I was at the doctor yesterday and got a question by text about whether green smoothies were unhealthy and I was completely perplexed why anyone would think that. Sure you can make an unhealthy green smoothie by loading it up with fruit and eliminating the kale and adding a little spinach instead. However the person who asked this question is a fan of healthy green smoothies with kale so I knew something was up. We texted back and forth for a while and I called her and we talked about it and got everything cleared up. Once I got home the email questions started flooding in and that was when I knew I had really needed to write this post for everyone who was confused but didn’t have anyone to ask about this. So much for my day off, darn it! *ugh*
I make my green smoothies with kale for a number of reasons but the primary one is that kale is the most nutritious raw and I think works well in smoothies from a flavor perspective. However kale (like all cruciferous veggies) is generally more nutritious when eaten raw which I will explain so that you understand the process and aren’t confused the next time someone writes something to the contrary. When I first heard about this controversy the first thing I thought of was the comment I wrote yesterday about being careful what you read in popular books, clearly the same thing goes for what you read on blogs.
All vegetables contain protective micronutrients and cruciferous veggies are no exception. If you were to look at the cells of kale under a microscope you would see each one contained a number of separate items included myrosinase (enzyme) and glucosinolates (organic compound) inside a vacuole (membrane). Think of myrosinase and glucosinolates like chickpeas and tahini before they are combined into hummus. When kale leaves are intact (whole) the myrosinase is physically separated from the glucosinolates. (1) However in order for the cruciferous veggies to form indoles and isothiocyanates the myrosinase and glucosinolates need to be combined. (1) They can be combined by blending, chopping or chewing. (2) Obviously blending does a more complete job of breaking down the cell walls releasing the maximum amount of myrosinase so it can combine with the glucosinolates. However it is also important to know that myrosinase is inactivated by heat (cooking) (3). If you cook whole or roughly chopped kale you are getting significantly less of the indoles and isothiocyanates (ITCs) because of the reduction in myrosinase. The human digestive tract does have some myrosinase activity but the absorption of the ITCs is substantially lower for cooked cruciferous veggies according to numerous studies. (4) Glucosinolates are water-soluble so boiling is not a good way to prepare cruciferous veggies. (5) Boiling (2), steaming (3) and microwaving (6) are thought to completely inactivate myrosinase making the cruciferous vegetables far less nutritious.
Why should we care about our absorption of ITCs? ITCs are critical for optimal human health since they not only kill cancer cells (7) but they remove carcinogens in both the phase I and II processes. (4) Additionally ITCs are anti-inflammatory (8), induce cancer cell-cycle arrest (7) and have been shown to eradicate H. pylori (9) (a major cause of stomach cancer). A better question might be why wouldn’t you want to absorb as many ITCs as you possibly could?
Why do we need indoles? In the acidic environment of the human stomach indole-3-carbinols (I3Cs) combine to form acid condensation products (10) which include: dimer 3,3’-diindolmethane (DIM) and cyclic trimer (CT). The biological activity of DIM and CT differ from I3C but I3C is typically credited for the effects. Both these (DIM and CT) form to a lesser degree when myrosinase has been inactivated by cooking. (11) I3Cs have been found to inhibit estrogen activity and thereby are thought to decrease estrogen-sensitive cancers (think breast and prostate). (12) Additionally I3Cs also have anti-angiogenic (13) properties (stop the growth of blood vessels to cancerous tumors) and they induce cell death (14) (apoptosis). I3Cs have been shown to eradicate precancerous cervical lesions in some women. (15) I don’t know about you, but I say bring on the I3Cs and as many as possible which means more raw cruciferous vegetables please.
First, I apologize for all the endnotes but when something is put out without any backup I worry that no one will have any confidence in the information that is being provided. I will not be boring you with endnotes often going forward but I thought it was necessary in this post.
