"Low levels of light are good for you, while high levels are bad for you." (2)
There is perhaps a favoritism in the PBM community that more intensity is better. In our Western mindset, bigger is better, hence high-intensity products (>=100mW/cm2) imply more value and "faster" treatments.
The law of reciprocity (Bunsen-Roscoe law) would tell us that we can achieve the same dose and effect regardless of intensity or administered timeframe. For example:
To reach a 6 J/cm^2 dose would require about:
- 1 minute at 100mW/cm^2
- 14 minutes at 7mW/cm^2
However, in experimental trials looking at Biphasic Dose, too high of intensity shows a lack of improvement. Indeed if we hold dose (J/cm^2) constant and only change the irradiance (mW/cm^2), researchers find a biphasic response curve! (2) Further, too high of an intensity >100mW/cm^2 would also risk thermal degradation effects. (1)
You cannot trick your body to absorb photons faster than nature intended. If you try, you can end up with a counterproductive result. (2) The sun offers the red and near-infrared light of about 23.6 mW/cm^2 (depending on environmental factors). (3) It might be best to emulate nature when possible.
"Photobiomodulation exposure to visible and IR-A light which emulates the conditions of natural sunlight in wavelength, intensity, and dosage can be beneficial to the skin." (1)
Have you ever taken high doses of Vitamin C all at one time? Might lead to a quick trip to the bathroom - your body cannot absorb all of it at once! When you want to raise blood levels of Vitamin C comfortably, you space out the dose over a longer period of time.
"Another guiding principle is that repeating the treatment daily (or even more or less often) until the wound is healed or the disease remission is observed is better than only one single application of LLLT. LLLT can be compared to a nutrient food for the human body; an adequate daily intake is best." (8)
So what does the science say?
In "Infrared and Skin: Friend or Foe" the author offers that lower irradiance <50 mW/cm^2 is less likely to induce detrimental effects. (1)
Irradiance from 5 to 50 mW/cm^2 is generally used for stimulation and healing, while higher irradiances can be more for nerve inhibition and pain relief. (7)
A dose of 3 J/cm2 was significantly more effective when delivered at 5mW/cm^2 than 50mW/cm2. It performed the same as 30 J/cm2 delivered by 50mW/cm^2. Indicating that the duration of treatment also plays a role. (2)
When treatments are administered too closely together, the cumulative dose can reach an inhibitory range. (6) This could even mean that in the first days of treatment could have a beneficial effect and then cross over into a biphasic region. (7)
A 12-week study, understanding that the effects of PBM in muscle tissue lasts up to 48 hours, chose to use a low intensity of 8.1 mW/cm^2 for 15 seconds per treatment, 3 times a week. They cautiously designed a low dose to avoid the cumulative biphasic dose. And they were right, this course of treatment improved overall muscle fitness versus the placebo! (9)
The application of 5 J/cm2 with 4 mW/cm^2 had a significant effect on wound tensile strength, while the same dose delivered at 15 mW/cm^2 had no effect. (7)
The dose required to promote a therapeutic response is <10mW/cm^2, a dose 100 times this amount was found to have detrimental neuronal effect in mice. (4)
Hart et al. were so concerned about high dose biphasic response for brain PBM, that they were considering the additional exposure from Sunlight on patients adversely affecting clinical trials. (3)
Human stem cells had a peak response at 3 J/cm^2 with 16 mW/cm^2, where higher doses had diminished benefits. (5)
One study on collagen production found that medium irradiances and exposure durations were effective - 1.2 mW/cm^2 for 145 sec. (1)
What does this all mean?
Intensity does matter, but more isn't always better! Do you cook your turkey low and slow, or set the oven to broil and let it burn on the outside and be frozen on the inside?
There are certainly applications for higher irradiances. We would like to see a distinguishment in Red Light Therapy for a "Recommended Daily Allowance" as opposed to a "Therapeutic Dose". An RDA for light!
If you own your own PBM device and plan to use it daily, you do not need to chase therapeutic doses all the time. By using PBM consistently and responsibly, your body is getting the photo-prevention effects. Similar to taking a consistent dose of Vitamin C for proactive reasons. As you know too many antioxidants can have negative effects, the same mentality should be applied to red light therapy.
If you already own a high-intensity product, no need to fret! Take an extra step back and use it slightly longer (or shorter!). Ask your manufacturer to provide accurate data on irradiance less than 20 mW/cm^2 and distances.
If you own the GembaRed Groove, this is great news. You can use it conveniently close to the body, on a chair, couch, bed. No need to fear too much irradiance, less is more!
What do you think? Does the law of reciprocity strictly apply to PBM, or do the benefits change based on intensity and time-frame administered?
"High doses of laser energy can focus on the postoperative pain, but to obtain a good healing process, lower power and longer time are more advantageous." (6)
This quote is reinforced by the Arndt-Schulz curve describing the biphasic curve as "stimulatory" and "inhibition" regions. Stimulatory doses can promote growth, while inhibitory doses can be used to manage pain. However for the long-term benefits of Photobiomodulation, we want to stay in the Stimulatory zone.
PBM: Photobiomodulation is the biological effects of light.
Irradiance or Intensity: Power Density of light, typically in units of mW/cm^2 (milli-Watts per Centimeter Squared)
Dose: Energy Density when a light is applied to an organism, typically in units of J/cm^2 (Joules per Centimeter Squared)
Biphasic Dose: Too low of a dose is not effective, a too high dose is also not effective. A peak response may lie somewhere in between.
Disclaimer: All information in this article and website are intended for educational purposes only. It is not intended to treat, diagnose, or cure any ailment. Please consult with your doctor or trusted wellness practitioner before starting any new health activity including Red Light Therapy.
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