Vitamin D: Over the past decade, dozens of studies have revealed many important roles for vitamin D, the nutrient that skin cells produce when they are exposed to sunlight. The recommended daily intake of Vitamin D is 600 IU per day, although recommended levels are under review. If you avoid the sun or live in the northern half of the U.S., ask your doctor whether your vitamin D level should be tested.
Katz DL, O'Connell M, Yeh MC, Nawaz H, Njike V, Anderson LM, Cory S, Dietz W; Task Force on Community Preventive Services. Public health strategies for preventing and controlling overweight and obesity in school and worksite settings: a report on recommendations of the Task Force on Community Preventive Services. MMWR Recomm Rep  2005;54(RR-10):1–12.

It doesn't matter how many pushups you can do in a minute if you're not doing a single one correctly. “There is no point in performing any exercise without proper form,” says Stokes, who recommends thinking in terms of progression: Perfect your technique, then later add weight and/or speed. This is especially important if your workout calls for performing “as many reps as possible” during a set amount of time. Choose quality over quantity, and you can stay injury-free.
Oaks BM, Young RR, Adu-Afarwuah S, Ashorn U, Jackson KH, Lartey A, Maleta K, Okronipa H, Sadalaki J, Baldiviez LM et al. Effects of a lipid-based nutrient supplement during pregnancy and lactation on maternal plasma fatty acid status and lipid profile: results of two randomized controlled trials. Prostaglandins Leukot Essent Fatty Acids  2017;117:28–35.
If motivation is your hang-up, change your exercise routine every 14 days. A University of Florida study discovered that people who modified their workouts twice a month were more likely than to stick to their plans compared to those who changed their regimens whenever they wanted to. Boredom didn’t appear to be a factor; it seems people simply enjoyed the variety more.