hepatoma cells [96
]. In vivo
inhibition of metastasis was shown in a follow-up study by injecting SK-Hep1 cells into athymic nude mice that had been treated for two weeks with placebo or lycopene or β-carotene. Mice treated with lycopene or β-carotene had significantly reduced numbers of metastasized tumors in the lungs, and smaller cross-sectional area of the tumors [97
]. Lycopene, but not β-carotene, also decreased the positive rate of proliferating cellular nuclear antigen (PCNA), the concentration of vascular endothelial growth factor (VEGF), and protein expressions of PCNA, and MMP-9 (matrix metalloproteinase-9) in lung tissue.
Nutragenomic research using microarrays found that lycopene modulated the expression of 391 genes in estrogen-positive breast cancer cells, but not in estrogen-negative breast cancer cells or fibrocystic breast cells [98
]. Genetic pathways affected included apoptosis, cell communication, MAPK and cell cycle, xenobiotic metabolism, fatty acid biosynthesis and gap junctional communication.
There are so many pathways that respond to carotenoids that it is quite possible that a triage mechanism is present. Some molecular switches may be inhibited or activated at low concentrations of certain carotenoids while other pathways are abnormally active until the concentrations are much higher. It is unlikely that all cellular pathways are equally affected at the same concentration of carotenoids. So, while there may seem to be a threshold above which some outcomes are not affected, it is much more likely that there are molecular pathways and systems in the body that would benefit from higher plasma levels of carotenoids.