Here is the bottom line for me. Consuming cruciferous vegetables is important for our health. Based on the science I have read blending raw kale is not only healthy but is the healthiest way to consume your kale. As with everything else I will continue to stay current on the latest science on this topic and if anything changes I will let you know. Until I see scientific research that tells me that raw pureed kale is unhealthy I will continue to enjoy green smoothies (made with copious amounts raw kale I might add) most days of the week. Dan and I both had green smoothies for breakfast this morning if you are curious. ;-)
1. Holst B, Williamson G. A critical review of the bioavailability of glucosinolates and related compounds, Nat Prod Rep 2004; 21(3):425-447
2. Shapiro TA, Fahey JW, Wade KL, Stephenson KK, Talay P. Chemoprotective glucosinolates and isothiocyanates of broccoli sprouts: metabolism and excretion in humans. Cancer Epidemol Biomarkers Prev 2001;10(5):501-508
3. Conaway CC, Getahun SM, Liebes LL, et al. Disposition of glucosinolates and sulphoraphane humans after ingestion of steamed and fresh broccoli. Nutr Cancer 2000;38(2):168-178
4. Lampe JW, Peterson S. Brassica, biotransformation and cancer risk: genetic polymorphisms alter the preventive effects of cruciferous vegetables. J Nutr 2002; 132(10):2991-2994
5. McNaughton SA, Marks GC. Development of a food composition database for the estimation of dietary intakes of glucosinolates, the biologically active constituents of cruciferous vegetables. Br J Nutr 2003;90(3):687-697
6. Rouzan G, Young SA, Duncan AJ. Hydrolysis of glucosinolates to isothiocyanates after ingestion of raw or microwaved cabbage by human volunteers. Cancer Epidemiol Biomarkers prev 2004;13(1):125-131
7. Zhang Y. Cancer-preventive isothiocyanates: measurement of human exposure and mechanism of action. Mutat Res 2004;555(1-2):173-190
8. Heiss E, Herhaus C, Klimo K, Bartsch H, Gerhauser C. Nuclear factor kappa B is a molecular target for sulforaphane-mediated anti-inflammatory mechanisms. J Biol Chem 2001;276(34):32008-32015
9. Galan MV, Kishan AA, Silverman AL. Oral broccoli sprouts for the treatment of Helicobacter pylori infection: a preliminary report. Dig Dis Sci 2004;49(7-8):1088-1090
10. Hertzer HG, Senft AP. The micronutrient indole-3-carbinol: implications for disease and chemoprevention. Drug Metabol Drug Interract 2000;17(1-4):159-188
11. Shapiro TA, Fahey JW, Wade KL, Stephenson KK, Talalay P. Human metabolism and excretion of cancer chemopreventive glucosinolates and isothiocyanates of cruciferous vegetables. Cancer Epidemiol Biomarkers Prev 1998;7(12):1091-1100
12. Jordan VC, Gapstur S, Morrow M. Selective estrogen receptor modulation and reduction in risk of breast cancer, osteoporosis, and coronary heart disease. J Natl Cancer Inst 2001;93(19):1449-1457
13. Chang X, Tou JC, Hong C, et al. 3,3’-diindolylmethane inhibits angiogenesis and the growth of transplantable human breast carcinoma in athymic mice. Carcinogenesis 2005;26(4):771-778
14. Chinni SR, Li Y, Upadhyay S, Koppolu PK, Sarkar FH. Indole-3-carbinole (I3C) induced cell growth inhibition, G1 cell cycle arrest and apoptosis in prostate cancer cells. Oncogene 2001;20(23):2927-2936
15. Stanley M, Chapter 17: Genital human papillomavirus infections-current and prospective therapies. J Natl Cancer Inst Monogr 2003;31:117-124
How to Make a Healthy Green Smoothie:
Making a healthy green smoothie is easy and you all know what I am going to say based on what you see me prepare. First include a lot of raw kale in your smoothies. Frozen banana added to the mix makes the smoothie creamy. Including walnuts and flaxseeds for healthy fats assists your body to absorb the fat-soluble vitamins in the kale. Cinnamon is also added to our green smoothies for additional antioxidants as well as flavor. Vanilla is included because it smoothes out the sharpness of the greens allowing me to add much more of them to our smoothies. You can also add frozen cherries and cocoa powder for flavor and antioxidants. That is how you make a delicious and healthy green smoothie.
Calories Burned Chewing:
This is just a side issue but I am mystified that anyone would believe chewing kale could result in 100 calories being burned. *scratches head* It takes 1 mile, either running or walking to burn 100 calories and that is using the largest muscles in the human body. Let’s think about that for a minute. If you walk a mile in 15 minutes you are using your legs and buttocks to generate movement and that burns 100 calories. The muscle mass of those muscles is what at least 100 times greater than your jaw muscles, and probably more? So that would imply that you would need to chew for at minimum of 1500 minutes (15 times 100) to burn 100 calories. Since there are only 1440 minutes in a day I find it impossible to see how this math works.
I appreciate that so many of you wrote to ask me what I thought about green smoothies. Your faith in my ability to cut through the controversy and give you the truth has really touched me. I would prefer to give up my day off if it means that I can clarify things for you. I wanted to include plenty of endnotes so that you could follow the science for yourself if you choose, or at least realize that this wasn’t just “my opinion on the matter” but rather there is hard science which backs up everything that I wrote. Now I am going to try to enjoy the rest of my “day off”.
If you ever see anything that confuses you in the future just ask me. If I don’t know the answer I will do my research and find out for all of us.